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Hot water boiler size

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Member Posts: 53
So, I have a quick question that bugs me all the time. Whenever we do a boiler replacement, we size the boiler by doing a heat loss calculation, especially if its high efficiency. Now my question is, let’s say were replacing a boiler and would just like to size it to the amount of baseboard radiation installed in every room. Would it be safe to measure the amount of element in the house and times it by 600? (example 90 feet of element x 600 = 54,000BTU DOE needed) ) because to my understanding doing a heat loss calculation by room gives you the amount of BTU needed to heat that room.

• Member Posts: 6,505
edited September 2019
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What if the room doesn’t have the proper amount of existing radiation?
Also your basing that output on 180 degree water.
A heat loss will tell you how many btus you need for the room.
The radiation will only tell you how many btus you can get based on any SWT.
If anything too much radiation will allow you to lower the SWT.

There was an error rendering this rich post.

• Member Posts: 53
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So, lets say its a cast iron hot water replacement running at 180 degree water customer says heat is fine. To replace the boiler snice the radiation is existing is it a good idea to measure the radiation.
Or should we do it like this

Length x width=room sqft
sqft x 50= BTU needed ( 50 BTU per sqft needed for zone 4 )
BTU ÷ 600 = ft of radiation needed
• Member Posts: 5,741
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> @Dave8699 said:
> So, lets say its a cast iron hot water replacement running at 180 degree water customer says heat is fine. To replace the boiler snice the radiation is existing is it a good idea to measure the radiation.
> Or should we do it like this
>
> Length x width=room sqft
> sqft x 50= BTU needed ( 50 BTU per sqft needed for zone 4 )
> BTU ÷ 600 = ft of radiation needed

I’m confused, are you doing heat loss calculations or are you using rules of thumb?

50 per sq ft is crazy high and isn’t a proper heat loss calculation.

I’m in zone 5 and not even at 40, in a 100+ year old house with original windows and virtually no wall insulation.
2014 Weil Mclain EG-40
Boiler pictures updated 2/21/15
• Member Posts: 53
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That's what my plumbing supply house uses to determine heat loss they said zone 4 is between 45- 50.

Is there a better way to figure out size of boiler thru amount of baseboard radiation.

Also where to I get an accurate heat factor for zone 4?
• Member Posts: 5,741
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There isn’t a factor. You do a Manual J heat loss calculation on the building. Everything else is a guess, and at 50 per sq ft it isn’t even an educated guess IMHO.

Do a google search for Manual J and read up. In addition there are several free programs out there to run the calculations. Slant one has one that works fairly well if you have a tablet or smart phone.
2014 Weil Mclain EG-40
Boiler pictures updated 2/21/15
• Member Posts: 53
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> @KC_Jones said:
> There isn’t a factor. You do a Manual J heat loss calculation on the building. Everything else is a guess, and at 50 per sq ft it isn’t even an educated guess IMHO.
>
> Do a google search for Manual J and read up. In addition there are several free programs out there to run the calculations. Slant one has one that works fairly well if you have a tablet or smart phone.

Ok maybe I wasn't specific, I was wondering if you do a total heat loss on a house and come up with the amount of baseboard radiation that every room needs and then find the total BTU output that the boiler needs to be but then you measure the amount of radiation the house has and it's less then the calculation wouldn't the boiler be oversized to the amount of existing radiation.
• Member Posts: 5,741
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If that happens, and the house has never had issues heating, then your heat loss calculations are wrong.

And yes the boiler is oversized for the radiation and shouldn’t be installed as you can’t do anything with the excess boiler output.

If the radiation does 50k and you put in 100k boiler, you heat the house with 50k. If the radiation does 50k and you put in a 500k boiler, you heat the house with 50k.
2014 Weil Mclain EG-40
Boiler pictures updated 2/21/15
• Member Posts: 23,415
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Dave8699 said:

> @KC_Jones said:

> There isn’t a factor. You do a Manual J heat loss calculation on the building. Everything else is a guess, and at 50 per sq ft it isn’t even an educated guess IMHO.

>

> Do a google search for Manual J and read up. In addition there are several free programs out there to run the calculations. Slant one has one that works fairly well if you have a tablet or smart phone.

Ok maybe I wasn't specific, I was wondering if you do a total heat loss on a house and come up with the amount of baseboard radiation that every room needs and then find the total BTU output that the boiler needs to be but then you measure the amount of radiation the house has and it's less then the calculation wouldn't the boiler be oversized to the amount of existing radiation.

OK. Let's be quite specific. Step one. Do a heat loss calculation on the house. I use the Slant/fin application, which works just as well on a computer as it does on a tablet or PC. It will quite happily give you both a total heat loss for the structure and a room by room heat loss.

Step Two. If you don't mind a bit of advertising (the thing is, after all, put out there for us by a manufacturer!) it will also recommend the amount of baseboard you need for each room.

Step Three. Select a boiler which matches the total heat load of the house. Don't oversize. Don't undersize either; try to get as close as you can.

Step Four. If some rooms appear to be under radiated, add radiation if you can. Ideally all the rooms on any one zone should be able to deliver the needed heat with the same water temperature (assuming reverse return).

Now. If we suppose that the house appears to have inadequate radiation already in place, talk to your client. If they are happy with what they have -- the house heats well enough -- as @KC_Jones suggested, go back and revisit your calculations. We all make minor (or major!) errors from time to time. If it looks like you calculations are OK, talk to the client some more. Do they run the house cold? Do they just ignore some cold rooms? What's the story?
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England