Posted By Bob I on 31 Dec 2012 03:04 PM
Lee: high efficency gas furnaces, unlike minisplits, make lousy air conditioners. So for your efficiency comparison calculations, especially with minisplits, you need to include the cost of A/C.

Right, that is why I made the comment, "If A/C is desired, that might tilt the decision toward minisplits in spite of the higher operating costs for heating."

I can remember camping in your area one time over Memorial Day. When we arrived Friday afternoon at the campground, there was some ice that a previous camper had thrown out. When we left Sunday afternoon, the ice was still there. On Memorial Day! Do people need A/C in that climate? Maybe on July 4 and 5th? Certainly not on Memorial Day. Like to froze my *** off thinking that it would be warmer, and not having heavier clothes. And we kayaked in some chilly weather.

The analyses of "dual fuel" heat pumps is interesting. Not worth it for me, but with propane or the right mix of electric vs gas rates it would be.

Posted By Lee Dodge on 31 Dec 2012 03:27 PM

Posted By Dana1 on 31 Dec 2012 02:29 PM
...snip...
A 30K max with a sub-15K min would be pretty reasonable for these smaller to-mid sized houses with design condition heat loads in the 20-25K range.  I'm sure the house with a 14K load @ design condition would be just fine with that (hypothetical) 15K/30K furnace.  And it would be both more comfortable & efficient than a 60K single stage bang-bang or 60K/30K two-stage.

My natural-gas furnace is a mod-con 60 kBtu/hr max / 21 kBtu/hr min, and I would not mind trying something a little smaller, as a thought experiment anyway. However, I have not seen the efficiency curves versus load, so there is no reason to expect an efficiency improvement with such a change.

I had the temperature set down yesterday since I was out in the snow, and it reached 60 degF last night around 4:30 AM. I noticed that the furnace came on at 4:30 AM this morning to try to reach 67 degF by the 6:15 AM set time. It starts out cranking at full output for about 1/2 of the temperature recovery and then throttles back after that. I suspect that the efficiency is better at the lower firing rates, due to both lower air pressure drops through the ducts and longer residence times in the heat exchanger, but that is only a guess. Going to a higher average firing rate by downsizing the furnace could be counterproductive in trying to increase efficiency. I really need to see (or generate) an efficiency versus output power curve to be able to answer that question.

It's fair to say that at 2x oversizing there isn't an efficiency hit, but at 4x oversizing it might be- it depends on many factors.  (Work done at Lawrence Berkeley Nat'l Labs back in the mid 1980s indicates there may even be an INCREASE in efficiency at 2x oversizing for bang/bang single stage condensing furnaces. I can look that up for you if you like.)

Most multi-stage or modulating condensing furnaces run at higher efficiency at their min-fire, and the benefits of setback strategies are potentially (but rarely) negative, since they almost always kick into the lower efficiency high-fire on the recovery ramps. In your case 4x oversizing may be good enough to simply keep it from hitting high-fire most of the time, since the setback temp is probably only rarely achieved due to the high mass of the house (?).  But with a set & forget strategy and a furnace with 15K min-fire output it would also NEVER kick into high-fire mode with a 14K design condition heat load. You're in a position that you can probably instrument your setup to track it in both setback and set & forget modes for several weeks at a time and measure it's performance against HDD weather data to see if the setbacks are actually buying you anything with your modulating furnace's and your house's mass/heat load  characteristics.  Low mass high loss houses actually hit the setback temperature values far more often than high-mass high-R houses, but a high mass house that loses any significant temp in the overnight would almost always induce a high-fire response on the recovery ramp.  I'd be interested to see how yours measures out, even if it's just a single data-point, and an atypical house.

One of the guys in my office lives in a town-house heated with a 2-stage 60K/30K condensing gas furnace (like almost every other unit in the complex where the original ~100k single-stagers have bitten the dust), and he has hacked the internal controls to keep it from ever running at 60KBTU/hr.  Yes, the recovery from setback is now longer, but not enough to matter.  He's never measured or calculated the heat load, but it's almost certainly under 20K, and may be under 15K after the insulation and air sealing he's done over the past 3-5 years.  (The biggest and leakiest unit in the complex probably still comes in under 30K, despite crappy leaky 1980s construction.)  I'm sure he'd be happy to see a 30K-max 2-stage on the market by the time the current unit is toast.

Lee: Air conditioning in New England is almost all latent load, and almost always peaking in the July/August time frame. But it's DEFINITELY a necessity from a comfort & health point of view.

Memorial Day Weekend is early- it's often a skiing day for those who like to hike for turns in the White Mountains (the top of which is a paltry ~6500' of altitude, but cool enough that the N-facing cirques often harbor patches of skiable snow until late June.) Early June usually comes with some heating degree-days. Air condition season doesn't start in earnest for another 5-6 weeks after that. But come July when daily high temps are in the low to mid-80s and the dew points are bumping on 70F it's not very comfortable at sea level. And 90s with dew points in the mid-70s are seen every year in many non-coastal parts of New England. It's the humidity more than the sensible temps that drive the desire for air conditioning here.

Some very rough estimates tell me that a low mass house has to lose about 30K btu to achieve a 10F setback. Whether that happens (and how long it takes) is highly dependent on insulation and outdoor temperature.