Furnace sizing calculations
I'm planning to replace my 30-year-old gas furnace (160,000 Btuh input, 128,000 Btuh output). The contractors who’ve come have all based their sizing only on the old furnace or on my (1867) house’s size (2200 sq.ft.), and their recommendations ranged from 91,000 to 115,000 input Btuh for a 95% efficient new gas furnace. Since they didn’t do it, I’ve used HVAC-CALC, and have come up with around 85,000 input Btuh [And since I’m not certain about some of my R-value and infiltration assumptions, I also did a scenario with super-optimistic assumptions (70,000 Btuh) and one with very pessimistic assumptions (100,000 Btuh).] But the real confusion is this: I've also done a careful analysis of my house's actual heating gas usage over the past 3 winters, and therefore have a pretty consistent figure for therms per degree day. Using this information – and assuming that my old furnace is still operating at 80% efficiency☺ - I can derive the peak heating load by applying the therms/degree-day value to, say, a -5 degree day (I’m in the Boston area, where it’s been that low only 1 day in the past 10 years), and come up with a peak requirement of only around 60,000 input Btuh for a 95% efficient furnace. To me, that's a significant difference. I don’t know why the gold-standard Manual J-type calculations and assumptions would be 40% higher (I know they have a safety factor built in, but I don't think it's that high), but I'm tempted to put more faith in working from the actual consumption of my battered old furnace. Is there a way to resolve this dilemma? What value should I use to properly size a new furnace? Hi Vic, you have real data and they are trying to fit you into the standard box, so I like your numbers a lot more. Plus, what are the drawbacks? If your furnace is a bit too small, you will have to bundle up on the coldest of cold days. In NE we are prepared for that. And it gets better. It sounds like you have done some efficiency improving on your old house, an 1867 building didn't have a lot of insulation to start. But, you will probably be doing more, and thus an oversized furnace today will be way oversized tomorrow. And there's more. A high efficiency furnace is only highly efficient in the steady state, so it has to come up to temperature and then it is more efficient. An oversized furnace that short cycles will not spend as much time in that highly efficient mode. I like to build in my own fudge factor, or at least know up front what they are adding. The fact that manual J has to be used with your eyes closed speaks poorly for it's required use. In other words, a lot of contractors/customers rely on it's determination without knowing how it gets to the final answer. Glad to review the numbers you are entering to see if you missed anything. Just wanted to add, have you sealed all of the ducts on that system? Bud Bud, thanks for your advice and information. You’re right, the house is tighter than when the furnace was installed. I didn’t mention that we set back the thermostat for 8 hours each night, which lowers my therms/DD calculations, but I assume that’s only in the 5% range. And no, I haven’t yet sealed the ducts in the basement – that’s on the calendar for this month (they are insulated, but not sealed). I haven’t factored that in yet, but I assume that, once sealed, that should reduce the therms/DD by 5-10%? Your comment about furnace efficiency reminded me of another question: if I get a 2- or 3-stage furnace, doesn’t that at least partially reduce the short-cycling issue? Or are these furnaces much less efficient in the lower stages? I’ve been told that most of the time, they’d be operating in the low mode. If you’d like to comment, here’s how I came up with my personal furnace-size estimate: (1) from December to March, if I back out the summer-time gas consumption (for hot water and cooking), I get a fairly consistent monthly value of 0.20-0.23 input therms per degree-day. (2) I took 80% of the highest value (0.23) to get 0.184 output therms/DD (pretending that my furnace is still 80% efficient) (3) For the worst-case scenario of a day at -5 F, I get 13.4 therms/day (we keep the house at 68 F, so 73 x 0.184 = 13.4) (4) This translates to 0.56 therms/hour, or 56,000 Btuh (at 100,000 Btu/therm) (5) To achieve this would require a 95% furnace with 59,000 input Btuh Does this calculation hold water? Thanks. I have a chart for the reduction in HDD for stat setback. Just calculate/estimate your average house temperature. For example, a 5 degree reduction in average house temp yields a 19% reduction in the hdd number. A 68 degree house temp would be 8% lower than the base 70 degrees they use. Your reasoning sounds good and yes the multi-stage furnace will help, although I am unsure of the measured efficiency at each stage of operation. Good question to ask them. I would want to see it in print as opposed tooh ya it's the same at all levels, which means they don't know. Air sealing a forced hot air system can be a biggie, especially if any of the ducts or returns are in unconditioned or semi-conditioned areas. Also, balancing the supply and returns for each area is important. Too much of one or the other can move air into or out of the home and under some conditions causing moisture problems inside the walls. That's scary isn't it, a potential 60,000 btu furnace instead of a 160,000 and it may end up being a bit too large. I'd like to be there when you tell the furnace guy that is what you want. I bet he will refuse or require you to sign something removing his responsibility. Let us know what you decide and definitely come back this winter and tell us how it is going. Bud
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