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Sizing my boiler using historical gas use - Sanity check and sizing advice?

Hi Everyone! I've been reading these forums a lot but this is my first post. Thanks in advance for your help.

I'm replacing my 60+ year old gas boiler (270,000 BTU input, 216,000 BTU output, 80% efficiency) and 24+ year old water heater (50 gallon tank, gas).

The house was built in 1957, is about 3300 sqft with fin tube radiators throughout. The lower floor is about 1500 sq ft with kitchen/dining/family rooms is below grade on one side, walk out on the other. Upper floor is about 1800 sq ft. Double pane windows in good condition, R-19 in attic, walls insulation unknown, and based on blower door test, the whole house could use some air sealing. Heating has 2 zones: Zone 1 has 134 ft of radiators covering 2 floors, Zone 2 has 51 ft of radiators for "guest rooms" over garage. No cooling installed yet, though we have quotes for and plan to install mini-splits in most of the house in the next year or two both for AC comfort and humidity control.

We found the boiler has a faulty valve and the water heater has a bad pressure valve. We decided to replace them both rather than repair them since they are both on their last legs. I want to be able to choose the replacements and not be forced into upgrade to whatever is in stock when they fail. We moved in this summer, so we don't have any experience with winter in this home. I do have two winters worth of gas bills though from the previous owner.

I'd like to replace the boiler with a mod/con with indirect water heat. I've had 5 quotes from HVAC contractors in the area. Two were useless (couldn't answer my basic questions) so I won't even mention those. One wasn't big on HE boilers so quoted a standard boiler. The other two seemed competent but I'm worried they oversized the boiler. We live in Pittsburgh area and Weil-McLain seems to be what everyone wants to install. I'm going with WM products because I figure it is probably better to get a product the HVAC installers are familiar and comfortable with (even if it is not the best) than to push for another brand that they may not know well.

#1. Quoted Weil-McLain ECO 155 with Aqua Plus indirect 45 gallon
#2 Quoted Weil-Mclain Evergreen 155 with Aqua Plus indirect 45 gallon

I've done my own heat loss calculations based on the past gas usage. We have gas water heat and a gas dryer so the calculations below exaggerate the load a bit. The problem is I don't know what thermostat set point the old owners used or if they were out of town a lot, kept the heat really low, etc. To account for this, I did the calculation at several balance points to get an idea of the range. I've done this calculation for several months of data across several balance points, but I'm just including one representative month.

I used this article as a guide for my calculations.

Balance point: 65F
Design temp: 7F

Jan 2018: (12/8/18 to 1/10/18)
Gas use 351 therms
351 therms*80%*(100,000 BTU/therm) = 28080000 BTU
[email protected] = 1378
(28080000 BTU/1378 HDD)/(24 hours/day) = 849 BTU/degree-hour
Balance point - design temp. = 65F-7F = 58 degrees
849 BTU/degree-hr * 58 degrees = 49,245 BTU/hr
Equipment sizing using ASHRAE 1.4x sizing factor = 68,900 BTU/hr

I repeated the same calculations at a 60 degree balance point and 55 degree balance point, figuring that if the old owners kept a very low temperature setting, the lower balance point would capture that and I'd at least know the range of options.

At 60F balance point: 51,333 BTU/hr, 71,866 BTU/hr for equipment size
At 55F balance point: 54,104 BTU/hr, 75,746 BTU/hr for equipment size

So based on these calculations, I'd be tempted to go with something like the ECO 110 (or Evergreen 110), or maybe even the ECO 70, and not the ECO 155 for our house.

I have the option of improving air sealing, adding insulation (which I plan to do regardless) and using mini-splits once they are installed for supplemental heat. So I would have plenty of ways to reduce heat loss even if the boiler were slightly undersized.

Now, I've read ( that oversizing on the upper end isn't as huge a hit on efficiency and performance as some believe, and that the main downside is paying for more equipment than you need. However, if you oversize the minimum output rating, you can get short cycling instead of modulation, and may have too high return temperatures to condense. I don't want to pay extra for a mod/con if I'm not getting the benefit of modulating or condensing!

Following the calculations in that article, I figured the average winter temp for Pittsburgh is around 30F, or roughly halfway between the design temp (7F) and the balance temp (65F). So if we assume a design load of 52,000 BTU/hr, then the average heating load is 26,000 BTU/hr.

For an ECO 155
Will it modulate?
Min boiler input 28,000 BTU/hr, so min output is 26,600 BTU/hr
This is about the same as the average heating load of 26,000 BTU/hr, so if I understand correctly, we'll get some modulation but not all the time and almost none in the milder months.

Will it condense?
26600 BTU/hr / 185 ft total fin tube = 144 BTU/hr
Yes, it should condense most of the time because this is below the 200 BTU/hr per foot needed for condensing.

Will it short cycle when condensing?
Zone 1: 134ft * 200 BTU/hr per foot = 26,800 BTU/hr should be fine
Zone 2: 51ft * 200 BTU/hr = 10,200 BTU/hr will short cycle if guest zone only, but based on house config, it isn't likely we would only heat this zone.

Doing the same calculations for the ECO 110 and ECO 70, both will condense and shouldn't short cycle for zone 1. ECO 110 has min output of 20,900 BTU/hr, which is below the average load of 26,000 BTU/hr. Is this still too close to get long burns? ECO 70 has a min output of 13,300 BTU/hr, so I beleive it would modulate most of the time.

So, my main questions are:
1. Is my load analysis correct/reasonable?
2. If it is correct, what size boiler makes the most sense? I think I can make a case for the ECO 110 with the contractor but I'm sure he would balk at the ECO 70.

I should also say that I used Coolcalc and the Weil-McClain boiler sizing guide to estimate heat loss for our house. The numbers I ended up with were 108,000 BTU and 95,000 BTU, respectively. However, I'm at all confident that I chose the correct construction characteristics in either calculation, so I don't have much faith in these numbers. According to the WM guide, the ECO 155 would be the right size based on these heat loss values.


  • Jamie HallJamie Hall Member Posts: 12,712
    You certainly have used one approach to the problem, however there's really no substitute for a proper heat loss calculation for the structure. Slant/Fin has a nice program for that, and it's not that hard to use.
    Br. Jamie, osb

    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.

    Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • flat_twinflat_twin Member Posts: 248
    edited September 2018
    The fuel usage calc gave me 49k btu in our old 1800sf two story home. This was based on fuel oil usage across the entire heating season during a mild winter. The Slant Fin app Jamie mentioned worked out to 64k btu. It's very detailed regarding house construction. I'm told there's about a 20% fudge factor built in. If I knock 20% off my 64k it becomes 51k btu.

    We're going on two years with a WM Eco 110. Zero complaints or problems with the boiler. Just over a year ago we replaced the electric water heater with a 40 gallon indirect. Again, no problems. Recovery time for hot water is about 10-12 minutes.

    I'm certain the Eco 70 would have been enough but was talked into the Eco110 by the contractor. The lower minimum fire of the smaller boiler would have been nice and allowed more continuous running. Fortunately there's a lot of iron and water in our old single zone radiator system and it acts as a buffer against short cycling. For example, I turned on the heat last week during that cold snap when Hurricane Gordon moved thru Ohio. With temps in the upper 50's I had 10-12 minute run times every hour. I get more or less continuous running when it's colder than 40 degrees.
    You touched on condensing in btu terms. I only know the boiler return temps need to be under 140 to condense. Our boiler water temps range from 128 on the coldest days to 88 degrees when it's mild. Your finned baseboard will respond differently.

    I should add, our radiator system is rated at 100k btu at 180 degree boiler supply temps, allowing us to run in the condensing range throughout the heating season.
  • HVACNUTHVACNUT Member Posts: 3,275
    Your using waaay too many variables. Like @Jamie_Hall wrote, download the Slant/Fin app. Apply the design day for your region and the temp you want to your home to be at said design day, and your off to the races. That's it.
    The contractor should be doing his own heat loss regardless.
    Not real experienced with installing mod cons, but outdoor reset, a 10:1 turn down and making sure the indirect is piped and wired to the boiler, not the SH manifolds and zone board that I see too often.
    It's been said here numerous times, the contractor is just as, if not more important than the equipment.
    BYW, a national OSHA code has just been inforced that all homeowners must be at least 25 ft from where the work is being performed and questions are limited to 1 per hour. Ah not really.
  • EBEBRATT-EdEBEBRATT-Ed Member Posts: 6,940
    As @Jamie Hall & @HVACNUT pointed out you need to do a heat loss. Your other appliances, age and efficiency of your old boiler will skew your results by a wide margin.

    You boiler seems way way oversized, You combustion efficiency could be 80% but your seasonal efficiency is probably much lower do to short cycling and standby losses.

    It's a big investment. You want to get it right
  • danceembracedanceembrace Member Posts: 2
    Thanks for the feedback. I used the Slant/Fin app as suggested and got a heat loss of 65,000 BTU. So like flat_twin, it is in the same ballpark as my gas usage analysis with a bit of a fudge factor worked in.

    flat_twin: My radiators have a total of about 110,000 BTU/hr at 180 degrees too. We should get good return temps most of the time and condense most of the season too. It seems like I'd be fine with the ECO 110. Now to convince the contractor to downsize.

    HVACNUT: Yes, I know contractors *should* be performing a heat loss analysis too, but very few do. Even if they do, it doesn't mean they don't add size "to be safe." The contractor I'll probably use measured all the rooms and looked in attic, etc, so he has the data to do the calc. I'm following up with him to talk on Monday.

    The WM Eco models have a 5:1 turndown, not 10:1, which is why I'm concerned that oversizing will put me out of the modulation range on the low end.
  • HVACNUTHVACNUT Member Posts: 3,275
    Well your well armed to make sure the job is spec'd correctly.
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