View Full Version : New Boiler and Indirect water heater question...

03-14-2012, 05:06 AM

I have a question about replacing my old boiler and hot water tank. Our boiler is original to our house (mid 80's) the hot water tank is under 10 years old. We have gotten 2 quotes for an high efficient boiler and indirect water tank. Both are at opposite ends of the spectrum. The first is for a Burham Alpine 105 with 105,000 BTUH input and a SSU45 Superstor and the second is for a Burham Alpine 150 with 150,000 BTUH input and a SSU80 Superstor. My question is do we need an 80gal tank? is 45 to little? We have gas heat. 4 zones. Our house is about 2500sqft with 3 1/2 baths. We have not done a heat loss yet. Do you have any thoughts? Our old system is an old Burnham boiler with a 40 gal tank. We have 2 adults and an infant presently living in the house.


03-14-2012, 10:16 AM
The larger system is almost certainly oversized, and the smaller one quoted is likely as well. You don't need to upsize the boiler based on the size of the indirect. Generally, it is set up as a priority zone, and all of the available heat will go to reheat the water when that is required. If you don't have a big soaking tub, you can likely get by with the 45g tank. But, if all three showers might be running, and those users like to take long, hot ones, you may prefer something larger. Sounds like that is unlikely, at least in the near term. You could compromise and go with say a 60g if you wanted to hedge your bets. The good thing is that, other than size, indirects tend to be quite well insulated, so you don't have a lot of standby losses like you could (relatively speaking) with a gas fired stand alone WH.

Without a good heat load analysis, sizing the boiler is all kind of useless.

03-14-2012, 01:20 PM
What Jim said. I live in a not-super-insulated 1923 house about that size in Worcester, and the -105 would be nearly 3x oversized for my design condition heat load, and I have a colder outdoor design temp (http://www.energystar.gov/ia/partners/bldrs_lenders_raters/downloads/Outdoor_Design_Conditions_508.pdf) than you. Get a room-by-room heat load.

Also since it has 4 zones, what it the total amount of radiation (baseboard, radiant floor, whatever) on the SMALLEST zones?

If the minimum-fire output of the boiler isn't small enough you can short-cycle it into low efficiency running it at condensing temps if it can't dump that heat into the room at those temps, cutting into the efficiency of the boiler. With any system you'll have fewer problems and can get higher average efficiency by going with the SMALLEST modulating-condensing boiler that actually meets the design condition heat load.

At 120F fin-tube baseboard puts out about 200BTU/foot, @ 140F it's ~300BTU/ft, @ 180F it's ~600BTU/ft.

The min-input of the Alpine-105 is 21KBTU/hr, so at ~ 120F running in condensing mode it's min output is ~20,000BTU/hr.

If your smallest zone has 40' of baseboard, the balance point would be at 20,000/40= 500BTU/ft, which takes water temps of 165-170F. But at 120F the baseboard is only emitting 40 x 200= 8000 BTU/hr....

...so you have 12000BTU/hour more heat going into the system than the baseboard can deliver into the room. What's gonna happen?

The temperature of the loop rises until the boiler turns off because it's overshot the outdoor reset curve temp, then refires when the temp has dropped sufficiently repeat a gazillion time until the thermostat stops calling for heat. How bad is it?

In most boilers the temperature hysteresis is a fixed number of a handful of degrees- lets call it 5F. In 40 foot baseboard with 3/4" pipe you have about 7lbs of water, double that for the boiler mass and distribution plumbing, for about 15lbs of water as the total thermal mass of the zone. At 12000BTU/hr (3.3 BTU/second) of excess heat, to raise 15lbs of water that 5F degrees only takes (15lbs x 5F)/3.3= 25 seconds, after which the boiler stops, waits, refires repeat. Running it that way throws away a fixed amount of heat with every flue purge & ignition cycle, and wears out the boiler.

The min-mod output of the smaller Alpine 80 is ~15000BTU/hour, so a 40' zone that balances at (15000/40=) 375 BTU/hr. The baseboard at 120F dumping 8000BTU into the zone, so your excess now is only (15000-8000=) 7000 BTU/hr (or 1.9BTU/second). The minimum burn time then is (15x 5)/1.9= 39 seconds- still not great, but substantially better than 25 seconds. But if you bumped the temp to 140F the baseboard would be delivering (300BTU/ft x 40=) 12000BTU/hr, so the excess is only 3000BTU/hr, (0.83BTU/second) and your min-burns are 72 seconds, which is much less of a disaster, whereas the bigger boiler with the 20,000BTU min-output would still be short-cycling like crazy on zone calls.

Unless you can run the boiler at 140F or less with burns of well over a minute (5 or fewer burns per hour would be GREAT), you won't be able to get the condensing efficiency without abusing the boiler, and that's why smaller=better, especially with low-mass heat emitters on a multi-zoned system.

But do the math on your actual zones to figure it out. If you have big old-fashioned radiators you have a lot more water-mass buffering it, and the min-burn times soar, not a problem. If you have radiant slab floors, even better! But if you have even one or two stubby zones shorter than 40' of baseboard you have to think about how you want to go about fixing "the problem". And it IS a problem even for the Alpine-80- a BIG problem for the Alpine 150! Adding mass is one way, adding radiation is another, combining zones is yet another, but ideally you'd be able to set the temperature floor of the curve at ~120F for baseboard, if you want to hit 95% efficiency.

With 3 people and even a 50KBTU/hr boiler behind it you won't need to upsize the tank, if the indirect is controlled as a "priority zone". A 50K boiler is nearly 2x the burner of a typical 40 gallon gas-fired standalone. You'd only need to upsize the tank if you were installing a big soaking-tub or spa or something.

03-14-2012, 01:56 PM
BTW: With the exact meter-reading dates and the therms used it's possible to put a fairly firm upper bound on what the whole-house heat load is, using the boiler as the measuring instrument. The base-65F heating degree days can be added up for a weather station near you on degreedays.net, and you can convert therms/degree-day into BTU/degree-hour, and multilply by the number of degrees between your heating design temp (+11F) and 65F (so you're looking a 54 heating-degrees. A mid-80s atmospheric-drafted boiler ran at about 80% efficiency steady-state, so 25 years later call it 75%.

eg: Say, between 5 Feb and 6 March you used 132 therms, and you looked up and found out that was 628 heating degree days. That's 132/628=0.21therms per degree-day.

A therm is 100,000BTU, so that's 21,000BTU/degree-day

There a 24 hours/day, so that's (21000/24=) 875BTU per heating degree-hour...

...as source fuel BTUs, but in an old boiler running at ~75% efficiency only 3/4 of that was heat applied to the house, so the house needs:

0.75 x 875= 656 BTU/degree hour as boiler output

Your 99% design temp in Weymouth is +11F, so at (65-11=) 54 heating degrees your whole house heat load is not more than:

54 x 656= 35,424 BTU/hour

That's a realistic number for a reasonably tight '80s house with 2x6 framing and double-pane windows. If the place leaks air like crazy, the basement has no insulation and you have a bunch of single panes it might be over 50KBTU/hr @ +11F, but it would be unusual for a house that size to be over the 73KBTU/hr DOE output of the Alpine 80 if it's insulated, and the windows actually have GLASS in them. :-) But run the numbers on a wintertime gas bill- this method of estimation only under shoots if you've been running auxilliary heating like a wood stove or something fairly often. Most of the time its an overshoot (due to ignoring the hot-water usage.)

If your numbers come up even 2x that in the example (possible, I suppose), the Alpine-80 is still your boiler, assuming you stick with Burhnam.

If 3x the example (not very likely) go ahead with the Alpine 105, (or better yet- fix the heat leaks on that house- it's an efficiency-disaster, truly!)

It would have to be almost 4x that amount to rationalize going with the Alpine 150.

03-15-2012, 06:46 AM
I have a Burnham Alpine with a 50 gal Burnham indirect tank in my cape. It's oversized by a lot. The tank really isn't a problem because as stated above the standby loss is very little. I go away skiing for 4 day weekends and return to find that the tank has only cycled once during that period on non-use due to heat loss.

The boiler is a different issue. After doing a LOT of reading here, on other sites, and the Alpine manual I've gotten my unit to stop short cycling for the most part. This all could have been avoided if I had spent more time in the beginning of this project finding an adequately qualified installer that would do a correct heat loss calc and correctly size the unit needed. With the heating season coming to an end, that is my suggestion. You won't need heat again soon until October. Get 10 estimates if you have to. Get 20. Ask them for detailed heat loss calcs and their justification for choosing the model they did. The numbers won't lie.

Dana is right to ask about zone sizes too. I switched to hydronic in my house and put in two zones on the first floor. After this heating season I'll be combining them to increase the load. I'm also pulling out all the fin tube I put in and replacing it with cast iron baseboard that I've been collecting off of Craiglist. That should totally cure my short cycling issues. If you don't have enough heating mass you'll never heat rooms with 125 degree water which is what my boiler made most of this mild winter. In that case the unit will heat the water to ~125, then turn off. The pump will stay on as long as the zone is calling for heat. Then when the water reaches ~110 the boiler will cycle on again. That isn't a situation you want to be in.

That being said, I've been very happy with the Alpine unit. I would recommend it as an alternative to the much more expensive Buderus models.

If you really want to test the contractors you're getting quotes from, you can talk to them about smart pumps. I've read about some installs using them and the ability for the pumps to sense temperature and vary the GPM output of the pump for each zone has yielded even more money saving results.

03-15-2012, 09:31 AM
Every properly installed hot water boiler - be it for a new house or retrofit - must be properly sized to the heat load. Every savvy contractor will produce the load - or a sample thereof - before ordering your new boiler. An indirect water heater (the best way to heat water when a boiler is the main heat source) is sized to the DHW plumbing load. An indirect-fired water heater must have the heating capacity to match the output of the boiler or short-cycling will occur.

Over-sizing boilers is unfortunately standard practice in the industry. Over-sizing indirect water heaters, is of little consequence, other than the initial up-front cost of the indirect.

To clear up some confusion; The Burnham Alpine is a wall-hung, low-mass, sealed-combustion boiler, which uses the Gianonni stainless steel, water-tube heat exchanger. As such, great care must be taken in the design of the near-piping to avoid low-flow and subsequent flashing (pings, hisses, rumbles, etc.). The water-tube boiler is also susceptible to plugging from old system debris e.g. rust from old radiators and other ferrous metals. Flushing the old system and installing a "y" strainer may be in order.

The use of ECM "smart" pumps, is an advanced design skill and should be avoided on the secondary side of any water tube condensing boiler without considerable experience. Though the percentage of energy savings can be above 50% (we use them all the time) the real savings in a typical residence is in the dozen dollar per year range.

As mentioned, when converting from a high to low-mass boiler, load size matters. In any conversion the radiation should be measured and zone loads taken into account. We use low-load strategies (no, not the over-sold buffer tank) to control boiler cycling and proper load management. Though cast iron baseboard will add mass (contributing to load) it does not guarantee proper load (heat load calculations do this best).

Mixing up mass and load is a common mistake. We heat many homes with very low "mass" radiant ceilings, which will never operate above body temperature here in Minneapolis.

As for finding a contractor it would be unwise to wait until November, in fact March is the perfect month to buy a new boiler. 20 new boiler installation quotes is not only irresponsible it is an unnecessary waste of your time and theirs. Contractors have to spend money to give you a "free" replacement boiler bid. For every boiler the contractor doesn't sell, he must add the cost of the estimate to future boiler jobs, driving up the cost for everyone.

We suggest a better way.

http://www.badgerboilerservice.com/contractor.html (http://www.badgerboilerservice.com/contractor.html)


03-15-2012, 01:31 PM
No argument from this corner with anything BadgerBoilerMN offered up!

But doing the napkin math on both the zone-radiation and the fuel-use against heating degree days & design temp is a dead-easy backstop against ridiculous or not-well considered proposals, which are the most common kind, when offered "free". The notion that ANY house that size in Weymouth MA needs a 137KBU/hr-output boiler like the Alpine 150 is preposterous, whereas the 96KBTU/hr-out Alpine 105 is merely ludicrous.

Maybe in an extreme cold snap and it actually hit -72F or -113F or something it might need the output of the -105, but that's not exactly a rational outside design temperature to use for Weymouth.

Get a room by room, zone by zone heat loss calc done (even if you have to hire BadgerBoiler to do it), have a hydronic heating designer (more than a licensed plumber) make the necessary adjustments to the system design, and put it out to bid.

Just dropping in either of those boilers will likely cost you more in the unnecessary additional up-front cost of a larger boiler, and in reduced efficiency than the cost of a heat loss calc & system design adjustments to make it work right from the get-go. (Not everybody is up for playing "junior hydronic designer" like some of us. ;-) )

03-20-2012, 10:48 AM
Thanks so much for your input!
So after a bunch of quotes we finally got a heat loss done. It came to 50000. Now we were told by other companies that it would take a few hours and cost about 3-400$ to have it done. The company we went with it took them about 15 minutes? Does that sound about right? Not sure about the cost. Just a refresher we have a mid 80's garrison just south of Boston. It is 2500 sq feet approx. Windows are new and it is very well insulated. We have 4 zones of heat. The company suggested a Bosch 100 boiler and indirect water heater. Any thoughts?


03-21-2012, 07:57 AM
I don't see this as a 15 minute task unless he already had the measurements of all windows, floors, ceilings, number of fireplaces, insulation levels, type of door/windows, the direction each wall was facing, etc. all ready figured out and ready to go. If so, he may have been able to input into the software and got a number within 15 minutes. Did he give you a print out? Something room-by-room with each heat loss/gain?

It sounds like he made a couple quick measurements and did the: "Well, a house like this is about 20 BTU/ft^2 and there is 2500 sq ft, so let's call it 50,000 BTU."

There are some Manual J type software that you can download and try yourself. There is a common one that runs $50 to use. There are also some free ones that aren't as detailed or powerful. However, these calculations are only as good as your measurements and how well you know how the house is constructed (R values, window construction, air leakage rates, etc.).

I bet that you could get a credit for the heat load calc if you had the same company do the install. Even if you didn't, the cost of the calculation is worth it by saving money upfront on the unit as well as down the road. The time and cost that the other companies are saying for a proper heat load calc seems to be about right. It takes time to measure everything and figure out insulation and all that.

03-22-2012, 04:37 AM
nukeman is right.

There are new apps. for heat load calculation and even respectable radiant floor loads (lower than forced air, so one program does not fit all), but these programs are intended for "block" load calculation, meaning whole house. When we size boilers for new or old construction the radiation is measured against the load calculation which, we will not do for free. Our bids almost always say; condensing boiler sized to the load. The reason for this is simple; we are usually the only company that performs a heat load for a replacement boiler. Most are done by the installer's salesman at the local supplier, if at all. If a good contractor wants to charge for a load calculation, pay him (after seeing a sample with his name on it). Once you have a proper heat load, you can better sort out the proposals, saving time and money in the long run.

If a contractor has some experience with Manual 'J' and a good program (our Wrightsoft with various modules now in the $3000.00 range for 2 seats) and he is performing a block heat load for a residence in the area he grew up in and has spent the many hours it takes to set up a template for the application he may knock out the loads in a half hour or two. Given the time invested in learning, set up and being the only guy in five that even knows what a Manual 'J' is, the smart guy should get the job.

Ask for a sample of a recent heat load analysis: http://www.badgerboilerservice.com/images/SampleHeatLoadAnalysis.pdf

Ask for a picture of a similar installation and don't take the low bid.

After getting the right "size" boiler, the next most important aspect of any boiler replacement is installation. Almost all of the serious condensing boiler problems are now and have always stem from poor installation.

03-22-2012, 02:24 PM
What they said- you can't do a room-by-room Manual-J in 15 minutes or anything like it. And recommending a boiler with 2x the output they CLAIM it needs even from their crude calc is negligence/ignorance or both. Without a LOT of baseboard on EVERY zone you probably wouldn't be able to run it in condensing mode without short-cycling it to death.

With fuel use and weather history data you can come up with whole-house number in that amount of time, but that would still leave open all the design issues around lowest water temps you could achieve without short-cycling.

04-27-2012, 06:29 PM
I'm also looking at replacing the current cast iron monster with a more efficient system. I was quoted a 110K Triangle Tube Prestige Solo; I'd like to double-check that with you folks.

I live in a 1900-era house, which has had cellulose insulation blown in and storm windows. It's not at all drafty in the winter. I'm also intending to replace the windows, but that hasn't happened yet. This is in Madison, Wisconsin.

From my utility website, I have the following:

2011: 870 therms, 7,137 heating degree days. So with your math:

870 / 7137 = .12 therms/degree day, or 12,190 BTUs/heating degree day, or 507 BTUs/hr. I believe I have a 60-70% efficient boiler, which means it produced 304 BTUs/hr. I don't know what the design temperature is, but if we say 0 degrees, that's 304 * 65 = ~20k BTUs. Seems from that that a 110K boiler is massively oversized.

This was off a free estimate, and we've had experience with the company before. Should I ask for a manual-J before they go ahead?

04-27-2012, 06:46 PM
110K is likely way oversized. Dana is better at calculating this stuff, but your calc seems reasonable.

04-28-2012, 07:11 AM
jad is right;

Every proper HVAC system including hydronic heating systems start with a proper Manual "J" heat load analysis. This is the only way to specify any boiler for new construction or old retrofit. Your decision to replace the old "hot stack" cast iron boiler with a high efficiency condensing boiler (100F stack) is a wise one). Many heating contractors are very good mechanics but poor at doing the math.

If you can't find a local who can produce a proper heat load then hire it done by a professional from anywhere (ASHRAE standards now cover a good part of the world so any competent designer can help) and direct your local contractor to follow the specification. This absolves the local boiler installer of the unwanted responsibility and you will have a system that runs more, lasts longer, keeps you more comfortable and lowers your fuel bill.


I'm a pro, but not an engineer ;-).

04-30-2012, 02:27 PM
The 99th percentile design temp (http://www.energystar.gov/ia/partners/bldrs_lenders_raters/downloads/Outdoor_Design_Conditions_508.pdf) for Madison is -6F, not 0F.

Estimating the efficiency of big-old boilers is a very squishy art. If you kept the place at 65F or above all winter and only used 870 therms it's probably doing better than 60%. But even if you used the SOURCE FUEL BTUs of 870 therms, that's 508 BTU/degree-hour 71 heating degrees (the difference between 65F and -6F), or 35,560 BTU/hr, which is barely above the minimum-firing rate for the Solo 110.

Assuming against all reason that the combustion efficiency of the beastie-boiler is as high as 80%, your true heat load at design temp is 28.5K or less.

Bottom line, yes the Solo 110 is likely to be grotequely oversized for the load, and the smallest unit in any vendor's line would keep up with your design condition load, since all put out more than you need at high-fire.

To have a heat load that small on a 100+ year old house means it probably has a low surface-area to floor area ratio (more of a cube shape rather than a sprawling 1 story layout ), few bump-outs/dormers adding to framing losses, a modest window to floor area, and probably not a huge amount of floor area. Tightened up with blown cellulose + storm windows, a 1500-2200 foot cube-shaped house can easily have a heat load less than 30K @ -6F. (An 1800' cube-house example: ~70F delta-T on a 2x4 walls with a U value of 0.1BTU per square foot per degree come in at 30' x 30'x 20'=2400 feet of wall area comes in at ~17K for walls. Add 900' x U0.05= ~3K for the ceiling, add 3K for windows & doors, 5K for air infiltration and you're still only at 28K, not 30K.)

The other things that might skew the fuel-use number would be if you had a lot of electrical power being used (running 3 Tivos and small rack of server-computers 24/365, are you? :-) ) or if you have been using wood stoves or other supplemental heating. If you're out most of the day and using ultra-deep setbacks, letting it get down to 50-55F while you're gone would do it too.

If the utility company's fuel use or heating degree numbers are wrong, that would be of some concern too. It's better to use a very close degree-days.net weather station for the HDD number, and work it from a mid-winter bill where you know how warm you kept it.

"Free estimates" never involve a real heat load calculation, but they can easily eyeball the house and come up with a model that would for-sure never leave you cold, but will ALWAYS be oversized. Based on your fuel use numbers, a Manual-J done by a competent 3rd party would be necessary to rationalize going with anything but the very smallest of any mod-con line. (The min-fire output of the Solo-60 is a much more appropriate value based on the fuel use estimated heat load.)

If you're keeping the place toasty and the beastie-boiler is truly only going through 870 therms over 7000 heating degree days it's not clear that swapping it out for a mod-con is going to be a very good investment. Even if it saved you 300 therms/ year (which it might), at a buck a therm it may not pay off within the lifespan of the mod-con. It may be better to wait until you actually NEED a new boiler, or until fuel prices rise substantially.

BTW: Reasonably tight older windows fitted with tight storms will often outperform code-min replacement windows. Replacing windows with high performance windows is usually only cost-effective when it NEEDs replacement for mechanical issues anyway. At that point it pays to consider carefully your performance point and window type based on which side of the house it lives. A higher-U higher SHGF may be cheaper and of more heating season benefit on the south side, whereas lower-U low-E high performance windows offer more comfort & value on the east/west/north sides of the house. When adding storm windows, going with low-E storms on all but the south side and going with uncoated glass stroms on the south facing windows also reduces heating fuel use, even if it adds a modest amount to the design condition heat load.

05-01-2012, 07:30 AM
We are aggressively setting the temperature back, so that might be it. The morning/evening temps are 68*, during the day is 65*, and night is 58*. I don't believe we are very well insulated (if nothing else, double-hung windows with no weatherstripping and uninsulated sash pockets). Does it make sense that the setback has that much of an effect, or am I working from bad inputs?

In any case, I'll definitely have a heat load calculation performed. I would like to know if my "So here's what I worked out..." has any basis in reality, though.

05-01-2012, 11:11 AM
With reasonably tight storms the empty sash weight pockets and lack of weatherstripping can be of remarkably minimal consequence. Stack effect infiltration from leaks at the attic & basement are much bigger contributors to heat loss than air leaks that happen in-between. The former are a 24/7 drive whereas lateral leakage rates are mostly wind driven.

As for the "basis in reality", for a rough cut, what's the size & shape of the house, and how many windows & doors? Even if the nightime setback is 58F, I assume it only gets cool enough to trip that setpoint on the coldest nights of the year (?). Setbacks of that magnitude would only result in single-digit percentage savings, whereas your total annual domestic hot water that uses the same fuel would usually be into double-digits. If you were regularly keeping the place at 50F for most of the day you'd be well into double-digit annual savings though. If you can verify the gas company data using your own heating degree day data from a third party source against fuel use during a winter billing period you can find out pretty easily if their numbers are WAY off. No matter what they're not going to be 2x-3x off, which is what it would have to be to rationalize installing a 110KBTU/hr mod-con.

Blown cellulose tightens up walls by quite a bit- something on the order of 90% tighter than low-density batts or low density blown fiberglass, and the installation process fills in all voids & anomalies, and plugging crack-leaks with cellulose. The only way to get tighter in a retrofit is with ultra-fine new-school blown fiberglass dense-packed to 1.8lbs/ft^3 or higher, or to use expanding polyurethane foams (or non expanding injection foam.) It's better than you might think, independent of absolute R value.

When having the heat loss calc done, insist on using 68F for the interior temp, and -6F as the design temp. If they use a different number, make them explain why. I've had heating professionals tell me with a straight face to use values that were more than 10F lower than my local 99th percentile design temp, which is something I'm very loathe to do. Since heat load tools tend to overshoot by double-digit percentages even using the correct inputs, adding another ~15% error to the high side is a ridiculous thing to do.

05-02-2012, 07:42 PM
1600 square feet, 2 stories. There's a bump-out that's about 150 square feet per floor, the rest is a cube shape.

4-6 windows per room, all double-hung. The storms aren't great. I definitely don't feel drafts in the winter, though.

Blown-in insulation in walls and ceiling, plaster walls.

I don't think the boiler ever runs in the middle of the night. It can take a while to get the house back up to temperature, though, and we'd like to keep it warmer if possible because we've got kids now.


05-03-2012, 10:50 AM
An 1800' mostly-cube, reasonably tight, reasonably insulated would almost always come in under 50KBTU/hr @ -6F, and could easily be under 35K.

How many windows total? Typical double-hungs are about 10 square feet, and w/storms have a U-factor of about 0.4-0.5 (R2-ish). The cellulose insulated walls run about U0.1, or about 1/4 to 1/5 the heat loss per square foot of the windows, so the actual window count/size matters.

Assuming you have 20 windows total @ 10' per, that's 200 square feet of window. Assuming they're U0.5 that's (200' x 0.5=) 100 BTU/hr per degree-F, and about 70F delta-T at design condition, for 7000 BTU/hr for windows. (In zero wind they will perform better than U0.5, and with blinds, shutters or curtains, better still.)

Assuming have at least 7" of cellulose in the attic the U-factor for the attic is less than 0.05, but let's call it U0.05. and 900 square feet of attic floor gives you 45BTU/hr per degree F. Times 70F gives you ~3000 BTU/hr out the attic.

With the bump-out on the footprint you probably have a total wall perimeter of ~150', give or take, maybe 25' tall, for 3750' of area, less 200' of window & door, call it 3500'. At an average of U0.1 that's 350 BTU/hr per degree-F, x 70F comes to 24,500 BTU/hr for wall area.

Assuming you have 2' of exposed uninsulated foundation for that 150', for 300' of U1.0 assuming it's about 55-60F down there, not 65F+, call it 60F delta -T tops, for 60x 300 = 18,000 BTU/hr.

Add it all up and you have 52,500 BTU/hr, at design condition, assuming minimal air infiltration (add 3-5K, just to be sure- more if you have known drafts), and an interior temp of 66F or greater. Subtract 250 BTU/hr per adult human, maybe another 1000BTU/hr for background electricity use (that can be calculated if you know your active plug load at night.) It's under 60K, and probably in the neighborhood of 50K that needs to be supplied by the boiler to keep it at 65F at night.

This is probably worse than worst-case, if you have any confidence in your annual therms/HDD numbers, but it's roughly how heat loss calculations work. If you're holding the line at 58F instead of 68F at night, that means your delta-Ts are about 60F, not 70F, and the 52.5K number drops to 45K. If the attic is ~ R40-ish rather than ~R20-ish your attic losses are ~1500BTU/hr lower. But whatever the true number is, your as-operated heat load at your setback temp is well under the output of the Solo 60.

And, if you put 2" (R8) of rigid EPS foam up against the foundation walls,cutting & cobbling some over the foundation sill & band joist, foam-sealing the seams & edges for good air-tightness then trap the wall foam in place with a 2x4 studwall with UNFACED batts, your basement losses will be cut by at least 90%, from ~18K down to less than 2K, cutting your whole house heat load to less than 40K @ design temp even with a 70F interior. The basement stays warmer & drier, and the first-floor floors would be noticeably warmer in winter too. Uninsulated basements & band joists can be more than 25% of the total heat loss on houses that shape, and are often the easiest most direct big efficiency upgrade to retrofit, after attics.

05-07-2012, 06:14 AM
This last one is a very valid observation. Though windows are often the first thing to be replaced, they are just as often in last place for return on investment. More often a new properly sized condensing boiler can save double-digit percentages in fuel bills, something new windows will very rarely accomplish.


05-07-2012, 11:58 AM
Windows... let's see. I'm going to categorize since they vary.

"Classic" double-hung (uninsulated sash pocket, no weatherstripping): 12.
"Upgraded" double-hung (1" foam in the sash pocket, nylon pile weatherstripping): 5
1 vinyl slider, 1 vinyl awning, 5 modern casements, 6 modern double-hungs.

It's a very window-ful house. I love the light, but it does make it hard to find room for bookshelves :)

All of the classic/upgraded have metal-framed storms in good shape. The rest don't, but are double-glazed at least. We've primarily been replacing the casements which _were_ wind tunnels; I expect to put in modern double-hungs only where the wood frames are really rotted out.

Oh, and heavy curtains, but hey.

WRT the foundation, there is actually no exposed uninsulated foundation. There's 5' of buried concrete wall (well, 6" or so showing) and then stud wall on top of that; the stud wall is insulated.

So. I just talked with the heating guy and he's happy going to a 60K boiler; but it's clear that he has no interest in running a manual-J and I'd like the insurance. Anyone want to run one for me? I assume this is something where I can do the measurements and so on?

05-07-2012, 02:16 PM
Your 100 year design frost depth in Madison is probably something between 54-60", and you are still losing a large amount of heat out the foundation.

The band joist + foundation sill is probably about 150 square feet of ~R2/U0.5, for a heat loss of about 4500BTU/hr @ 60F delta-T (assumes a cool basement.) The 6" of bare concrete is ~75' of R1/U1 for another ~4500BTU/hr. So it may be closer to 9K than 18K, it's still a pretty big number, a number that is fixable on a cost-effective basis without tearing your house apart.

From the "better than just a WAG" heat loss estimator adjustment point of view, the windows in excess of the 20 I used in my quick example adds another ~ 3K, but the reduction in above grade exposed foundation area probably knocks off ~6-9K, so the net is still at 6K lower than 52.5K I came up with previously, for something on the order of 46.5K, and it might well be under 40K.

That makes going with the solo-60 even more of a slam-dunk.

If the slab is 5' below grade, fine, it may not be worth insulating the floor unless you're finishing it as a room, but the foundation walls are DEFINITELY worth insulating and air sealing. Losses to the cold earth with is more of a seasonal steady-drain than a peak-air temp drain. Crudely modeling the below grade 5' as 55F dirt, and the foundation as R1 for a 10F delta-T, that's still 750 square feet of sub-grade R1 at 10F, for 7500 BTU/hr. Soil types vary- it could be more, could be somewhat less but it's not much less half that- maybe 12K rather than 9K would be the better guesstimate to use on your basement load (ignoring the slab for the time being.) It's still well-worth air-sealing and insulating the foundation a number reasons beyond the (still cost-effective) fuel savings aspects, comfort and indoor air quality among them. Foundation sills & band joists are often the single largest air leak in the house, and being at the bottom of the stackup, contributes a gia-normous fraction of the stack-effect driven infiltration.

BTW: Storm windows tighten up the air leakage of old un-weatherstripped double-hungs by a LOT. But if you can still detect air leaking at the sash pulleys it's worth buying a sack o' these (http://www.energyfederation.org/consumer/default.php/cPath/21_392_72). The installed view looks something like this:


05-07-2012, 02:36 PM
If you want to do your own heat loss calc, you might try running Harold Kestenholtz's I=B=R tool:

http://www.livepast60.com/heatpro/heatfla/flashpro.html (<online, 1-room version)

Or download a standalone to play with: http://www.livepast60.com/heatpro/estimation_programs.htm

Watch out for excessive air-change numbers- the defaults of this (and many tools) is much higher than reality. A pretty-tight house can have 0.1 air exchanges per hour or less when not actively ventilating, but play around with it.

(Kestenholtz wrote one of the very first heat loss tools for computers some 30 years ago. It's not much different from Manual-J methods.)

05-07-2012, 03:12 PM
I'll definitely look into sealing the basement. The band joist is plastered over (did I mention plaster, plaster everywhere??) so getting in there hasn't been a priority. Should be one, though.

And the problem with the double-hungs is air coming through the access ports as well as the pulleys. It's okay - I'm fairly handy, and putting in weatherstripping is easy. The only part that takes a long time is stripping off the old paint and reglazing.

Okay, one estimate coming up, and a 60K.

Thanks everyone for all their suggestions!

05-07-2012, 03:24 PM
If you tighten the place sealing every obvious air leak, it's often worth hiring a pro to run a blower door test with or without infra-red imaging to nail down the rest of it. Around here (central & eastern MA), that service runs north of $500, but in combination with expert air-sealing the comfort level go up while the heating loads come down.

If your place is tight enough that it stays north of 35% RH @ 68F indoor temps throughout the winter you may need to actively ventilate, but that would generally be a GOOD problem to have.

With the band joist plastered over, if the plaster is in good shape it may be doing a decent job of air sealing (better than 95% of homes that age out there) but if it has cracks, maybe not. Either way, installing rigid foam and foam-sealing the edges (or giving it a 1-2" shot of closed cell spray foam) would guarantee it. Insulating at least the upper half of the foundation to R10 or better has pretty good payback in your climate zone. R5 XPS (1" blue or pink board) and an unfaced-batt insulated studwall trapping it in place is about an R15 wall, after the thermal bridging is factored in, and still reasonably cost effective.

05-14-2012, 02:24 PM
This is a good point. After I insulate an attic, the next target is the rim joist where enormous infiltration is a given in vintage homes. Only after these are addressed, the frost line is the enemy of comfort and economy.

05-24-2012, 04:29 AM
if you use gas for heating domestic hot water, you have to deduct domestic hot water portion of gas usage from your gas bill.