Boiler Sizing Question

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Nathan Hjelm

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I am getting ready to replace an old boiler at my house and I want to make sure I am sizing this right. For reference my house was built in 1963 and has 2400 ft^2 of heated space and is located at 7300 ft.

This is what I have right now:

- 2 zones. One with ~24' of slantfin baseboard (lower floor). One with ~40' of slantfin baseboard (upper floor).
- A 1991 slantfin Galaxy 85000 btuh boiler 80% efficient boiler.
- A 55G 1989 kenmore water heater.

This is what I am looking at:

- Split 39' into two zones. One with 25' and the other with 21' (adding 7' of baseboard to even things out). This will give me three zones each ~23' long. I am also replacing ~11' of the old radiators as well on one loop. The new radiators are slantfin MP 80 which output ~ 530 btuh/ft @ 150F (condensing) at 4 gph. All the older radiators are MP 30 which output ~ 330 @ 150F
- Weil Mclain WMB-155C

The old boiler has really struggled with heating out house in the winter. The design temperature for my area is ~ 1F and the calculated heat loss for the house at that design temperature is ~ 64000 btuh (using the Weil Mclain worksheet). Crunching the numbers the current boiler is (1.0 - 0.04 * 7) * 0.80 * 85000 = 48960 btuh (so definitely small).

My primary loops calculation:

- Loop 1: 13' * 330 btuh/ft + 11' * 530 btuh/ft = 10120 btuh
- Loop 2: 14' * 330 btuh/ft + 7 * 530 btuh/ft = 8330 btuh
- Loop 3: 25' * 330 btuh/ft = 8250 btuh

The new boiler output should be 155000 * 0.944 * (1.0 - 0.04 * 7) = 105350 btuh when condensing. The boiler has a turn down ratio of 10:1. This would put the lowest output @ 10535 btuh which is a little high.

How does this math look? Am I oversizing the boiler for my application? I am mainly after the 155C because of its higher DHW output (which is way higher than the old kenmore: 300 G/hr @ 120F vs ~ 60 G/hr @ 120 F first hour).

Also, any thoughts on the boiler choice? This unit has a decent price ($2200 on eComfort) and seems to fit my needs.
 
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NY_Rob

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Something doesn't sound right with your radiation...
A 2,400 SqFt home and it only has 64' of radiators?

If you assume a heatloss of 20BTU's/SqFt you need 48K BTU's of heat output.
Std Fin-tube puts out 510BTU's per foot with 170F water.

48,000/510= 94' of fin-tube.

64' of fin-tube can only put out 32,640 BTU's with 170F water.

Maybe your current boiler isn't the problem, maybe you don't have enough radiation?
 

Dana

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The W-M worksheet uses only the crudest of crude methods, and is guaranteed to oversize. Most insulated and tight 2x4 framed houses with at least storm windows over wood-sashe single-panes would come in around 15 BTU/hr per square foot of conditioned space @ 0F, so odds are your 2400' house has a design load of about 36,000 BTU/hr, give or take 10%, unless it leaks a lot of air (which can be fixed) or has an unusually large amount of window area.

At your altitude you need to derate the boiler output to compensate for that cheap low-oxygen air you're feeding it, and the pressure/heat content of the gas has to be dialed back to reflect the lower pressure, to avoid running too lean a mixture. Is that what your " (1.0 - 0.04 * 7) " factor supposed to be doing? A 4% per 1000 rule of thumb has long since gone the way of the dodo- 2% is closer to reality for atmospheric drafted boilers, and with mod-cons it's totally out the window. The derating varies with boiler design, and particularly when the controls have smart feedback (like most modulating condensing boilers do.) Most manufacturers will specify how to do that appropriately with their equipment.

If the calculated output of ~49,000 BTU/hr is correct (seems low for an 85K boiler, even at altitude- I would expect most 85K-in 80% atmostpheric drafted boilers to deliver ~60 KBTU/hr @ 7000 feet of altitude) and the load is 36,000 BTU/hr (WAG, based on experience and crummy rule of thumb) that's an almost ideal 1.36x oversizing factor for a 36,000 BTU/hr load. (ASHRAE recommends 1.4x.) It'll run VERY long cycles during colder weather, but still keep up with the load, at +1F or even -10F, as long as you have sufficient radiation to emit that much heat. That may seem like it's "struggling", but in fact it's the most efficient way to run a non-modulating boiler.

Dropping in a 2-3x oversized modulating condensing boiler would be a mistake. It would be prone to short-cycling itself into lower efficiency &/or a shorter lifespan on zone calls, since there isn't sufficient radiation to emit even the minimum modulated firing rate on any single zone. With your zone radiation you want something with a min-fire output UNDER 10,000 BTU/hr, and under 8000 BTU/hr would be even better.

Since you have a heating history on the place do your self a favor. Start with a fuel-use heat load calculation, (you can even skip the altitude compensation issues, since altitude factor and distribution losses etc all tend wash out of the math, since they're not measured separately), then run the napkin-math calculations on sizing a mod-con boiler against your whole house load and zoning plan.

At your mid-winter incoming water temps you probably won't be satisfied with the combi version of this boiler, especially if you right-sized the boiler for the space heating load. But even the -155C is going to take forever to fill a bathtub in winter, though it'll support a single shower with some margin. An indirect tank sized for the biggest tub you need to fill, operated as the priority zone off a smaller boiler is almost always a better way to go.
 
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Nathan Hjelm

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We recently remodeled the lower floor. While I was cutting out the old slantfin 30 radiators I opened up the walls to find many uninsulated sections which we filled with R15 insulation. When the house was built the town was growing rapidly and the inspectors (both electrical and plumbing are/were done by the state) were either overworked or on the take. We had noticed drafts since after we moved in (it was conveniently warm when we did the walk-through-- $%&**%#). I wouldn't be surprised if there are more. Will this have much effect on the gas usage? That zone was usually set to 60F at night to reduce usage.

Yes, the (1.0 - 0.04 * 7) is the de-rate. From the WMB manual:

"In USA Input rates are derated 4% for each 1000 ft. above sea level, beyond 2000 ft. in accordance with National Fuel Gas Code, ANSI Z223.1/NFPA 54 -latest edition, and/or the Natural Gas and Propane Installation Code, CAN/CSA B149.1."

I kept seeing the 4% number so I assumed we have to derate the old boiler as well. This is why I came here to ask :). I know a fair bit about plumbing but not much about heat load calculations. I am a little gun-shy about getting a local plumber to do an estimate as the last time I got a quote > $14k for a direct swap with a newer non-mod/con slantfin boiler + WM indirect tank. Much of the boiler trim needs to be replaced though. Needs a new autofill, pump, new zone values (at least new motors) and I want to split the long zone (which will take me < 2 hours since the piping is all accessible in the basement). The zone heats the bedrooms + a large den which is a waste when we are asleep.

I too am a little surprised about the small length of radiation in the house. But the boiler is running between 180 and 200 with a single Taco 007 (never messed with it) and the MP 30 documentation shows 610 btus/ft at 180F.

To clarify the house is a 2-story long-face split level with a new metal roof and original cedar siding. I assume the worst when it comes to heat loss :).

Given your feedback the WMB-80 (though it still seems a little small) plus a triangle tube smart 40 indirect might (I had already been looking at them) work. The WMB-120 is probably overkill but looks like it might better match the input of the TT indirect. The cost is in the general ballpark I am willing to spend.

I will download my last couple years worth of utility bills and see how the math works out. Thanks for your feedback!
 
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Nathan Hjelm

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We had a bit of a warm winter this last winter but I think I have this calculated. In the month of Feb we used 139 therms (this is the best I can get from my utility using the app but IIR the readings were 1/31 and 3/1 so it should be good). The boiler is 80% efficient so of the 139 therms 111 went to heating the house. This gives 11 MMBTU. During that period the total degree days @ 63F was 620 (much lower than usual but it's all I have to work with). This gives 17935 BTU/degree day and 747 BTU/degree hr. The design temperature for Los Alamos is 6F (a little higher than I thought) which is 34F below what we had on average in Feb (63F - 620F/28days = 40F) so 747 * 34 = 25398 BTU/hr. Oversizing that by 1.4-1.7 gives me a range of 35557-43176 BTU/hr. That suggests that the WMB-80 is more than sufficient for my needs. Even if the 4% per 1000/ft is correct the boiler will provide up to 50000 BTU/hr or heat. This implies that the boiler will keep up with demand even if the temperature dips down to -27F (40F - 50000/747) which does happen but not for more than a day or two at a time. Dana, does this look correct?

The next question is will 50000 BTU/hr be enough output for an indirect water heater?
 

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50K BTU's should be sufficient for an indirect.
Most stand alone gas fired water heaters are in the 32-40K BTU range.
 

Dana

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A typical standalone 50 gallon tank such as yours has a ~40,000 BTU/hr 80% burner, delivering 35,000 BTU/hr into the water (at sea level). An 80K condensing boiler like the WMB-80 serving a 50 gallon indirect operated as the priority zone would recover in less than half the time. If served by a 50K condensing boiler it would recover in about 0.85x the time of the standalone (still faster, just not amazingly faster.)

By giving the water heater priority with the zone controler the indirect gets the full output of the boiler during the recovery period, temporarily suppressing calls for heat from the heating zones. Unless you have 5+ people taking back-to-back showers the space heating zones won't be off long enough to really matter.

I'm not sure why you're using the average temp- it needs to be the heating/cooling balance point temp, the heating degree-day base. You used 63F instead of the more commonly used 65F, which is fine. The balance point of tight 2x4 houses with double-pane is usually a bit under 65F.

With an outside design temp of +6F and a presumed balance point of 63F you have (63F - 6F= ) 57F heating degrees, for in implied heat load of 57F x 747 BTU/F-hr= 42,579 BTU/hr @ +6F.

An ASHRAE 1.4x oversizing factor would then be 59,611 BTU/hr for their "optimal" boiler sizing. At 747 BTU/F-hr a boiler with 59,611 BTU/hr of output would have you fully covered down to (63F- (59,611/747)=)~ -17F before it begins to lose ground (assuming the radation is sufficient for delivering that much heat.)

An AFUE test 1.7x oversize factor would be 72,384 BTU/hr of output, which would have you covered down to (63F- (72,384 / 747)= ) ~ -34F.

Even though it might hit -27F on a very clear cold night, you will literally NEVER experience -27F for "... a day or two at a time...", or even 6 hours. The temperatures rises from the overnight low very rapidly after dawn. I suspect the daily high is only very rarely below 0F in Los Alamos (maybe not more than once or twice within the lifecycle of a boiler) even on nights that dip below -25F.

The WMB-80 would be enough boiler for your load and typical mid-winter extremes no matter what the altitude derating factor is for that boiler. If they're still using 4% per 1000 for the WMB series it means they've never bothered to do the math,or run a real test, they're just relying on the old school none-too-relevant model. There are many mod-cons out there that get zero derating for altitude.

Assuming at non-condensting temps it delivers 87% combustion efficiency (it's probably slightly better than that) it's output at sea level is about 70,000 BTU/hr. Using a 4%/1000 derating it's good for at least 52,500 BTU even at 180F (non condensing) output, or close to a 1.25x oversize factor even at that unrealistic derating. If that's real (not likely) you'd be good down to (63F- (52,500/747)=) -7F before it even begins to lose ground.

Using a 2% derating (probably also conservative, but maybe) you'd be over 60K, and in the ASHRAE sweet-spot zone.

The similar Westinghouse WBRUNG080W boiler (same boiler as the HTP UFT-080 ) may be cheaper and easier to install with an indirect than WMB-80, since it comes with hooks such as a pre-plumbed separate port for running an indirect, and some internal controls to manage it. It can also be pumped direct with most zoned system, no need to set it up primary/secondary. The WMB-80's heat exchanger has as somewhat higher pumping head than the WBRUNG080W and would probably need to be set up primary/secondary (unless the installer did all the design math to verify it can be done with a single pump.
 

Nathan Hjelm

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How were you planning on venting the new boiler Nathan?

If I were to go traditional I would use the existing type B vent. If I go mod/con I can run two 2-3" pvc sch 80 pipes parallel to the old vent or use the old vent a chase. I would use a concentric vent at the end. I have really good access to the space above the boiler from the attic and there is space for 3 8" vents in the wall.
 

NY_Rob

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Be sure to check with the local AHJ (Authority Having Jurisdiction) to see if you can concentric vent (both intake and exhaust) through an existing vent?
 

Nathan Hjelm

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Be sure to check with the local AHJ (Authority Having Jurisdiction) to see if you can concentric vent (both intake and exhaust) through an existing vent?

Ah, didn't think of that. My state Construction Industries Department is the AHJ and they inforce UPC with some modifications. Looks like there is nothing relevant overriding UPC when it comes to ventilation. Still probably easier to run the new vent and leave the old one as is. I have a good roofer who will give me a good deal for cutting and sealing the new roof penetration.
 

Nathan Hjelm

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An alternative I have seen to using an indirect tank is to use a combi-boiler with an electric water heater (disconnected element) with the DHW on the boiler attached to a recirc pump on the hot water tank. Like this: http://www.houseneeds.com/learning-...o/takagi-tankless-water-hot-water-high-volume

The reason I ask is the 80C is $100 more and an electric tank water heater is ~ $500. This is a bit less than ~ $1000 for the indirect. In theory the heat input should be identical to using an indirect if the boiler uses all 80k BTUs to heat the DHW. Is this setup worth looking into?

BTW, Dana. Thanks for suggesting the Westinghouse. That is a good looking unit. Unfortunately I can't seem to find a place that sells it with free shipping which makes it slightly more expensive than the Weil Mclain. Online has the larger one but not the 80 :-/. I will double-check locally though.
 
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Dana

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The Westinghouse labeled version of the HTP-080W can be had for < $1.7K + $200. (<$1.9K total.) You may be able to land the HTP version for similar or less money buying direct from HTP.

If it takes even ONE more pump to build the system around a WMB-80H than the WBRUNG080W (probably will, if it has to be plumbed primary/secondary) the ~$1.8 K eComfort pricing on the WMB-80H makes it more expensive as a system, and more complicated to assemble. It's probably too close to tell which is going to come in cheaper without component pricing on the full the system.
 

Nathan Hjelm

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I couldn't fit an indirect water heater into my budget ($1000+!) so I did end up going with a small electric water heater as an indirect. I got the A.O. Smith Signature 38-Gallon from Lowes for < $400. Because the tank does not have a heat exchanger I bought the WMB-80C boiler and piped a loop through the on-demand DHW side of the boiler with a brass Taco-006 pump pulling from the cold water input of the tank and returning near the top of the tank (piped in at the TPR valve with a tee). The lower hydrostat in the water heater is set to 130F and is wired to the indirect water heater priority zone in a Taco 5-zone control module.

It has been a warm winter so far. The lows have been in the high teens only a couple of nights but so far everything appears to be working well. The 73 btuh output has been sufficient to keep all three zones at 69F and the hot water supply has kept up with the demand. After morning showers the priority zone runs for ~ 5-10 mins after the last shower before the tank water is back to 130F at the bottom of the tank. The gas fired tank water heater didn't even come close to meeting demand in the winter. No surprise as it was an 1989 40k btuh 50% efficient heater (20k btuh DOE, 58 gallon first hour delivery when new).

At this point the only thing I could change is to add a mixing valve to the hot water output on the tank and modify the EEPROM of the boiler to allow for 140F water on the DHW supply (the default max is 131F). Probably not worth doing unless we run into supply issues as it gets colder.

@Dana Thanks for your help in sizing the boiler. Its clear the WMB-155C would have been overkill many times over. The burn times on the WMB-80C are very long and usually at low fire (as expected). It only kicks into high fire when the zones switch from the away setting (62F).
 

Dana

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Not that it really matters now, but the steady state efficiency of the old tank water heater was ~80%, not ~50%, despite an EF test of ~0.50. Under an EF test the burner is mostly in standby mode most of the time, and with an atmospheric drafted water heater the standby loss rate is pretty high due to convection through the center-flue heat exchanger. When using a water heater as a combi-heater the net as-used efficiency (AFUE-style) would pretty close to it's steady-state ~80% once the burner's duty cycle hits the 30% range or higher. But simply as a water heater it's recovery time after a long shower or tub fill would more than 2x what you have with the -80C. The WMB-80C has enough output to deliver a 24/7 low-flow shower (but with no margin to spare for space heating- so don't test that theory out... :) )

The WMB-155C is complete & utter overkill for the loads of at least 19 out of 20 houses out there, and the WMB-80C is a pretty good fit for at least 18 of those 19 houses, as long as there is some external buffering for the domestic hot water. Your buffering solution using an un-powered electric water heater is pretty good, and with a tank big enough to fill your biggest tub it will serve you well!
 

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Now one year later. The boiler is running like a champ. Over the summer we never ran out of hot water (running out was a common occurrence before replacing the water heater). We have had nights where I have been asked to turn the heat down :).. Now, that was really really rare with the old boiler. Looking forward to seeing how this setup handles a cold winter (looking like that is what we are in for).

For those who are not plumbers, I did learn something new. When I installed the new DHW system I installed a backflow preventer on the cold water side (seemed like a logical thing). This combined with no expansion tank on the DHW side (the old system had none) lead to some leaking from the pressure relief value in an under-sink tank. So, the moral of the story is if you install a backflow preventer (where one didn't exist) and you don't have an expansion tank you probably should add an expansion tank (cold water side). I added one today (2 gallon) and it seems to have resolved the issue.
 
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