How big of a boiler do I need?

jim lockard

You may want to check the find a Pro listing. Their are guys from your area that visit here. Best Wishes J.Lockard

saxman251

What btu size boiler should I get

I currently have a weil-mclain CGM5 boiler running at 140Kbtuh. I need to replace it with a direct vent model because of an issue with my chimney (costs more to fix that than replace the boiler!). I have a 1400 sq. foot house (2 stories + full, unisulated attic) with 260 sq ft of radiation area (old cast-iron radiators), and have calculated the heat loss of the house to be somewhere around 75000 btu/h. I am looking at a Weil-Mclain CGs6, but after reading a bit, I am beginning to think that is way to big a boiler for me. Currently there supply and return lines that go to the boiler are 1.25" which is reduced from the 2" overhead, asbestos covered lines.
Any input would be greatly appreciated!

Thanks,
Ryan

Brad White_22

I would re-check your radiation EDR

and your heat loss. If you do have 260 EDR of cast iron radiation, it will only emit 62,400 BTUH if run with 2 psig steam (240 BTUH/SF EDR). If water as it seems you have, at 150 BTUH/SF EDR, (170 AWT) the heating output to the space will be only 39,000, far short of your 75,000 calculated loss. Even with steam you could not heat your house with that little radiation, if your figuring is correct; you have hot water so are well below that still.

Until you get a handle on the discrepancies, I would hold off making a decision.

Not knowing where you live, I can offer that my own house here in Boston is similar to yours. Design temperature is 7 degrees F. House is 1450 SF and with a calculated heat loss of about 55,000 BTUH (insulated walls and attic, storm windows on single pane windows). If your walls are not insulated I can see the house being 75,000 BTUH in a similar climate, by the way.

Given a choice I would prefer more than enough radiation and have a boiler sized for the design heat loss. No point in having a boiler with more capacity than your radiation and piping allowance. But unless you have at least enough radiation to drive the heat loss, you are SOL

Alan(OnVacationInOregon)Forbes

Lots of numbers

If you have 260 [] EDR, then 260 x 170 = 44,200 BTU's. And then you go 44,200 BTU's divided by 1,400 [] house = 32 BTU's/[] which seems pretty reasonable, except that we don't know what part of the country you're from (which might help).

But then you calculate that heatloss of the house at 75,000 BTU's which brings us to 54 BTU's/[], almost double than what you have radiators for. So, this brings me to the conclusion that you don't have enough EDR and no matter how big the boiler, you're not gonna' heat your house.

Are you playing with us?

Mark Hunt

Old gravity system



This is an old gravity system so the EDR multiplier would be 150 btus/sqft of EDR.

150 x 260= 39000. 39000/1400= 27.85 btus per square foot on design day. That too is probably more than needed.

In any event, what is in there now is 3 times bigger than what is needed.

I suggest you find a contractor in your area that can perform a real heat loss. If it is as I suspect, you can take advantage of a modulating/condensing boiler and outdoor reset. Running lower water temperatures will result in fuel savings. Correctly matching the load will be a big help as well. On average our customers report 30% to 40% in fuel usage reduction.

Mark H

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saxman251

Radiation EDR, heat loss, etc.

Thanks so much for the info, and no, I'm not playing with you guys! It would seem that I did indeed make a calculation error on the EDR (we can chalk it up to being up late and trying to use a tape measure). I have now double checked, and I have 373.55 sq. ft. of EDR, not the 260 I thought I had before. I have tried a couple of different heat loss calculators, and they come up with heat loss in the 54-58kbtu range. If I'm doing my math right this morning, it would appear that my heating output should be 63.5kbtu. Does this mean that I should look in the 60-70kbtu range for a boiler?
Thanks,
Ryan

Mark Hunt

That still


works out to about 39btus/square foot. This still sounds high to me especially if you have added insulation.

Regardless, I would recommend a modulating/condensing boiler and outdoor reset. You will reduce your annual fuel consumption by quite a bit.

Hope this helps.

Mark H

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Todd_12

type of system

What type of system do you have? Steam,hotwater,gravity?

saxman251

type of system

I have a hot water system that is not gravity (at least not anymore). There is a Bell & Gossett circulator on the boiler. The thing definitely heats the house; if I crank it up to 66 (I like it cooler in the house than most), it will get the temp up to 70 before it starts to come down again. I live in Allentown, PA (if that makes any difference). I've got 2" pipes in the basement, and the pipes going to and from all but the 2 skinnmy radiators are 1".

Guy_6

hot water

Keep in mind that you are falling into an area that is considerably more prevalent these days: Low heat loss, yet higher hot water demand. The need for BTUs for hot water generation surpasses the BTU load of the heat.
If you are using that boiler to supply your domestic hot water, either through a tankless coil, or an indirect water heater, you may want to look larger.
This sounds like an ideal candidate for a modulating, direct vent boiler.