Coldest day of the year

[MN Renovator]

I wish I would have viewed this site sooner, I went off to St Louis and didn't check before going and now that I'm back I wished I would have caught up on this thread first since this thread currently interests me more than the car forums.

Yeah, I bet it's about twice the required size even in that case. However, given enough air infiltration though leaky windows et cetera, and a much higher set point, it might be closer to the right size.

This furnace was a waste of money for whoever bought it (should have sealed up the leaks and bought a smaller furnace), and the blower is inappropriately large. Aside from that, though, is there any efficiency penalty associated with an oversized furnace?

Edit: I finally found the furnace ID tag, INSIDE the furnace. It's rated for 75000BTU/hr input, 78% AFUE, so about 58500BTU/hr (17KW) output. Do they sell furnaces as small as 30000BTU/hr?

The capacity of a furnace is barely a factor in the price of a furnace, the difference between the 75000 BTU furnace and a size smaller is about $75 today and the difference between that same size and a 45000 BTU furnace is $150. Of course a 78% AFUE is not comparable to a newer 95% AFUE unit when comparing input sizes, which is what the industry goes by, so a 60k unit at 95% would be close to your current oversized unit(and mine). So they didn't really lose much, the contractor who installed it probably based it on the fairly oversized 'rule of thumbs' that they go by in the industry. I found out in my area what the rule of thumb is and they go by 30-35 multiplied by the sq footage and then match that to the output capacity. According to that rough measurement, my 75000(57000 output) unit is either right on a slightly lower rule of thumb or was chosen to not massive oversize it based on their rule of thumb for 1600 sq ft of finished non-basement area. I'm also in need of a 30k output unit based on my -20f temperature design temp experience with no sun in the morning.

There are some smaller units. The problem is they are sold by companies that have their emphasis on CHEAP and generally it shows in the construction and reliability. For example, Ducane HVAC makes units primarily to be sold at a very low price to replace units in rentals, being installed by landlords who want the cheapest thing. I think they only have a 10 year warranty on the heat exchanger because I heard an HVAC guy say that they literally are a '10 year furnace'. Their smallest unit is 26.6k output first stage and 38k output second stage. ...not sure its worth going with them, I wouldn't even if I were heating a house that barely ever needed heating in Arizona.

I was trying to price out furnaces online... it's harder than pricing out EV batteries. Where do you buy furnaces? I struck out at Sears, Home Depot, and Lowes.

Furnaces are a tough one to buy as an individual and generally made tough to price out at a wholesale level too, but I think we have a few good places to look. I found this guy on ebay that when going through the eBay store, I can look through Rheem products and get a good idea of pricing. The furnaces were cheaper than I expected and the air conditioners(with coil) were more expensive than I thought.

The reason why they are tough to buy as an individual is that they aren't considered a DIY item by the industry, they require testing equipment to get the maximum efficiency out of them by measuring pressure and doing calculations for the proper CFM going across the heat exchanger, same sort of thing for the blower speed needed for air conditioning efficiency. If you get it wrong it will be either inefficient or things could get too hot and you could crack a heat exchanger. Keep in mind that if you want a 90% or higher unit, they are all condensing furnaces and require PVC exhaust venting because their exhaust has very high humidity and rusts out standard metal ductwork. I think this may be the only reason why 80% furnaces still exist since the initial cost isn't too much different IMHO but the retrofit would require PVC ducting which is sometimes tough, especially for flues that aren't a straight shot up and the homeowner or contractor isn't willing to let a wall be ripped down.

I think air conditioners are an easier DIY item and generally much safer since there are no concerns over CO issues. Could you DIY it, sure but I would definitely get a company out there for the $80 furnace tune-up deal where one of the line items shows that they will do combustion testing and adjustments to the unit, then hopefully any mistakes can be corrected.

Another important thing to note: Pretty much every manufacturer voids your warranty if it isn't installed by a qualified contractor, so if you DIY, goodbye warranty, even if its a simple switch, it is out of your own pocket. With A/C and furnaces some manufacturers require their own A/C and furnace to be paired to get the full warranty, something to keep in mind.

I took a peek in Rheem's product portfolio, and their ASHP and gas furnaces come in sizes starting around 42000BTU/hr / 8kW output. I also sent an e-mail to a local HVAC installer, but got no reply. I guess he's only interested in paying customers.

Xringer, that site has a good selection of boilers, but zero furnaces. I guess natural gas is more plentiful here than in New England.

If anyone comes across the Amazon dot com of natural gas furnaces, please do share.

This is the current source that I'm looking at.
Eagle Wholesale Supply items - Get great deals on items on eBay Stores!

Either that or use froogle.com and search for something like "Rheem 95% variable speed" ...or find the model you want. I want an RJRM, which I can search that on froogle and find the 45k 2 stage unit.

If you want an output of less than the single stage unit, they pull you into the 2 stage unit where you get a high fire of 45k, low fire of 31500, which is an output of about 43k and 30k. I like the idea of having the low fire running pretty much all the time but if it is -10f which happens about half a dozen times a year, I won't be able to have a decent setback to 45 or 50 degrees and have it pull it up to 70 degrees in a decent time period. If it is above zero, I'm sure its not too bad in lower output, could take 2-3 hours though.

If you want a smaller unit and don't want the electric bill to go up because the blower is running all the time, you should get a variable speed unit because they have an electrically commutated motor, (ECM motor), basically a brushless DC motor that is efficient when running at lower speeds. My current furnace uses a shaded pole motor(some use permanent split capacitor motors), these use the roughly the same power at full speed as lower speeds which isn't ideal when heating.

$1300 for the top end 95% unit(43k/30k BTU output) with all the features and a lifetime heat exchanger warranty.
...or $613 for an 80% 50k input/40k output builder grade unit with a 20 year heat exchanger warranty and pretty much no electrical energy saving features.
...or in between there is a 93.5% efficient 45k input non-ECM motor for $839

Since I'm planning to stay put, if I were to get a contractor to install a unit, I'd have them do a manual J letting them know exactly where my insulation is going to be before the next heating season, I don't see it possibly coming up with a result beyond 43k and if it did, I'd get a second opinion. I'd get 3 decent bids with competent contractors, if one wasn't decent, I'd step away and get another so I had 3 bids. I'm also looking to do a package deal with an A/C 2 ton or smaller(currently 20 amp breaker which seems to max me out at 2 tons, probably a 1.5 ton there, depends on the manual J calc though but 2 ton has higher SEER efficiency so I'd probably rather take a slight oversize to get the SEER)

You could also go with a modulating furnace, Carrier ICS(expensive), Rheem modulating, and York modulating furnaces are a bit pricey, it also seems that modulating furnaces bring down efficiency more than you might save with having shorter cycles, so I don't think they are worth it, even if you were to get the Rheem that goes down to around 22k(can't remember if that is input or output at the moment). Even though it is more efficient to cycle less, the AFUE rating is supposed to include transition losses and 90%+ efficiency furnaces have much thinner heat exchangers now that they are built in a tubular style and are built with better materials. My old 76% draft hood style natural gas furnace(no inducer blower) starts up and burns the pilot for 2 minutes, then runs the burners for 1.25 minutes before I have the blower kick on. New high efficiency units run the inducer blower for 30 seconds while it runs a self-test, ignites the burner (with most Rheem units it is instant with spark ignition) and 45 seconds after that you have the blower. IMHO not as much is wasted with this new units inside the furnace, mostly duct heat losses, insulation and air sealing can help that. I spoke with a guy who does blower door testing while I was in Saint Louis, he said he packed fiberglass batting around his ductwork and noticed a difference from that. I'll probably use rigid foam since most of my ductwork is flat if I try that.

[MN Renovator]

It was definitively the house at its least comfortable. Not saying it's bad, just that I know I can do better, with a well-sealed house where air flows are set up in a deliberate manner.

The house's HVAC relies on lots of air infiltration. Infiltration is my only mechanism of ventilation, circulation, and providing combustion air for the gas stove, dryer, furnace, and water heater. The furnace and dryer negatively pressurize the basement, which leaks quite a bit anyway. I'm not sure about how the water heater's flue works.
..and AC_Hacker's reply
I think that all the new combustion appliance with the exception of cooktops now require external supply air.

-AC_Hacker

They don't require it, in fact Goodman and some other manufacturers still sell units with open vents in their faces so their inducer motor sucks air from the house. Whenever my current furnace craps out on me, I'll be getting an outside air intake piped in. I'll probably leave the house sealed up as tight as I can get it because the water heater is about the only thing gas that would be pulling a suction with its flue but I think that even with all of my efforts at air sealing, I think that I won't be able to get it tight even to worry about it, even if the electric dryer is running at the same time. Worse case scenario, I leave the basement air leaks alone and have the basement door shut and then the rest of the house stays warm in January for less and the appliances get the air they need and on the occasion that I run the oven, the air exchange through infiltration will be enough to cover it. Most water heaters still don't have power vents and are natural draft units, I don't know of any stoves with outdoor intakes and gas dryers usually require a kit. ...unless there are regs in certain states, not in mine though unless its for a code reason such as a space requirement in case something is operating in a closet.