Oversized boiler, extremely high heating bills

[Ilavey]

Hey All,

We recently moved to Indianapolis, IN and bought a house built in 1960. We had been warned by the previous owners that the heating bills were very high (+$700/month), but the house was priced to sell and we were in love with the mid century architecture, so we pounced. We wanted to get through at least one winter to see how bad things really were before making any changes. The heating bills were as bad, if not worse than advertised. We're currently burning about 10 gallons/day of heating oil during January and February.

Some additional details about the situation for additional context:

• The heating is being provided by a Weil-McLain Oil-Fired Water Boiler 242k BTU with a 2 gallon nozzle on a Beckett oil burner.
  
• House is ~5,100 square feet
• There used to be radiant floor heating in the walkout basement. About 5 years back, the radiant floor began springing leaks and was decommissioned. The HVAC contractor rerouted the basement zone to the air handler used for our central AC to provide a heating coil for forced air heat to the basement.
• The basement is just exposed cement block with a stone exterior.
• The forced air is not zoned so when it kicks on for the basement, it also sends hot air upstairs. The previous owners recommended closing the vents upstairs but that just creates an immense amount of back pressure on the air handler and leaks through the vents anyway.
• The house still has original aluminum double pane windows which probably should be replaced. However, there are a lot of them and I think this is going to be very expensive.
• We had an energy audit done in the spring in order to understand insulation, air leaks, etc. They said the house had surprisingly good insulation for a house of the era. He did call out how much glass we have in the house and recommended we replace the original aluminum windows. He did a blower door test and the results were: 8.75 ACH and 6585 cfm.
• The boiler has very short runs times. I timed it today and it ran for about 3 minutes.
• We've kept the thermostats set to 68 degrees.

It seems like it would be a good idea to eventually tighten up the house, add additional insulation, and replace the aluminum windows. However, unless we address the boiler, my concern is we won't really see much return on those investments.

Am I right to think that even with the shortcomings of the house, the boiler is oversized? I've had a very hard time finding an HVAC contractor in the area that knows much about boilers. The majority of them I've spoken with have just recommended we abandon the boiler system and move to a furnace. Some of the contractors that seemed more knowledgeable did express concern that the current ductwork could be poorly set up for forced air heat. It's all center room ceiling vents as it was designed for AC. That being said, even those contractors appeared to be using very questionable sizing methodologies.

Appreciate anyone willing to read through this entire post! I know it's very lengthy but I'm hoping to provide enough relevant information.

 

[Ilavey]

I should also add that we are having gas run to the house as part of a kitchen remodel we are currently doing.

 

[Sponsor]

 

[Fitter30]

Number 2 oil 140k btu's per gallon 2 gallon nozzle 280k. Boilers are more efficient with longer run times what were the times during the bitter cold snap? Your oil supplier may have their own burner tech or able to recommend one. Windows, doors and insulation make all the difference in a house and should be addressed before a boiler. What is model of boiler? What water temperature are you running? Take a few pics of the boiler pumps and piping around it. One control that would help is outdoor reset which varies water temperature with outdoor temperature. Blower door test effective leakage area 4.57 sq ft and 8.75 ach is at the top of moderate leaky house 9 is very leaky. What heats the upstairs other that the basement air handler?

 

[Ilavey]

Thank you very much for the response!

Number 2 oil 140k btu's per gallon 2 gallon nozzle 280k. Boilers are more efficient with longer run times what were the times during the bitter cold snap?

I had never timed it previously, but the runs were still infrequent and very short.

What is model of boiler? What water temperature are you running? Take a few pics of the boiler pumps and piping around it. One control that would help is outdoor reset which varies water temperature with outdoor temperature.

• Boiler is Weil McClain GO7 3 Series
• It runs at about 165 degrees Fahrenheit
• I added some pictures to this thread. The boiler is actually offline at the moment as they are moving a baseboard as part of our kitchen remodel. That's what all the water on the floor is from when they drained the system.
• I know I have some type of setup that is supposed to allow the boiler to preheat water when the outside temperature drops below a certain threshold. Is this different than what you're referring to?

What heats the upstairs other that the basement air handler?

We have 134 feet of slant fin baseboard upstairs providing the heat which is split into two zones. That's the measurement of the actual copper, not the covers. There are no baseboard in the basement as they had relied on the radiant floor heating that was laid directly in the slab.

Windows, doors and insulation make all the difference in a house and should be addressed before a boiler.

This was a big part of my question. If my boiler is oversized, will any of these changes making a meaningful difference? From my reading on this thread, it sure sounded like my boiler was way too big for the house.
​

 

[JohnCT]

10 gallons of #2 a day on a 5100sq house during the coldest weeks doesn't seem ridiculously excessive. Big houses cost a lot to heat no matter how good the heating system is.

There are ways to install or fabricate internal "storm" windows to install during the colder months. Window losses really add up.

Also, 2 gph nozzle seems high even for a house that big. I would see what the lowest recommended firing rate is for your boiler and downfire it (adjust Beckett with combustion equip.) If nothing else, it should extend your run times quite a bit.

 

[Ilavey]

Is the file I uploaded what you're referring to as the minimum fire rate? If so, it seems like there is an opportunity to reduce the firing rate a little, but not a lot.

I had run the calculation for the BTU requirements for the house based on a few other threads I saw on this forum and came up with a BTU/HR requirement of 69k BTUs. This was based on the following article: https://www.greenbuildingadvisor.com/article/out-with-the-old-in-with-the-new

Is this the right way to look at it or does my house really need a boiler as large as I currently have?

 

[Fitter30]

Outdoor reset - outdoor temp 60* water temp 130* outdoor temp 0* 180* water temp
Oil nozzles are rated at 100 lbs pressure but when a burner is set up the oil pump pressure is adjustable. At 125 lbs 2 gallon nozzle puts out 2.24 gallons boiler could be over fired. There might be a old invoice or something written on the boiler. Boiler s have two temp controls a high limit safety and a operating. Limit can be set as high as 190* operating has two adjustments on and off. The difference between the two could be called differential. Setting the control 180* off, on 135* should give you more run time if you don't have outdoor reset. Return water temp has to be a minimum of 135*. Another poster Dana might want to contact if he doesn't chime in.

 

[Ilavey]

I remember my HVAC technician making a comment about the honeywell device utilizing some type of temperature from outdoors. This is set to 135*. The boiler seems to stay at a consistent ~165*. Is that the high limit temperature?

 

[Fitter30]

Honeywell control might be outdoor reset looks like it has two bulbs one strapped to supply of the boiler and the other outside north exposure. Operating and limit will be mounted on the boiler. The wire from outdoor reset should run to the boiler pull cover there wil be a number that is inked on the control or cover 7 numbers 2 letters starting with a t. Wouldn't surprise me if that control is either disconnected or has failed.

 

[Ilavey]

I was able to find it. It is set to 180*. Does this mean it should be operating at 180*? It seems to only heat up to the ~165* listed on the gauge.

 

[Fitter30]

I was able to find it. It is set to 180*. Does this mean it should be operating at 180*? It seems to only heat up to the ~165* listed on the gauge.

Boiler water temperature would vary on outside air temperature. 36* in Indianapolis 7:55p see what it is tomorrow

 

[Ilavey]

It's still at that 165* it has been sitting at. Is there somewhere on these devices that I should be able to see what outside temperature it adjusts at? Really appreciate your continued help with this!

 

[Dana]

From a fuel-use load calc you came up with ~70K (@ +6F, the 99% outside design temp for Indianapolis?). Is that the 99% heat load, or is that with the ASHRAE 1.4x oversize factor?

For a dumb rules of thumb sanity check, 70,000 BTU/hr divided by 5100 square feet is just shy of 14 BTU/hr per square foot. That's a bit on the low side for a 2x4 framed house if it were all above-grade, quite low for a high glass-area mid-century modern type house testing at 6585 cfm/50Pa. How much of that 5100 square feet is the walk-out basement? (Something like half, maybe?)

An 8" CMU block with a stone veneer has a U-factor of about 0.4 BTU/hr per square foot per degree-F, so the heat load of that exposed CMU is quite large, even in basement rooms that aren't actively fully heated. At 55F indoors, 5F outdoors (a 50F delta) every square foot of above-grade foundation wall is losing 0.4 x 50F= 20 BTU/hr. So if you have say and average of 2' of exposure on a 240' perimeter ( a 40' x 60' rectangular footprint) there's 480 square feet, times 200 BTU/hr is 9600 BTU/hr or about (9600/69,000=) 14% of the total calculated heat load, even if the basement weren't fully heated.

CMU walls are also notoriously air-leaky- those tiny cracks add up to a heluva lot more square inches than intuition might tell you. Air sealing the foundation sill to a CMU wall is also a problem, but usually solvable with closed cell spray foam.

Also, leaks at the bottom of the house are as important as leaks into the attic spaces from a stack-effect drive infiltration point of view. The leaks at the top & bottom are FAR more important than the window & door leaks, since the height between bottom & top are what defines the stack effect pressures.

So the middle-page (not absolute bottom) line is that it's WELL worth air sealing and insulating the basement walls all the way from the slab, up and over the top of the foundation, sill plate and band joist. Indianapolis is in IECC climate zone 5, where the IRC prescriptive is R15 continuous insulation, which can be achieved with 2.5" of foil faced rigid polyiso, 3" of cheap reclaimed roofing polyiso, or 4" of EPS (reclaimed or virgin stock.) The same performance point can be hit with 1" of foil faced polyiso trapped to the wall with a 2x4/R13 (unfaced) studwall. With a sheet foam or foam + studwall approach it's generally better to use 2" of HFO blown closed cell foam to air seal and insulate band joist, foundation sill, and foundation top all the way out to the top edge of the wall foam (or top plate of the studwall.) Alternatively, 2" of HFO-blown closed cell foam from the slab all the way up & over the top of the foundation, sill & band joist yields about R14. It's expensive, but much quicker to install. The 1" foam is necessary to avoid wintertime moisture accumulation on the above grade section of the fiber-insulated studwall using only standard latex paint on wallboard as the interior side vapor retarder. A true vapor barrier on the interior is risky, since that would trap ground moisture in the studwall. I've covered the details of different basement insulation approaches in several threads over the past several years, but if you're considering taking on that project I can go over it again, or you can dig those threads out using the search function on this site.

Contrary to intuition, replacing the boiler for something right-sized before fixing the thermal efficiencies of the house the isn't going to be the first-best investment. For a beastie boiler running very low duty cycles a 3 minute burn is a short cycle, and the standby losses to an uninsulated basement are huge. Short of replacing the boiler, installing a heat purging economizer to bypass/replace some of the aquastat controls would reduce the average boiler temp (reducing standby losses), increase burn times, and reduce the number of lossy ignition cycles. The air handler is probably ludicrously oversized for the basement zone, and since a walk-out basement's load characteristics don't track with temperature very well with fully-above grade floors it's best to It's probably better to decommission the heating coil and install panel rads (or fin-tube baseboard) sized correctly for the basement load. If insulating the basement walls, size the radiation for the "after upgrades" condition of the house, not the where-is-as-is uninsulated foundation clear-glass double-panes condition.

 

[Dana]

As a general rule the order of project payback (in both comfort and fuel cost) is:

1> AIR SEAL THE HOUSE, concentrating on the upper floor ceilings and the basement walls/floor/band-joists.
1b> Re-commisson the existing boiler with better controls, not just a tune up.

2>AIR SEAL IT EVEN MORE, concentrating on bigger leaks first. This may require blower door & infra-red directed air sealing to find the more subtle less obvious leaks. Until you can get it down to < 3ACH/50 true comfort is going to be illusive.

3> Insulate the most accessible (like bare CMU, or open attic floors) to at least current IRC code where ever possible.

4> Find and fix any defects in the framed wall insulation. This may take a bit if sleuthing with an IR camera (or IR thermometer, if you're very patient and diligent). Or, just bite the bullet and have somebody install dense-packed cellulose (3lbs per cubic foot min), or fiberglass (1.8lbs min) over any pre-existing batts. This is a bit of a PITA compared to dense packing empty cavities, but it's not super hard, even with mid-density R13s, pretty easy with R11s. (I've recently done it myself on a 2x4 framed 2200' house- it took about 1.5-2 man-days.) Dense packing blown insulation tightens up the house considerably, since the fiber follows the escaping air to clog the air leak paths, and the denser fiber reduces the convection losses through the insulation layer by about 99% compared to R11/R19 type density batts.

5>Replace the boiler, sizing it for the load of the "after all upgrades" (including windows, if that's in the plan) condition of the house. Since even the smallest oil-boilers meet or exceed the 69K output requirements of the house as-is, any replacement is likely to be oversized for the "after upgrades" picture of the house. It may make more sense from a comfort point of view to install right-sized modulating air source heat pumps, one for the upstairs zone, a second for the downstairs.

6>Upgrade the windows. In 1960 double-pane windows weren't commercially available, so those aluminum double-panes are probably late 1970s (post oil-shock) or early 1980s vintage. The U-factor on aluminum framed double-panes with clear glass (no low-E) is about 0.60 BTU/hr per square foot per degree F (yes, that's less lossy per square foot than your CMU wall). If the seals aren't broken (do any of them fog up?) you could hit current IRC minimum performance by installing low-E exterior storm windows over the fixed windows at a fraction of the cost of a full replacement. There are good l0w-E storm solutions for double-hungs too, but most mid-century moderns have casement or awning type windows, which don't have good storm window options.

FWIW: A handful of years ago I got called in to advise on what to do with a 4000' (2000 up, 2000 down) walk-out basement mid-century modern in central MA (outside design temp +5F) with a flat roof, minimal roof insulation (~R10 vermiculate + R2 fiberboard) and a failing radiant ceiling on the upper floor, with TERRIBLE air conditioning loads. The homeowners had already replaced the expanse of U0.1 single pane glass with U0.28-U0.30 double panes, an it was still miserable, both winter & summer. The prior owners had installed 9 tons of AC in the form of a 4 ton + 5 ton commercial building rooftop package unit and a crazy insulated duct sculpture above the roof(!) . It cooled the place, but sounded like a B29 on a take-off roll when running.

Even though it was ridiculously expensive, I advised them to build a new very low pitch roof over the old roof, creating an attic just deep enough to house new right sized ducts & mechanicals based on a professional third party Manual-J load calculation and Manual-D duct design. They mothballed the 250KBTU gas boiler, and installed a 60KBTU 2-stage gas furnace with 2 zones, using part of a closet as the duct chase to the lower level. The AC was replaced with a 2-stage 4 ton unit using the same air handler as the furnace. The comfort levels went from MISERABLE to SUBLIME. With the small air handler the system could barely be heard, with no wind-chill effects even at high-stage, and the room to room/floor temps rock stable with minimal drafts, even next to the taller windows. As it happens they sold the house last spring (down-sizing to prevent their adult children from returning ) and despite the high cost of their modifications they still made money on that house. Had they done it by replacing the mechanicals first they would have had a 100K furnace and 5-6 tons of AC, much noisier/draftier (and more expensive), with a lower comfort level.

Some light reading and video-ing while pondering your options, download the Nate Adams' freebie chapters and watch the short videos on

Home Comfort 101

HVAC 101

HVAC 102

Nate has an online video channel covering related topics in much greater detail, as well as numerous case studies on his Energysmart Ohio and other info on his Nate the House Whisperer websites. Being located in Cleveland his climate is very similar to yours, and some of the climate-specific details are relevant.

 

[Ilavey]

Thank you very much! This is super helpful and insightful!

From a fuel-use load calc you came up with ~70K (@ +6F, the 99% outside design temp for Indianapolis?). Is that the 99% heat load, or is that with the ASHRAE 1.4x oversize factor?

For a dumb rules of thumb sanity check, 70,000 BTU/hr divided by 5100 square feet is just shy of 14 BTU/hr per square foot. That's a bit on the low side for a 2x4 framed house if it were all above-grade, quite low for a high glass-area mid-century modern type house testing at 6585 cfm/50Pa. How much of that 5100 square feet is the walk-out basement? (Something like half, maybe?)

• This was the 99% heat load, however I did find an error in how I was calculating gallons of oil consumed during this period. I adjusted this and it brought the 99% heat load to ~77k BTU/hr with the 1.4x oversize factor bringing it to 108k BTU/hr. Just to make sure I'm not making any major mistakes, I'll add in my inputs to how I got here. We consumed 217 gallons of heating oil between 12/18/20 and 1/12/21. I downloaded the degree day data from a website to end up with 823.7 degree days during this period. Our boiler is rated at 85% efficiency and I used 6F as the 99% outside design temperature.
• I somehow have 5,100 square feet drilled into my head from when we bought the house, but this includes a 700 square foot garage that is not heated. It did have heating at one time, but this zone was decommissioned at the same time as the radiant floor heating in the basement. The actual conditioned space being heated is ~4,400 square feet. The above grade square footage is 2,591. This leaves the walkout basement with 1,891 square feet.
  
• I should also note that the upstairs is split into two separate zones. One for the living room, entry way, and kitchen. The second is for all of the bedrooms and bathrooms.

For a beastie boiler running very low duty cycles a 3 minute burn is a short cycle, and the standby losses to an uninsulated basement are huge. Short of replacing the boiler, installing a heat purging economizer to bypass/replace some of the aquastat controls would reduce the average boiler temp (reducing standby losses), increase burn times, and reduce the number of lossy ignition cycles. The air handler is probably ludicrously oversized for the basement zone, and since a walk-out basement's load characteristics don't track with temperature very well with fully-above grade floors it's best to decommission the heating coil and install panel rads (or fin-tube baseboard) sized correctly for the basement load.

• I see that you've recommended these economizers in the past, but I don't quite understand how they work. Is this something that would be a no brainer to install?
• Based on how the previous owners talked about the air handler situation, it doesn't seem like it was ever meant to be a permanent solution. They knew that they were going to be selling the house in the next few years and decided on the quickest fix. Anecdotally, this zone seems to be called for most frequently as it seems like that pump is always running when I go into our equipment room vs. the other zones which only seem to be pumping on occasion. Getting this zone onto baseboards and/or radiators was at the top of my list but I have yet to get quotes on the costs and wasn't sure if I should wait to do this in conjunction with a boiler replacement.

1> AIR SEAL THE HOUSE, concentrating on the upper floor ceilings and the basement walls/floor/band-joists.

• We do have batt insulation in attic in addition to about 10" of blown in cellulose insulation. Based on the research I've done online, they recommended removing all existing insulation, air sealing, and then re-insulating. Is that typically what's required to fix air leaks into the attic or should we have a third party auditor confirm that is where the leaks are happening first? My concern with dealing with insulation companies is that they have a vested interest in selling me new insulation.

2>AIR SEAL IT EVEN MORE, concentrating on bigger leaks first. This may require blower door & infra-red directed air sealing to find the more subtle less obvious leaks. Until you can get it down to < 3ACH/50 true comfort is going to be illusive.

• One thing to note is that while it's very expensive (in my mind) to heat, the house is actually very comfortable. I'd say within 5 minutes of turning up the thermostat up a few degrees, the zone is at that temperature and it easily stays at that temperature. I don't know if that's abnormal with a leaky house. Is it possible for an auditor to mess up the blower door test? He seemed to be doing a lot of napkin math for the inputs he was adding to his computer.

6>Upgrade the windows. In 1960 double-pane windows weren't commercially available, so those aluminum double-panes are probably late 1970s (post oil-shock) or early 1980s vintage. The U-factor on aluminum framed double-panes with clear glass (no low-E) is about 0.60 BTU/hr per square foot per degree F (yes, that's less lossy per square foot than your CMU wall). If the seals aren't broken (do any of them fog up?) you could hit current IRC minimum performance by installing low-E exterior storm windows over the fixed windows at a fraction of the cost of a full replacement. There are good l0w-E storm solutions for double-hungs too, but most mid-century moderns have casement or awning type windows, which don't have good storm window options.

• I attached a photo of one of the living room windows. Many of them do have broken sales and fog up. Based on how they are installed, they don't seem to be a good candidate for exterior storms. All of the windows are either double or triple sliding windows.
• I've seen a lot of conflicting information out there about the ROI of replacing windows. Does the state or type of windows I have change the priority level or should this still only be addressed once I've done steps 1-5?

Perhaps another additional piece that could be helpful. My electricity bills during the summer have not been on proportion with my heating bills during the winter. My power company is making changes to their website so I don't have data to the actual consumption, but my electricity bills were only $80-$100 more than what I'm currently paying for during the winter. I have a Carrier Weathermaker model #38tdb060300 A/C. The house cools relatively easily even during the hottest of days. Is it normal or strange that my cooling bills seem to be more reasonable than my heating bills?

Really appreciate your help with this. I'll also check out these resources you posted. While this have been financially very frustrating, I do enjoy learning about these topics and would love to be more a more competent home owner.

 

[Dana]

The sanity check using the new numbers, a 77K load for 4400' of conditioned space is 17.5 BTU/hr per square foot, which would be in the right ball park for a reasonably tight fully above grade 2x4 framed house with at least R11 in the cavities, and clear glass double-panes or your not-so-insulated walk out basement with a in insulted framed walls on the upper floor.

Heat purge economizers all have their own proprietary algorithms, but most basically draw heat from the boiler at the beginning & or near the end of call for heat, and try to lengthen the burn times by running a bigger difference between the (usually user programmed) low and (algorithm determined) high temp limits of the boiler, and lowers the average operating & standby temp of the boiler. Old school aquastat controls are usually limited to a max of 15-20F of differential, which limits the max burn time. If you can program a low limit to 140F (typically fine for oil burners if there's a stainless flue liner) and the max differential is 20F it can only swing from 140F-160F. But if a smart control can allow it to run to 200F that's a differential of 60F- 3x the temperature swing, but more than 3x the minimum burn time during a continuous call for heat, since the radiation puts out about twice as much heat at an entering water temp of 200F as it does at an EWT of 140F. The higher heat emittance at the high temp end of the swing means the rate at which the temperature is rising is slower.

Heating up quickly from setbacks isn't usually a good measure of comfort, but maybe it's what most people become used to given the typical 3x+ oversize factors for most existing systems. That level of oversizing ends up delivering repeated hot/warm flashes followed by extended cooling periods between calls for heat. It keeps up with the load, but the room temperatures are prone to overshoots & undershoots, and sometimes not all rooms on the zone get fully heated, depending on the radiation/ducting. A right-sized or modulating system keeps much steadier room temperatures, and delivers long steady heating cycles that get longer as the heat load drops. Heating with baseboards under big windows creating a warm updraft is quite comfortable when the baseboards are actually warm or hot, but when they're off there's a cold cascade of air coming down the face of the glass creating a cold draft that spills along the floor. With a right sized system the warming cycles are long, and when it's really cold the warming cycles would be on more than 2/3 of the time (71% of the time at the 99% outside design temp, if the radiation is sized for 1.4x the design load for the room/zone.)

Energy auditors screw up blower door tests all the time when calculating ACH/50 due to errors in calculating the house volume, but the raw cfm/50 number is much harder to screw up. 6595 cfm is a lot of air leakage.

If the seals on the double panes are failing replacement is really the best option, but don't even think about ROI- the payoff in energy costs is typically "not in your lifetime, or even the lifetime of the glass", but the payoff in increased comfort can be significant. That comfort payoff becomes obvious both in summer & winter, but it comes with a hefty price tag. From a resale point of view it can pay off in ease of sale and eventual sale price though. Houses with a large expanse of fogged up U-0.6 high heat gain window get discounted and are harder to sell.

Even with the seals broken the U-factor is still about 0.6. There are people who will drill tiny vent holes on the exterior pane to limit the amount of fogging, but if you're going to live there for a decade or more it's probably worth putting in higher performance glass simply for comfort. IRC code min is U-0.32, but double panes in the mid U-0.2s can still be "worth it". There are lots of decisions to make for the glazing type and low-E type, and you may want to use higher solar heat gain coefficient (the SHGC number) windows on the south & north sides to lower the heating bill and higher SHGC windows on the east and particularly the west sides, since it's harder to adequately shade the east & west windows that get sun when the sun is low in the side. South facing windows at your latitude can get at least partial shade from overhangs, and even in direct sun in summer the sun angle is high, with most of the heat reflected off the exterior surface. This is something worth analyzing a bit before making decisions on windows.

There is affordable double low-E double-pane glass with a hard coat low-E coating on surface #4 (the surface in contact with the room air) that run in the U0.20 range, but depending on the low-E coating on surface #2 (the interior side surface of the exterior pane) can have an SGHC of anywhere from the mid 0.2s to 0.6. A clear glass double pane typically has an SHGC of 0.75- it's all high-gain, and with the amount of glass you're looking at it's probably the major driver of your cooling load.

Any operable windows that can be converted to fixed windows will be both cheaper and less leaky/higher performance, so think about which windows you would ever really open up.

The 5 ton Carrier AC is also likely to be sub-optimally oversized for the cooling load even with the high-gain windows. A typical 4400' all above-grade house would have a 1% design load of about 3 tons, but with your windows it could conceivably be as high as 4. If you have optimal roof overhangs and shading factors it could be as low as 2 tons. The fact that it "...cools relatively easily even during the hottest days..." means that it's probably at least 2x oversized, if "..relatively easy..." means a low duty cycle &/or rapid recovery from setbacks.


This graphic is the square feet per cooling ton for dozens of Manual-Js performed by a consulting company in Decatur GA. Most of those houses were in the Gulf Coast states, with higher 1% design temps and higher latent load (humidity) than you would have in Indianapolis.

The two closest to the 4400' house size had a ratio of ~1500 square feet per ton, which would be 4400/1500= 2.93 tons. Unless you have an inordinant amount of of unshaded west facing window even on the walk-out basement level that's probably about where your load is.

 

[Ilavey]

Thank you very much for your continued responses. These are very helpful. I've started digging into the resources you suggested as well.

That makes sense on the heat purge economizer and the desire to increase the max burn time. JohnCT above also suggested downsizing the firing rate. It looks like there is a version of the WGO-7 called a "WGO-7R" which has a nozzle size of 1.6 resulting in a heating capacity of 196k (down from 242k). Is the also something you'd recommend in conjunction with installing the heat purge economizer in order to increase burn times?

In terms of the windows, it does sound like my money would be best spent air sealing and insulating the basement as you recommended. I'll do a search for the basement insulation threads you referenced above. We had it on our list of things to address in the house, but based on this conversation it seems like it would make sense to increase in priority. It sounds like this would also be the right time to decommission the heating coil in the air handler and install baseboards or radiators in the basement.

It's also good to know that my AC set up is also not ideal. I guess everything is relative when comparing $250 electricity bills for AC vs. $1000 oil bills for heating. The AC was installed in 2003 and the HVAC contractor recommended installing a heat pump when the AC reaches end of life which I imagine is coming soon. Based on what you're saying, it sounds like I should really dig into the size of this as well so they don't just replace a 5 ton unit with a 5 ton unit.

 

[Dana]

Before down-firing the boiler it's important to assess the venting. If it's on the large size for the firing rate and doesn't have a stainless flue liner a lower firing rate can (in some instances) cause excessive acidic flue condensation and destroy the chimney from the inside out. With a stainless liner, go for it.

A heat purging economizer will work about as well no matter what the firing rate is.

Before the AC has hit end of life, measure it's duty-cycle on summer afternoons when it's near the 1% outside design temperature. This is easier to do if you have some sort of data-logger that measures actual compressor on-times- do not assume that a call for coolth from a smart thermostat means the compressor is actually running continuously, even if the air handler is running continuously. Some of the methods & options are discussed in this bit of bloggery. (The author of that piece is the person who created the square feet per ton vs. house size graphic above.) An oversize factor of more than 1.5 is likely to have pretty lousy latent cooling characteristics due to the low duty cycle on all but the hottest days, but there are now modulating AC systems with enough operational range to make up for some of those shortcomings.

As with heating, the cooling load of a walk-out basement has very different characteristics than that of a fully above-grade floor. Walk out basements usually only get solar gains for a few hours of the day, whereas the sun usually hits some side of the above-grade part of the house any time the sun is shining. It's likely that the system would either need to be zoned with duct dampers (which comes with a bunch of design difficulties), or completely separate dedicated systems serving each floor. The sizing for those can't be determined by single fuel-use or duty cycle measurements. The gold standard is ACCA Manual-J heating & cooling load calculations, best performed by an independent third party (like a professional engineer or performance home architect), and not an HVAC company. The HVAC company Manual-Js usually exhibit an all too human inclination to be very conservative on the inputs and end up oversizing, often by more than 1.5x, then spec equipment that's upsized from there by 1.25x or more. Such oversizing guarantees that the home can be heated and cooled, but it's not maximizing comfort or value. Professional engineers on the other hand make their living and reputation by the accuracy of their numbers. Many HVAC companies go with even crummier rules of thumb, such as "A ton per 600 square feet" for cooling, or "35 BTU per square foot" for heating, leading to even bigger oversize factors.

For sanity checking your duty cycle & fuel-use numbers, it's good to run your own Manual-J(-ish) load calculations using some of the better online freebie tools such as CoolCalc or LoadCalc. Run the numbers room by room, for the house in it's current where-is-as-is condition, and keep tweaking the input parameters until it comes within at least 25% of the fuel use or duty cycle measured reality. These freebie tools tend to oversize by quite a bit, and even though you know the house is pretty leaky, assume both the house and ducts are completely air tight. When the tools are giving you numbers that are at least sort of credible the room by room and zone by zone numbers will also be sorta-credible, usually close enough to start penciling in equipment options.

In your climate it's unlikely that a heat pump ideally sized for cooling in a mid-century modern could be adequately heated with the same equipment, but maybe. A good place to start looking is NEEP's cold-climate heat pump search page. The short-spec single pagers for the equipment (eg the page for this 4 ton modulating full ducted unit) show the capacity at +5F (pretty close to the +3F design temp for Indianapolis), 17F, @ 47F, as well as the cooling capacity at 82F and 95F (the 1% design temp for Indianapolis is 88F.) As long as you don't upsize from your known oversized LoadCalc/CoolCalc somewhat tweaked load numbers it'll be big enough to cover the load comfortably.

It's worth mapping out the ducts & returns to first guarantee that it's at least sort of reasonable, and think about how it might be reconfigured into zones or two separate duct systems. Are all (or any of) the ducts mastic sealed & insulated?

Starting this analysis now gives you the luxury of time to both upgrade the house, re-measure and track any load reductions before committing to any new equipment. This is the sort of thing that only the rare whole-picture whole-house retrofit contractor (Nate Adams is one of those) has the time for, but it's in your interest to have it figured out and dialed in before making big upgrades.

 

[Ilavey]

I'll definitely take a look at running my Manual-J load calculations. Considering the age of my AC, I'd like to be prepared incase I need to replace in the near future.

I'm usually pretty good at finding my way around Google, but I've yet to come across anyone in the Indianapolis area that does this. Are there any centralized listings of home performance architects or specialists?

Is there a recommended manner for mapping out the ducts & returns when dealing with fully finished space? While our basement still has cement block wall, it is "finished" in that it has a finished ceiling so tracking ductwork is somewhat difficult. However, we did have to remove some drywall ceiling for some electrical runs we're doing for our kitchen remodel, and based on that I can tell you that the ductwork that runs throughout the basement is neither insulated nor mastic sealed. We also have a run of ductwork in our attic that is in fact insulated.

 

[Fitter30]

Mechanical engineering, hvac design and build contractors, mid century modern owner groups or facebook. Looking for the right company.