heat loss calculations for boiler sizing

[penfrydd]

Just figured my heat loss using slant/fin program. Came up with 115,000 btu/hr loss for an old farm house, 3600 sq ft, moderately insulated with new windows but lots of them. Does this sound reasonable? How much more should I add for domestic hot water?

Great forum. Lots of good info. Can't wait to get back to burning wood. Did it for 25 years, but took a break for the last 7 or 8.

 

[Nofossil]

115,000 BTU/hr seems really high. I have 3500 square feet in Vermont, well insulated but lots of windows. At -20 degrees, I use 30,000 BTU/hr.

 

[maurice]

I've got about 1800 sqft, and real 4" wall w/ blown in, 12" attic blown, old farmhouse, one window each room. Heat-loss professionally calculated at 80,000 btu. So I put in a 100,000 btu propane furnace years ago and we have been comfortable. (but now costly)

So I'd say your calculations are reasonable.

 

[Nofossil]

I've got about 1800 sqft, and real 4" wall w/ blown in, 12" attic blown, old farmhouse, one window each room. Heat-loss professionally calculated at 80,000 btu. So I put in a 100,000 btu propane furnace years ago and we have been comfortable. (but now costly)

So I'd say your calculations are reasonable.

So the question is: when it's really cold, does the propane furnace run almost continually? You'd be comfortable even if it ran only 40% of the time. A really good way to check is to look at your total season propane consumption. Divide by the number of days. Convert to BTUs and you have your average daily heat load. Your peak is usually about twice that.

My theory is that heating contractors and equipment vendors have every reason to estimate high. If you install something bigger than you need, you won't notice that it only runs half the time at most. If you install something that's too small, then you'll be cold and you'll be mad at them. Plus, they made a smaller initial sale.

Since fossil appliance don't lose a lot of efficiency when they cycle on and off, there's every incentive to be conservative and oversize everything.

 

[Bob Rohr]

Just figured my heat loss using slant/fin program. Came up with 115,000 btu/hr loss for an old farm house, 3600 sq ft, moderately insulated with new windows but lots of them. Does this sound reasonable? How much more should I add for domestic hot water?

Great forum. Lots of good info. Can't wait to get back to burning wood. Did it for 25 years, but took a break for the last 7 or 8.

"new windows but lots of them' could be a reason for a high load number. 20 -25 BTU/ ft in not uncommon for older homes. How much glass and what r-value?

Most load programs tend to have a bit or over sizing built in. Try another load calc program to double check.

You should not need to over size the boiler for DHW, unless you have very high demands. typically the boiler will drop into a DHW priority mode to recover the hot water. It shuts down the heating loads for 20- 30 minutes. Some newer boiler controls have a "time out" feature to prevent the boiler from staying in an endless DHW mode and risking a heating freeze up.

hr

 

[Sawyer]

I calculated out for 91,000btu/hr at –20 on 4600sq/ft (new construction) of heating area. We have a lot of windows also, 360 sq./ft of glass in the living room which is 32,000btu’s of our total loss. According to SlantFin we use more heat in our living room than Nofossil uses for his house :-(. We have 63’ of cathedral ceilings which increases the volume to be heated also. This was calculated with 6” blown walls and ceilings and triple pane glass. We heated the upper 2300sq/ft with 9 cords of hard maple in the Accucraft fireplace but only maintained 50-55 degrees in the lower level. We ran the boiler for the lower level when guests arrived.

 

[penfrydd]

Thanks for all the advice. I also consulted a friend who's an 87 year old retired heating engineer who used to do work for Department of Energy during the old oil crisis. He thought the slant fin program covered all that was needed. And the advantage to getting a slightly larger boiler would be to have a bigger fire at one time to get the water storage up to temperature.

 

[MrEd]

Thanks for all the advice. I also consulted a friend who's an 87 year old retired heating engineer who used to do work for Department of Energy during the old oil crisis. He thought the slant fin program covered all that was needed. And the advantage to getting a slightly larger boiler would be to have a bigger fire at one time to get the water storage up to temperature.

My understanding is more or less the same - if you have sufficient storage, than a bigger boiler is not really going to be a problem. I could have gotten by with a smaller one, but for the small amount of extra money I went with the Solo 60. I can't see the downside if all that extra heat gets sucked up by storage anyway. Without storage, and with no plans on getting it, I could see why bigger isn't always better. Time will tell, finishing up install this summer and next winter will be my first year using a gasifier.