calculating btu's

gary_28

to make a long story short... i presently have two systems in my home. an oil boiler for baseboard (three rooms) and domestic hot water and an oil furnace for forced hot air (most of the house). i'm going have a gas boiler installed that will be supplying the baseboard and it will be supplying a heating coil that will work in conjunction with an air handler for forced hot air. it will NOT be supplying domestic hot water. can i use the heat loss calculator on this site to figure out the boiler size? if so i didn't see anywhere to specify this type of system. do i leave the inputs blank when it asks about baseboard length or radiant tubing??

Rich L.

btu's

A btu's a btu Gary. It doesn't matter what the emitter is as long as the load is matched to the heat loss of the house.

Boilerpro_5

I disagree based on extensive studies....

if you have a typical forced air system in the United States, about 20% of the heat produced is lost before it gets to the rooms. In addition, there can be typical issues like pressure imbalances caused by the operation of the forced air fan that can multiple your air leakage rates. Studies have seen increases in air leakage as much as 600% when the fan comes on. Take a very close look at the condition and design of your ductwork before assuming that "a btu is a btu". I would recommend at least an aditional 20% or so additional heat load for your duct system beyond what is calculate by a convection hot water system. DO NOT base your heat loads on a radiant design, as the loads drop even further with a highly radiant system, due to reverse air stratification and the reduced heat loads it provides.

Boilerpro

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Rich L.

I respectfully disagree Boilerpro

When doing a proper heat loss on a house, no rule of thumbs here, all conditions, ie: windows, wall and insulation thickness as well as type, foundation materials and insulation, attic insulation, etc. play into the heat loss to give a detailed heating plant needed. If what you are saying about forced air refers to leaky, uninsulated duct work running through an attic or crawl space, yes that has an affect and is yet another factor that would go into the proper heat loss. Regardless of the emitter you need to replace the heat lost.

Now that said the emitter can and does greatly affect the comfort level of the occupant but does not adversly affect the btu's required in the conditioned space.

Best regards, Rich L

Gordy

I think the point that

Boiler Pro is trying to make is that the Btu's generated for the fan coil are going to lose 20 percent by the time it gets to the room where the btus are needed. You need to compensate.

Kind of like having a leaky water supply going to a radiant emitter. you would lose velocity, and the heated medium some where on the way to the point needed.

Sure a heat loss for a room is the same no matter what, but you have to get the btus needed to the emitter for that room.

Gordy

Boilerpro_5

However, it appears that it is not always the same....

because of things like hot air stacking at ceilings creating larger temperature differentials at indoor/outdoor surfaces and the stack effect creating by rising air which in turn creates low pressure zones at the floor, drawing in cold air and higher pressure zones at ceilings, forcing warm air out. Most more advanced load calculations assume that when you design for an indoor design of 70F at the height of humans, they assume the whole space is at 70F top to bottom and side to side. When hot air stratification occurs, as with any predominantly air heating system that is on/off controled, its alot higher than 70F at the ceiling when it says 70F at the thermostat. Yes it is all comfort related, but it also goes deeper than that because of the air leakage issues.
Good rules of thumb probably do a better job of taking these inconsistancies into account than detailed calculations because the calculations do not compensate for these very hard to pin down numbers.

Boilerpro

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gary_28

first i want to thank you all for your advice on this. i do appreciate your willingness to help. what i have discovered since starting this project is that in the heating and cooling industry if you ask 10 people their opinion you will get 10 different well meaning answers. just about all based on truths, so even tho many of you disagree, i do thank you for halping me. one question so i understand everything correctly. the boiler (has a max btu output) feeds a coil which has its own max btu output (of course depending on the boiler size) is that correct??

jp_2

Boilerpro, I have found

through measurements years ago in a drafty force air house that the floor to ceiling temp diff is only 2-3F.

stratification seems to occur more with BB heat or iron rads running at high temps. the FHA keeps mixing air temps, boiler systems can not do that.

I'd like to see your documents that suggest 600% more infiltration due to FHA systems? thats an awful big number.

Rich L.

match coil to load

Sorry for any confusion and I appreciate the fact you realize it's all well meaning. There are of course different routes (methods) that can be taken to reach the same destination.

Gary as I understand your project and question, you would need to first determine the heat load of the house. Then match the boiler to that. Next determine the amount of load covered by forced air and get a coil/ air handler/ pump sized to handle that portion. The remainder will go to your base board.

Hope this helps, Rich L

Boilerpro_5

I believe that it was the State of Idaho...

that did those tests, but it may have also been the US. Dept. of Energy. That 600% is very high, and was only one of a large number of homes tested, but it clearly points to a larger scale, if a smaller percentage issue...poor ductwork installation and heating system design that seems to dominate the US is costing millions in energy costs. Ductwork systems that are sealed and well insulated are almost nonexistant in my neck of the woods. In fact in the 12 years or so I have been in business, I may have seen only one system built that well, and I am not sure how well sealed it was as I don't have the test equipment for checking that The 20% ductwork loss is from Dept of Energy studies.
That's the problem with calculations. You may see only a 2 to 3 F difference. I have seen differences of 15 or more with on/off burners and blowers with heating ductwork located at the ceiling of a high loss space. Continuous fan operation (which uses more energy) and modulating output (either modulating steam or hot water into coils, or the recent modulating burner furnaces) help address these issues. It appears we are getting closer, thank goodness, but so many still opt for the most expensive furnaces......78 percenters that last about 7 to 10 years, and gobble up energy while providing extremely poor comfort.

Boilerpro


Boilerpro



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gary_28

you guys are great. rich, you and everyone else here seem to have answered my questions for now and it's greatly appreciated ! if an when i need more guidance i will be posting again. once again thanks everyone for the time they put into helping others on this site.