I would think for the installer, erring on the side of being say 20% too big is more of an issue of playing it safe. Worst case, the furnace runs a little less and is a little less efficient, something the homeowner will never bitch about. If theyt make the mistake of putting in one that's too small, then the customer is going to be screaming because the house won't maintain temperature on the coldest days. And that's a difficult problem to fix.
_____________________________-
Here in the south, thats not an issue... the problem here is finding a small enough furnace with a big enough blower drive to support the A/C. As a rule, I install furnaces that are 2 stage with a variable speed blower... especially with dual fuel/hybrid systems, and its a very rare occassion when the second stage of the furnace kicks in.
Based on where I've turned the regulator dial on my home furnace, and by judging the resulting flame height, I think I'm easily running at about
1/2 the full-throttle fuel flow. My furnace is a lennox g8-120-1, manufactured October 1976. Specs:
0 - 2500 ft altitude:
120,000 btu input
96,000 bonnet BTU orifice size 39 temp rise 70 - 100
1/4 hp motor (?)
3 1/4 inch pulley
The furnace is about 22" wide, 28" deep, and about 4.5 ft high. AC coils mounted in plenum above the furnace.
I changed the pulley to 4" a few years ago to get more airflow (higher fan RPM). Interesting that it says it's supposed to have 1/4 hp motor. The higher altitude spec says 1/3 hp, and higher static pressure (.75 vs .2 inches WC). I am at a lower altitude (850 ft above sea level).
We've had a few nights now where the over-night temp has gone down to
10F, daytime high 15F, and my furnace is still cycling (not 100% duty cycle). I have the thermostat set for 71F (electronic thermostat, not mechanical, 2 degrees hysteresis). I haven't started my humidifier (I don't always run it in the winter). Current indoor humidity is 20%. Today's low and high temp is 13F / 22F.
Bottom line is that it's amazing how much you can turn down the burners and still keep the house at the desired temperature, even given these low outside temps.
?
Turning the control knob on the regulator is pretty-much on the fly. If you mean that my thermostat can't do it, well I don't think that's necessary. For the next 3 months I'm looking at a pretty constant outdoor climate in terms of temperature, so constant regulation of the furnace burner output is not necessary.
I don't know what's normal for a 2000 sq foot (including basement)
2-story house, brick sided first floor, aluminum sided second story (no insulation under the siding) in terms of furnace BTU. My climate zone is basically Detroit / Cleveland / Buffalo / Toronto.
I think that the HVAC industry was probably targeting a 25% furnace duty cycle back in the mid-1970's, so a 120k BTU furnace was spec'd back then for a house like this.
I really don't think that a fully controllable / variable burner is necessary or is cost efficient for the average home. Two stage - maybe. Two stage aftermarket electronic thermostats are insanely priced compared to single stage. I bought a 2-stage programmable thermostat for a leased office about 10 years ago (to replace a mechanical thermostat) and I think it cost $300 at the time, compared to $50 - $70 for a single-stage "home" version.
I would install it myself, because I belong to the school of "it's not done right unless you do it yourself", and there's a lot of stuff I do for myself (car restoration, welding, plumbing, wood-working, concrete work, etc).
A new furnace might cost me $1500 max to buy, a 30% rebate of that is max $450.
It's not the money that would motivate me one way or another. What would really cut my energy bills is putting a 2" layer foam-board insulation around my second-story and getting new windows. I'll let everyone else pay through the nose for new furnaces so that they will use less natural gas resulting in a continued depression for natural gas prices which will mean that people with inefficient furnaces will continue to pay less anyways.
I've got several other home-projects in mind for the next few years. When I'm ready to alter my air-handling system to include the concepts I've talked about here, that's when I'll take a good look at my furnace and decide if I'll buy a new one (I probably will). But I'll still install it myself, and if that means I'll have to buy it on the internet because the local hvac dealers won't sell it to me unless they install it, then so be it.
Which unfortunately says nothing about whether the furnace is more efficient at the lower firing rate or by how much. However if you can fire it at that reduced rate and keep the house at the desired temp on the coldest days it does say the furnace was oversized to begin with.
Any differences in furnace efficiency in heating at full firing rate versus 70% would be so small that you'd have to have instrumentation to figure it out. Note that HVAC and manufacturing folks claims for two stage "efficiency" are really based on:
less cycling of the furnace, so less heat loss when it shuts down, etc
more even heat distribution, due to the blower running longer, possibly resulting in feeling more comfortable at a slightly lower tem.
So, essentially you have it throttled back in Fall and Spring and on full during winter. That's part of what a two stage furnace would do. Difference is that with a two stage furnace and thermostat, if you're out of the house in Oct and set it back to 60, it will fire at the higher rate. Even the 2 stage without a 2 stage thermostat will start at low, then move to high after an adjustable period, eg 5-12mins.
25% cycles when? on the coldest days? From all you've said, sounds like it's oversized.
I can buy a top of the line Honeywell VisionPro 2 stage Tstat for $100-125. Or a cheaper prgrammable one for $60. The delta for getting two stages on a good thermostat is maybe $25
Are there no gas company or state rebates/credits available? Here in NJ there are and they would chop another $600 off that price. From what I've seen posted here, there frequently are similar credits available in other states. That could make the cost of that new furnace $450.
Hard to see how you would not recover the cost in a reasonable time of getting a furnace that is:
A - Sized correctly B - 95% efficient compared to your current whatever, say 75%
Using your own numbers, ie $1500 cost, Fed Tax credit, no other rebates, it's $1050. IF you only cut your fuel costs by $200 a year, that's a fast payback. With any other rebates/credits it could be down to a few years.
Given a lower BTU output and a slower flame speed (slower combustion air induction speed) I am promoting more heat residency time inside the heat exchanger galley and a lower flue stack temperature. My blower fan is pushing the same CFM through the exchanger, so when taken together my heat extraction efficiency is going up. By how much, I don't know.
Isin't it common knowledge that oversized furnaces were normal a few decades ago?
Actually, it's throttled back right now, to at least half of full flow. That's my point - even given my current outdoor climate (10 to 15f min and 20 to 25f max outdoor temp) I'm getting enough BTU's to keep the house at 71 - 72F all day and all night. And even then, my furnace is not running at 100% duty cycle. Maybe only 65 - 75%.
To be honest, the burner setting I have now is the lowest that gives me a consistent and even flame pattern and acceptible ignition roll-out.
Fall and spring don't last long enough to warrant their own lower burner-output setting, which as I just mentioned I probably couldn't achieve anyways.
Yup.
Tell me what a 120,000 BTU input, 96,000 btu bonnet furnace is designed for. What size house.
Tell me where such a furnace ranks today in terms of size (small, medium, large, extra-large, etc).
First thing, I'm in Ontario (Canada).
Second thing, in order to get gov't rebates, you need to contract for a home efficienty test by an acredited tester who will evaluate your current situation (air leakage, current furnace type and efficiency, maybe a few other things) and he will do it again when your new furnace is installed, and then he signs off on the tests and then the gov't will send you money. I don't know if it's "federal" or "state" money (or both). I also don't know if the local gas utility will kick in any of their own money.
One of my co-workers had this done, so I'll ask him how much of a kick-back he's supposed to get.
You've also got to factor in rebates for the heating vs the cooling components as well. I wouldn't be interested in replacing my A/C system.
One of the experts here would probably be better at guessing that number. But that's all it would be without doing a proper load calculation. It depends on a lot more than just size, ie how well the house is insulated, number of windows, location, etc. However, none of the contractors have actually done that. The most one of them did was to measure the house sq ft. I'm not too concerned with that, because I have the experience with the current furnace, which IMO is an even better starting point.
I can tell you that for my house here in coastal NJ, near NYC, I'm getting quotes of either 100K or 120K input, 95% efficiency, depending on the contractor. My own opinion, based on experience with the current furnace, is that 100K is the right size. For example, it's 18F outside and this morning, when recovering from overnight setback from 60 back to 70, it's going up about 5.5F per hour. Also in 16 years, even on the coldest days, it never is close to running all the time. Current is 150K,
26 years old, so guessing it's maybe 77% efficient at this point, when new it was probably 80%.
The gas furnaces I've looked at from various manufacturers product lines all top out at 105K to 120K, at least in the 95% efficiency series. Don't know about the 80%, etc. Two reasons for that. First, is obviously with higher eff, you get more heat out. Second is today for a house the size of mine they would put in two smaller HVAC system, one up, one down, which does a better job, allows more flexibility, etc.
Part of the rebate here, $900 from the state, requires a home eff test too, but it's free.
Yes, you may get them for one part, but not the other, or both, depending on what rebates are available and the particular system. One problem I was not aware of is that on large AC, ie 5 tons, it's apparently harder to make them high eff, with 14.5 SEER or similar being more typical, which can make that part harder to qualify for. Also, I think here in NJ, we probably have some of the best rebates. As I said, I can get $1200 rebated locally and another
30% Fed tax credit. For me, that makes it a very good deal.
Nope. There is no self-compensating mechanism in the tax law. One dollar taken out here doesn't mean another dollar is automatically taken from over there. You might have a (technical) point on the deficit except spending in the last 50 years has bore no relationship that I can see to revenue.
Find me the part of the tax code that is even remotely self compensating for these kinds of things. If it was true, we would only be in about the pickle we are today. There remains no mechanism where a dollar saved here is replaced by a dollar from someone else.
And if my aunt had balls she's be my uncle. Who can possibly enter government spending remaining constant into this conversation and expect to keep any credibility?
Certainly it does, whether the additional tax is paid this year or by our (grand)children. Expendatures certainly don't go down because a tax credit was given! Whether they be rebates for "efficient" cars, air conditioners, or "green energy", that is money taken from the taxpayers at large and given to an individual.
Expenditures have always been independent of revenue. Again, there is no self-compensating mechanism here. Heck they don't pretend to "pay for things any more. A tax benefit to one person has no direct impact on tax increases for another. It makes the deficit go up, in many cases, but again, there is no discernable relationship between taxes and what is being spent (oh, only that were true)
Nor do they go up if it isn't. To do this, is an underlying assumption that a certain amount of your income is the property of the government and if any is given to you it must be taken from someone else. The only thing that you owe the government is what the tax laws (including credits, etc) require you to pay. Expenditures are not (even with the conceit of "paying" for things, related to revenues and thus taxes.
Wrong. Your assumption is that money if free. All the government has to do is print it.
There is a difference between normal deductions and things like energy credits. Energy credits are a manipulation, by the government, in the free market by transferring money from the treasury to an individual. They really do matter.
My assumption was that you were talking about paying more taxes, which as I mentioned doesn't happen. Now, you are talking inflation which is a whole 'nother kettle of wax.
Nonsense. Deductions are manupulation, too. Look at the mortgage deduction for instance.
HomeOwnersHub website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.