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Old 07-07-2009, 03:18 PM   #1
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Boiler size


advice please: we have a 3300 sq ft house, three stories. the original boiler is from 1945, 375,000 btus. one company told me that we need two 187K btu boilers hooked up in tandem, w/o measuring the house because in the old days, engineers determined the amount of radiation from the radiators and decided the btus. thus, no new calculations need to be made. another company will measure because new boilers are more efficient, etc, etc. do either of these views seem more or less stupid? should the house be measured and determined for new btus based on new boilers and what they're capable of?

thanks in advance.


Last edited by Bonnie Youngs; 07-07-2009 at 03:22 PM.
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Old 07-07-2009, 04:28 PM   #2
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Standing radiation can be used but usually it ends up oversizing the system. I am sure the has been some form of upgrades to the home since the original install. Man j or other approved method should be used before selecting any boiler. It is no different than forced air in that aspect. There are several contractors in your area that can preform this for you.

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Old 07-07-2009, 04:31 PM   #3
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thank you. all of our unprofessional calcs done so far have indicated 140-150K range. i'll be sure to ask from now on what method is being used to assess the need.

BY
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Old 07-07-2009, 04:39 PM   #4
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Boilers net IBR or output should meet or exceed your load. Good luck
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Old 07-07-2009, 04:43 PM   #5
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sorry, one clarification. if we need net 140K to properly heat the house, that means we need to buy a 'gross' unit of 160K at 87% efficiency. this is what you mean by Boilers net IBR or output should meet or exceed your load, correct?
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Old 07-07-2009, 04:47 PM   #6
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yes remember they are not 100% efficient
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Old 07-07-2009, 05:21 PM   #7
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Let's clear this up. When a manual J is done the contractor needs to measure all the doors windows walls etc. They run this through a computer program to determine the heat loss. You then size the boiler off the DOE output not IBR output unless the boiler is not in the house.
The efficiency does not play in any way in this step. The efficiency is only how much you will save when operating. If the heat loss is let's say 50,000 you choose a boiler with a DOE of 50,000 or close. If that boiler is 95% efficient or 50% efficient it will put out 50,000 if that is the rate. All boiler manufactures have the output listed at the given efficiency. The 50% efficiency boiler will give you the same amount of output if it is rated the same it just takes more fuel to get there. A 95% product will give you the 50,000 btu's at less fuel input.
The other thing we need to realize with boilers is the efficiency is never the same. Boilers are tested with an AFUE test. This test is done at 120 return water temperature and 140 supply water temperature. When the boiler is operating at that temperature that is the efficiency providing the piping and flow is correct. As soon as the water temperature increases the efficiency decreases and if the water temperature decreases the efficiency increases. Mod/con boilers can hit efficiencies as high as 99% and in theory over 100%. This is due to the condensing mode you can get heat out of the condensate which normally would not be taken out in the older boilers. This can account for about 9% more heat out of the gas. To take advantage of the higher AFUE efficiencies and the latent heat from the condensate the boiler water must remain below about 130. Many older and existing homes cannot heat in that range depending on the amount or type of radiation.
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Old 07-07-2009, 05:27 PM   #8
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ok, now my head really hurts .

i believe i understand what you're saying. in doing the measuring, then, as you noted, will the radiation capacity of the house be taken into consideration?
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Old 07-07-2009, 06:48 PM   #9
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Don't use the DOE rating.

Your house has large steel pipes.
Use the IBR rating it allows for teh extra heat the pipes will take from the water.

The house should be measured for its heat loss.
Then a boiler is selected to meet that heat loss. And by using the IBR rating, the piping gain is allowed for.

Seem a lot of older hopmes where teh DOE rating was used. And they had problems heating when it got to the lower outdoor temps.
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Old 07-07-2009, 06:50 PM   #10
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okay, i'll do more research on the IBR and do a heat loss analysis. at least i'll have lots of questions for tomorrow's estimate guy! thank you.
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Old 07-07-2009, 06:56 PM   #11
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My house was 1640 sq ft
With an addition/sunroom its increasing to ~3000 sq ft
The same boiler will heat the whole house still
New windows, more insulation in the addtions - including 2nd floor
Our boiler was about 2x as big as it needed to be
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Old 07-07-2009, 06:57 PM   #12
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what size boiler do you have? and where do you live?
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Old 07-07-2009, 06:58 PM   #13
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sorry, i see now, a cold state!
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Old 07-07-2009, 07:25 PM   #14
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Do not use the IBR. You must use the DOE. Some of the areas in the country are now requiring the use of DOE. The heat loss from the pipes are the difference between the DOE and the IBR. Heat loss programs are 15-20% heavy as is. If there is a problem heating when the DOE is used it is a problem in the piping not boiler sizing.
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Old 07-07-2009, 07:38 PM   #15
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Quote:
Originally Posted by tk03 View Post
Do not use the IBR. You must use the DOE. Some of the areas in the country are now requiring the use of DOE. The heat loss from the pipes are the difference between the DOE and the IBR. Heat loss programs are 15-20% heavy as is. If there is a problem heating when the DOE is used it is a problem in the piping not boiler sizing.
Which is why you use IBR.

The same as determining the size of a hot air furnace.

If the ducts are in an unconditioned space. You add that heat loss to the load. Instead of just saying its a problem with the ducts.

DOE calculates that all jacket and pipe heat loss go to the conditioned space(too bad it doesn't work that way).

IBR calculates that all heat loss from the jacket and pipes is not loss to teh conditioned space.
Which, with the infiltration of most basements is the way it really works.

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