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Old 01-27-14, 08:53 PM   #6
MN Renovator
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Hattiesburg, MS appears to be close to your place so I'm using the numbers that I found for Hattiesburg figuring it is close to you. Heating design temp is 27 degrees and cooling design temp is 94.

I'm assuming you are looking at the space heater running at about 45% of the time based on your kill-a-watt reading? What temperature was it outside when you measured this? When trying to size a heat load you usually want to go off of a day as close to your heating design temp as possible and you can extrapolate a little bit from that point. If you had a single 1500 watt space heater doing the job you could go with a 9000BTUhr mini-split to cover that load and it would have almost twice the heating power of your 1500 watt space heater. 27 degrees outside is not really a challenge for a heat pump, especially an inverter heat pump. If you are happy with the performance of the window unit

"During heating season, you can predict how low the unit will go regarding outdoor temperature by relating to the current window a/c unit's summer performance. For example, if the current unit does a good job up to 98 degF during the summer, the mini-split will do roughly the same at heating down to around 38 degF."

How do you figure this? In the summer most of the cooling load comes from the sun shining through the windows and in the winter the heat loss is through the glass, walls, ceiling, and floor. The heating requirements will be a lower BTU than the cooling requirements per degree. For example if it is 88 outside and my house is at 75 on a day with full sun, my house needs 15000BTUhr to hold 75 degrees. ...but in the winter 15000BTUhr of heat will take care of me at -2f if I keep it 68 degrees inside.

To put it simply, the heat pump manufacture will usually have a balance point chart showing the outdoor temperature and how much output it will provide at that temperature. If you are working just fine off of a 1500 watt space heater(5000 BTUhr roughly), a 9000BTUhr heat pump should be able to provide at least that much heat at your design temp. The AHRI Certification Directory site under 'variable-speed mini-split and multi-split heat pumps' will show you the output at 17 degrees, pretty much every heat pump will exceed a space heater with that kind of outdoor temperature.

The higher the HSPF, the lower the energy it will use to heat the winter. The higher the SEER, the less energy it will use to cool in the summer.

For a 1050 square foot house, if the 9000BTUhr mini-split turns out to be inadequate, my suggestion would be to seal up the place better first, then add more insulation.

If you are really planning to put two in the house, I wouldn't go with anything bigger than 9000BTUhr because 18000BTUhr will be more than plenty for the whole house in the winter.

I don't know your summer cooling load though, that is harder to calculate and isn't just a matter of how much insulation is in the walls, windows, shading, which direction the windows face, etc are large factors and the Manual J cooling load calculation is a bit complicated for that. If your 8000BTUhr window AC can't do the job with all of the doors open in the house, you'll need to go bigger for cooling, add a good exterior shade cloth or other shading to the outside, or otherwise block more sun to get the load in a good place.
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