Boiler decision - help wanted

[abernat]

Some background: I own a 1600 square foot house in Madison, WI. When we bought it it was set up as a two-flat, upstairs and downstairs. Each floor is heated with a boiler and cast iron radiators. The current setup is a pair of Bryant (?) boilers, 112K BTU in and 93K BTU out. I have no idea how old they are, as they came with the house (so > 5 years) and we have no documentation.

Recently, we added insulation to the walls (3.5" cellulose, dense-pack) and attic (10"). We've been steadily replacing windows but there are a large number of original windows. We plastic them in the winter. Before this there was no wall insulation and minimal (2-3") in the attic.

Since everything I've seen says that our boiler setup is now massively overkill, and we definitely get nowhere near 100% utilization even in the middle of winter, I'd like to make this system more efficient. However, the budget is _really_ tight right now as my wife is out of work. So I'm looking to make do with what I have. My two options are as follows, and I'd like advice on which one to select.

Option 1: rework the system to use a single boiler. If the system was oversized to begin with (pre-insulation) then a single boiler should be able to handle the load easily.

Option 2: purchase a used boiler and switch to it. This just came up. The replacement is a <brand unknown for now> 140K in, 90% efficient, PVC vented unit, 7 years old. The PVC venting is definitely attractive, although it would not mean we could seal the chimney up as we still have a gas water heater. The 90% is more attractive. Unit cost for that is a cheap $600.

I'm also looking into an intellisense unit. However, budget is a factor.

Which option will be the more efficient? Any other things I should look at?

[jadnashua]

Combining the systems is probably a good idea unless you expect to split the house again. You might want to consider an indirect WH when you get a little money in hand, much more efficient than a typical standalone tank, and faster recovery, too. 93/112=~83%, so not all that much to gain on a new one unless it is also able to modulate. A modulating, condensing boiler is generally designed to cold start, and makes use of an indirect more efficient since during the summer, it only needs to turn on when you need to reheat the WH tank, not stay on, when you have little need to space heat. Mine generally only turns on once a day during the summer, after morning showers unless I'm doing laundry.

[abernat]

We are not planning to re-split the house, no. It started as a 2-flat and we're recombining it into a single family home.

WRT efficiency, the opinion seems to be that efficiency drops over time. My wife thinks the current boiler is ~25 years old, so would it still be kicking out 83%? Is there any way to tell? We have been getting it serviced every year, but that's only for the past 3-4 years, and I have no idea what happened before then.

[jadnashua]

The efficiency is a maximum theoretical number, and especially with oversized units, you'll never reach that - same is true with a new(er) one. Those numbers are based on the unit constantly firing, and on an oversized unit, you'll get a lot of inefficiency with it turning on for a short time, cooling off, then restarting. So, just combining the zones so the one has to run longer will help all by itself. Total efficiency doesn't need to drop over time. Depends a little bit on if you have leaks and need to add water frequently. Ideally, once originally filled, you never have to add water, and any minerals that were in it have deposited themselves. If you have to add a lot, you can get deposits thick enough to affect the heat transfer, and thus the efficiency. If the burner is checked and adjusted and cleaned, there shouldn't be much difference over time otherwise.

There's no guarantee the used, newer boiler is working properly. If I was going to go to the trouble of replacing, I'd want to go with a new one. Through the end of this year, (it might get extended again) you can get up to a $1500 tax credit for efficiency upgrades. Now, to qualify, you need a quite efficient unit, and that extra efficency costs extra. You'd qualify with an indirect WH, too. That's a 30% credit on hardware, up to the max of $1500.

[abernat]

Unfortunately, I think a new one is out of the budget. Even with the 1500 credit we're still talking multiple thousands. Also, our taxes are likely to be low this year (one of the benefits, I suppose, of not making a lot of money) and so we wouldn't even see the total 1500.

So it looks like option 1 is the way to go. Use the current equipment, and up the load so that we don't have two units sitting idle all of the time.

Thanks!

[Thatguy]

Option 1: rework the system to use a single boiler. If the system was oversized to begin with (pre-insulation) then a single boiler should be able to handle the load easily.

What's the cost to rework?
What's your yearly cost to heat?

Neglecting the changed costs for maintenance/repairs for 1 vs. 2 boilers:

If it's $1500/yr for a 60% efficient two boiler setup then a mythical 100% eff. setup would cost you $1500 x 0.6 = $900/yr.
If the single boiler setup is 80% efficient then this setup would cost you $900/0.8 = $1125/yr.
If the cost of rework is $2000 then the rework would be paid off in $2000/($1500-$1125) = 5.3 years.

[abernat]

Reworking is free; I'll be doing the work myself. I'm simply replacing existing piping runs and adding a manifold to the one boiler. The other will be disconnected and abandoned in place. Well, okay, it'll cost piping, but that's in the small number of hundreds. And I know I had short-cycle problems with the downstairs boiler that this will hopefully fix.

The plot thickens: the $600 replacement is a Munchkin 199M boiler in "mint" condition (from the seller). The web indicates that the 199M is a 92% modulating boiler; this seems to match with the PVC venting (doesn't that require a condensing unit for safety?). It also appears as though it supports an indirect tank. Given this new information, would it make sense to snap this up? A 7-year-old mod/con vs a (guessing) 25-year-old 80% efficient unit?

A side question - I was looking at indirect storage tanks, and they are *very* expensive. Is it possible to use a normal water heater as an indirect storage? I can't see how the plumbing would work, but it would be significantly cheaper.

[Thatguy]

Replace the $2000 with $600 and use your own numbers to see how long it takes for you to make up the purchase price.
A water heater may lose 50 w continuously through the insulation and it probably makes sense to wrap it in a thermal blanket.

[abernat]

I definitely see where you are going. One complication - in real terms, in a place like Madison (frozen north, heh), how much does the "modulating" nature get us?

As of last year we spent ~$100 in gas, per month, for the downstairs boiler. This was including a _ton_ of short-cycling as we kept a single bedroom heated due to a new child. The upstairs also reported ~$100 in gas. So $200/month for 5 months = $1000 heating costs per cold season. If that's at 80% efficiency then going to a 90% will save us $100. However, since I know neither boiler ran very often, I'm wondering if the effective efficiency was lower (is "effective efficiency" a term?). If we were actually getting 60%, then we'd be saving $350 a year going to a 90%. If the modulation helped, we might get better than that.

I simply don't know enough about boiler behavior to tell. I've seen a lot of references here to oversized boilers and horrible efficiency - is that a situation I'm in? Am I looking at the 6-year range (at which point I'd rather stick with what I have and know works) or a 2-year range (at which point the upgrade is a no-brainer)?

[jadnashua]

A modulating boiler works best if it is burning just enough so that it essentially runs all the time to match the heat loss. You get better comfort, and higher efficiency. The cooler you can run things, and match your load, the higher the efficiency. So, if you need say 20K BTU, and the thing can modulate to that, that's where it will run. The lower it runs, the higher the efficiency.

Now, why did they take that thing out? You have no idea if it still works properly, and probably have a taillight warranty - good until you can see their taillights.

[Dana]

I definitely see where you are going. One complication - in real terms, in a place like Madison (frozen north, heh), how much does the "modulating" nature get us?

As of last year we spent ~$100 in gas, per month, for the downstairs boiler. This was including a _ton_ of short-cycling as we kept a single bedroom heated due to a new child. The upstairs also reported ~$100 in gas. So $200/month for 5 months = $1000 heating costs per cold season. If that's at 80% efficiency then going to a 90% will save us $100. However, since I know neither boiler ran very often, I'm wondering if the effective efficiency was lower (is "effective efficiency" a term?). If we were actually getting 60%, then we'd be saving $350 a year going to a 90%. If the modulation helped, we might get better than that.

I simply don't know enough about boiler behavior to tell. I've seen a lot of references here to oversized boilers and horrible efficiency - is that a situation I'm in? Am I looking at the 6-year range (at which point I'd rather stick with what I have and know works) or a 2-year range (at which point the upgrade is a no-brainer)?

The efficiency you get out of a mod con has more to do with the temperature at which you can run the radiators and still stay warm, not so much the outdoor temp or anything else, as long as you can establish a reasonable minimal burn time (or modulate for some very long burn times). If the two cast-iron beasts were keeping the place warm, a 199K Munchkin is probably at least 2x oversized for your peak load (and may be 3-4x) but your AVERAGE load during the heating season on a binned-hourly basis will be about 1/4 to 1/2 the peak. It's not always -7F (or whatever your outside design temp is), and even if your radiation requires 160F or 180F water to deliver the peak load, odds are it can deliver the average heat load at 140F or less, which WILL get you into the condensing range with a 20F delta-T on the radiation.

The lowest-modulated output of a 199K Munchkin is about 2/3 that of your current boiler, and if set up with a buffer tank, wouldn't short-cycle, and would deliver better than 90% efficiency as compared to the likely sub-70% efficiency of your dual cast iron setup. And since you're talking about an indirect HW heater, if that buffer tank was a TurboMax or Ergomax "reverse indirect" set up as a buffering pre-heater for an electric tank, the boiler would be doing all of the heavy-lifting on the DHW front during the heating season, while still running low temp enough to get the condensing benefit:



The smaller ErgoMaxes are under $1500, and have enough mass to keep a 199K Munchkin out of short cycling itself into an inefficient range. If it turns out in practice that you need more mass in the buffer, you can plumb a cheap electric tank heater (not hooked up electrically) in series with the boiler loop on the return path between the reverse-indirect and the boiler.

You can't really use an electric heater as a indirect. You'd need an extra pump and an external heat exchanger, and some controls, and by the end you'd have paid as much for Rube-Goldberg contraption of an indirect that had less capacity. What you're buying with an indirect is a well-designed heat exchanger & control arrangement, not just an insulated tank.


Even with a 4x oversized mod-con you'll likely cut your fuel use by at least 30%, as long as the min-burn is on the order of 10minutes or longer. If the min-burn is 5 minutes or less you'll probably reap only 25% savings. But a lot depends on the mass, size & type of radiation. Big water-filled radiators can still put out a lot of heat with 90-100F water, where you'll see combustion efficiencies north of 95% (even with a 92% AFUE boiler), whereas fin-tube baseboard really craps out once you drop below 120F. But as long as the RETURN water coming back to the boiler is 120F or less you'll be getting better than 90% raw combustion efficiency, and if your min-burn is 10min+ the AFUE will pretty much be your average combustion efficiency.

Odds are pretty good that the combined output of your dual-cast-iron is at least 3x your true heat load, and that the smaller one could carry the entire load by itself. You might start by hacking the system to run it all on the smaller boiler, and use the fuel use tracked against heating degree-day weather data to measure your actual whole-house heat load over the next season while saving up for a mod-con. Measure it in therms/ccf/decatherms, whatever they bill it out as, not dollars, since boiler input/output is measured in BTUs or kilowatts, not dollars. With only one boiler on the system it'll be possible to apply the proper derating for oversizing, and it'll be very real efficiency boost as well, even with short-cycling during the shoulder season. The December-February billing will have the least error, but the heating degree-day data must be to the EXACT meter reading dates no the billing period dates or calendar months.

For an example of how that type of measurement can be done see http://www.terrylove.com/forums/show...e-Size-too-big

[abernat]

Thanks for the response! Things are making a lot more sense now. One last question based on Dana's post. If we're using big-iron radiators, is there still any benefit from the buffer tank? I wouldn't expect boiler-fed DHW in the near future, unfortunately. The purpose of the buffer tank is simply to increase the thermal mass in the system, right, which makes the heat-and-cool-off cycle take longer?

As a completely side note, I finally understand why the return temps have to be cool for the condensing part to work. Thanks!

[jadnashua]

the buffer tank forces the system to run at least some minimum time, and acts as a storage vessel for it so you don't lose it. As said earlier, ideally, the burner could modulate, run all the time, adjusting its output to exactly meet your needs. Depending on the type of radiators you have and a compatible boiler, you could find that running at a very small output and low temperatures would keep the room a nice even temp as big iron radiators work just fine barely warm. Baseboard stuff just doesn't work unless you have the water fairly warm, but the larger surface area and radiant nature of an iron radiator works just fine at lower temps. Adding an outdoor reset for that type of boiler helps, too. It senses the outside temperature, knows the inside temp, and can figure out what outlet water temperature you need to keep the house the most comfortable with the minimum amount of energy.

[Dana]

Thanks for the response! Things are making a lot more sense now. One last question based on Dana's post. If we're using big-iron radiators, is there still any benefit from the buffer tank? I wouldn't expect boiler-fed DHW in the near future, unfortunately. The purpose of the buffer tank is simply to increase the thermal mass in the system, right, which makes the heat-and-cool-off cycle take longer?

As a completely side note, I finally understand why the return temps have to be cool for the condensing part to work. Thanks!

That depends on how it's zoned & controlled, and the output of the boiler relative to the thermal mass of the zone(s), the amount of hysteresis in the system, etc. The behavior of the system will tell all. If the burns are significantly shorter than 5 minutes per shot when the house is just maintaining temp, yeah a buffeed primary/secondary where the buffer is the hydraulic separator (which IS the configuration using a reverse-indirect) can cure a world of ills:



Short of adding mass, you can also get quite a bit out of economizer boiler controls like the Beckett Heat Manager or Intellicon HW+ (either of which is a far far easier DIY than installing a mod-con, or even an indirect.) For gas-fired appiances be sure to keep the programmed min-temp on the economizer at 130F or above (140F+ if oil). These devices work by utilizing the thermal mass of the boiler itself running it at a high hysteresis to maximize burn lengths. Five gallons of water in the boiler running with 50F of hysteresis is buffering the same amount of BTUs as 25 gallons in a buffer with 10F of hysteresis. Boilers tend to have much lower insulation levels than buffer tanks though, so it's not exactly the same. If you're looking for a cheap way to boost the system efficiency with a 2-4x oversized boiler and a low mass system these are pretty good. If all of your radiation is high-mass radiators and it's only making 3-5 minute burns in temp-maintenance mode (as opposed to coming back from an overnight setback), a $150-200 economizer and an hour or so of messing around with the boiler controls would be a good investment, and will likely deliver double-digit percentage fuel savings.

Insulating all of the boiler & heat distribution plumbing in the basement to a minimum of R4 is cost effective if you're going to keep it for at least another 5 years, which I would, if it's doing the job, and has some life left to it. A ~100K mid-efficiency boiler isn't likely to be an efficiency-killing 5x+ oversized for your load, and with some control tweaks &/or buffering mass you can probably get it to run close to it's AFUE rating.

At typical mod-con fuel/air mixtures the dew point of it's exhuast product is in the low to mid 120s F, which is why it takes 120F or lower return water to make the heat exchanger cool enough on the exhaust side to make that happen. At the very beginning of condensing it's about 88% efficient, and climbs pretty rapidly with declining temps to the ~92% range. But to break 95% you'll need to run cooler still. Radiant floors done with concrete slabs make this pretty easy, and are self-buffering (it's a lot of thermal mass). Staple-up radiant under wood floors typically requires higher temps and have much lower thermal mass, but you can still hit the mid-90s average by using extruded heat transfer plates on the sub-floor tubing, or using an above the sub-floor system such as WarmBoard (tm). With high-mass radiators you still do pretty well at low temp too- much better than the bare ~90-91% you might squeak with fin-tube baseboard.

As for the rest of it, you've been busy pounding in cellulose and swapping windows, but have you done a blower door test and air sealed the places? It can make as much of a difference as replaceing ALL of the windows, for the price of a couple(!). The foundation sill & rim joist is the biggest most-often overlooked air leak, and important for breaking up the whole-house "stack effect". Seal the bottom of the stack (the basement), and the top (the attic), and you' broken the back of the convection forces- air leaks in the middle then become less important (unless you live in a high-wind area.) Insulating the foundation is another big step often not taken. You have to be careful how you do that in order to avoid mold & rot risk, but it's quite doable, and would typically cut the total heat load in a place as chilly as Madison by 15-20% (not to mention your first story floors would be more comfortable with a 65F basement than with a 55-60F basement.) Before investing $10K in a mod-con installation finishing out the thermal envelope of the house might yield similar fuel savings, but higher comfort levels, and the mod-con you end up installing down the road can be significantly down-sized. Like you I've been improving the thermal envelope of my 1923 stick-built bungalow that I've been tightening up over the past 8 years a bit at a time. My outdoor design temp is ~10F higher than yours, which would yield about a 15% reduction in peak heat load (all else being equal) but I have ~20-25% more square feet that you're talking. As of year or so ago my design-day heat load was measuring in at aroiund 30KBTU/hr, which could be handled by the very smallest mod-cons in most manufacturer's lineups. But that was only AFTER insulating the foundation & rim joist, and taking some moderate air-sealing measures, fixing the big leaks (which weren't all obvious.) There's still plenty of room to go, but unless you live in a mostly glass house I'm pretty sure you'd be able to get your heat load down there (or lower) without breaking the bank, if they're not already there. Of course that would make just one of the boilers 3x oversized for the whole house but that's not an efficiency disaster that 6x oversizing is, and would be fixable with more mass and/or economizer-controls. (My boiler was close to 5x oversized before I bit the bullet and did the system over pretty much from scratch.)

[abernat]

I will look into insulation. I was poking at this mod-con because it's $600 - which makes a damn cheap gamble. At this point it's all about the "cheap".

I'll check the foundation and sill. The basement is entirely plastered (as in wall covering, not drunk, unfortunately) which makes spotting some of these spots difficult. We had a blower test done pre-insulation (leaky as hell) and post-insulation (moderately less leaky as hell). Most of the obvious infiltration was a set of non-sealing casement windows which have since been replaced via Craigslist.

The house is pretty glassy. We average 4 windows per room and about 6 sq ft per window. It makes finding furniture tough, let me tell you. They're mainly single-glazed double hung windows (TODO: pull trim and insulate weight pockets) with aluminum storms. We cover 'em with plastic each winter.

Again, the thought on the mod-con was "$600? Why not". I've also looked at the ~$150 Intellicon and if you think it would have roughly the same effect I'll go with the less complex. I don't think we were seeing 5 minute burns, but I doubt they were over an hour long at a time. Unfortunately, I never measured. I definitely don't remember the furnace burning as a "frequent" event, though.