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[quote=Anonymous]Something you have to realize in these discussions: every house is unique. Every house is hand built. There is no such thing as a "normal" or "average" house.

As to the questions on sizing and heating vs. cooling, there are no rules of thumb. In our climate some houses are heating dominant, some are cooling dominant. Proper sizing of equipment involves doing what's called a "Manual J," which is a computer model of the energy use of your house. Code now requires it in most places. Doing a proper Manual J means going into every room in the house, measuring the dimensions of every room and all the windows and exterior doors. Also going into the attic and basement and looking at the construction. If someone wants to quote you a system without doing a Manual J, they're just guessing. Even if they just want to replace existing equipment with a similar size. In fact, especially if that's what they want to do.

The Manual J uses what's called the "design temperature," which is basically the hottest and coldest temperature we regularly see. In Washington, DC, the design temperature for heating and cooling are 21F and 92F.

Since the cooling temperature is only 20F from room temperature and the heating temperature is 50F, you'd think we'd need much more heating than cooling. But this is where the uniqueness of houses comes in. As part of the Manual J you have to figure out the solar gain and account for it. That means measuring the dimensions and orientation of every window in the house, and factoring in the solar gain factor for the type of glass in it. These aren't hard calculations, but it is work. You also have to figure the removal of humidity in the summer, which takes cooling capacity.

I had a Manual J done for my house. The results were that the heating load is 27,600 BTU/hr, and the cooling load is 29,800 BTU/hr. This is Washington, DC. The solar gain is almost half of the cooling load, 14,000 BTU/hr. I have lots of windows and my house faces south.

As part of the process I had the air infiltration of my house measured, using what's called a blower door test, a fan is mounted in the front door and pressure and flow are measured. My house is much tighter than most. This means that air infiltration is low, which means that I need less dehumidification in the summer than typical.

In my case, since the loads are so close, a heat pump sized for my winter heating load keeps the house comfortable year-round.

But every house is unique. [/quote]

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