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Old 11-19-11, 12:35 PM   #6
MN Renovator
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50% over the manual J? I don't understand that at all, that is far too much over. Usually you size to the manual J which already has a small amount of fudge factor built in, albeit a small one. If the manual J is done right it should be accurate enough to use the same size or if it is right on a size, say a 50k output furnace has a man. J of 48k, one size up would be fine if an owner insisted on a setback. I think setbacks are great but I'd rather have a 30k unit and then just lose the setback when I'm within 10 degrees of my design temp which is less than a week out of the winter.

With a heat pump you usually have to size for your air conditioning load at a minimum so you are covered there and if your heat load is reasonable you'd be set. In my case natural gas is so cheap that the balance point for a heat pump would leave me with a 24k air conditioner because that is what my house needs for 78 degrees inside at 95 degrees outside(experience with my 23k at 95 outside) and once the temperature gets low enough to where the heat pump can't keep up and is defrosting often I would be on natural gas anyway. Problem is that it that it is 30 degrees out right now and my furnace has would have run so little if I wanted things at 70 degrees that if I wanted the increased efficiency of a heat pump the savings are limited for me and the cost would never be made up unless natural gas prices changed significantly for my area. If I was running off of oil or propane I'd be all over heat pump. Of course I'm talking about the central air heat pumps including the Carrier Greenspeed aren't worth it not to mention its extreme cost, an mini-split inverter heat pump like the 12k Fujitsu would be a different story as the A/C and heating savings might make a bit more sense and the unit cost is small and I'm not running the expense of swapping the current A/C and heat equipment to go that route.

...ok, I just realized I'm off topic to the boiler discussion. How are you sure that your under 50k as a worst heat load. If you are 18750BTU/hr on the worst month and that is a number reduced by sun through the windows and warmer days, have you done any measuring or calculating to tell you that you are under 50k? My January was 76therms with the house at 50 degrees outside of load testing because that is the temperature I'm comfy at with pants and a hoodie, add a blanket from time to time. Average temperature was 11.4f so 50 degrees - 11.4 = 38.6 degrees of rise. If it was 72, that would be 60.6 degrees rise and about 57% more fuel on paper to 119 therms $117 versus $63 for 76 therms. ...scary comparison, I haven't done the math to calculate the savings yet and didn't figure I was saving that much.

My point 119 therms = 11900000 BTU/730 hrs(month)=16301BTU/hr. In reality it was 7600000/730=10410BTU/hr. Similar climates but probably different sun exposures so the numbers aren't as comparable as I'd like to think they'd be. Plus the numbers might not be as linear as multiplying the temperature differences otherwise 18750/16301=114% * 27700 BTU/hr = 31578. ...I wouldn't bank on that number, too much fudge. I suppose for a heat pump calc if I used 50 degrees further into the future and my average was 10410, then a Fujitsu 12k mini-split shouldn't have much of an issue from the time the sun shines on it from the southwest in the winter mornings until it starts to get cold at night for most nights. Most heating happening when the sun is down and its cold though, which is when you have no solar water heating or solar window heating benefits but if I had a setback at night and let the heat pump recover most of it during the day, that might help. ...again sorry I'm blabbing off topic about heat pumps. Maybe it could apply though, what are your electric and natural gas rates? Mine are 75 cents up to 110 therms and electric is $.11 in the non-A/C months.
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