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Posted by HeatPro on May 02, 2008 at 16:12:57:
In Reply to: Is historical usage data better than Manual-J for sizing boilers? posted by dana on May 02, 2008 at 15:44:36:
Insulating an uninsulated 1923 house would reduce the heat loss more than 50%, so you understand from the meter readings that the original IBR heat loss method has some 'buffer' built in. As co-author of the H-22 IBR manual, I am aware of that. Also as the original programmer and distributor of the first IBR heat loss computer program, Several utilities and contractors explained that it was excessive. The Manual J method over-estimates for duct loss. I stopped using the IBR method to correct the over-estimation due to the 'walls with windows' infiltration method that gets wilder as the house sq ft increase beyond 1000 sq ft.
My present heat estimate program eliminates that over-estimation, by using the 'crackaqe method' I taught in the IBR Advanced hydronics course (which hasn't been taught in 15 years.)
After you insulate your house, you'll see short-cycling all the time unless you either change to a modulating boiler or add mass to the radiation. If your radiant floor heat is due to adding some inches of gypcrete or concrete to the floors you will have to account for that mass heat-up and cool-down time. A slab floor can take hours to heat up and has a long swing-cycle as the floor temperature changes (one of the problems to solve with controls for fast weather changes.
An air handler doesn't have that problem, as there is no real mass to all the air in the house. I suggest you keep the air handler method, but keep the radiant floor at a low temperature, like 62F at a constant temperature and let the air handler vary the temperature for set-back savings.
As fuel costs rise, it will be important to have the ability to set back unoccupied rooms and just hea the rooms you enter to let them fall when you leave. A central air system or slabs don't let you do that readily, so you will be paying for the occasional use of unoccupied rooms. If you want to get rid of the air heat, use the tubing and spreader radiant method below the floor then add baseboard to the rooms to let them go up and down while the floor temp remains constant.
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