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taylor46952
03-02-2007, 06:48 AM
I've received three quotes for a new furnace and heat pump system. My house (2100sq. ft.) has plenty of insulation for the region, new windows, new doors. two of the quotes include a 100,000 BTU (80%) furnace, and the third includes an 80,000 (80%) BTU furnace. The third is also the only one who ran any heat load calculations on the house. I grew up in a home without gas heat, so my knowledge of BTUs is lacking. Are 20,000 BTUs enough that I'm losing the efficiency of the furnace by having to large of a furnace for the house? To ask in another way, am I losing money if I choose a 100,000 btu furnace if an 80,000 btu furnace is enough? (other than the 100,000 furnace cost a little more upfront?)

Question #2: I think the load calc. stated my house needs 62,000 BTUs of heat, an (80%) 80,000 BTU furnace is going to provide 64,000. Is a 2,000 BTU cushion enough that I'm not going to have a cool home in a few years when the furnace begins to lose it's efficiency?

Thanks for the help.

TedL
03-02-2007, 08:19 AM
In general, longer run times are more efficient.

What's your location and the design temp?
What do you have now? BTU input, estimated efficiency and/or age?

03-02-2007, 02:21 PM
First, there are furnaces readily available that are in the mid-90% efficiency, and with today's energy costs, might be a better choice. Some of them are dual stage, so they can run at lower outputs when the full output is not needed.

What was the design temp for the heat-load? Around here, it is in the order of -10 degrees F. Basically, the heat load indicates how much heat is lost at those worst-case temperatures. If the furnace is sized for that and the outside is at that temperature - the furnace will run continuously and be able to maintain the design temperature. If it is colder than that, it will not be able to maintain that temperature. If it was say -15, the house would stabilize a little cooler than the design temperature. On a warmer day, it will only have to run intermittently. The best comfort happens when the thing runs continuously and it also makes the humidifier and filtration system work better too.

One factor is if you use a setback thermostat. If you let the house cool off on a -10 degree night, and it doesn't have some reserve, it will raise the temperature very slowly. Conversely, if the thing is oversized, it will only run for a short time, which isn't as efficient.

Another thing to consider is a variable speed fan. This can make the most use of the heat you pay for. When first starting up, the vents are cold and the heat exchanger isn't hot yet - turning on the fan full force means blowing cold air out. Starting the fan off slowly means you not only don't do that enough to notice, but it is less noticable when it happens, too. It also slowly scales down when the heat goes off, extracting all of the heat you paid for rather than flowing out the flue.

Last, if the furnace is serviced on a regular schedule (cleaning, checking the air/fuel mix/CO, etc.), it should not lose efficiency.

taylor46952
03-02-2007, 07:39 PM
Not sure what the design temp was, but I feel confident that the guy knew what he was doing. I'm in Central Indiana, so I doubt it was to low. We have pretty moderate winters. Rarely dipping below zero. All three quotes were for variable speed blowers. It sounds like from you guys that, assuming he ran the calculations right, an 80,000 BTU furnace should be able to keep up 0.K.

I guess I'm just unsure of whether the difference in BTUs is going to make so much of a difference that it should be a major issue other than up front cost?... if that makes sense. Would 20,000 BTUs to much cause a furnace to shutdown prematurely enough to see a significantly higher gas bill from a properly sized furnace?

Indiana energy cost are pretty inexpensive... not sure why but electricity is dirt cheap. Probably the easy access to coal from southern Indiana and Kentucky... anyway, with our moderate winters the heat pump will do most of the heating, until the temp lowers enough to kick on the gas furnace. The price difference between 90% and 80% would just take to long to make up for what the more efficient run time savings would add up to.

Thanks for the responses.

taylor46952
03-02-2007, 07:44 PM
currently have about a 15-20 year old single stage gas furnace... about 65% efficient. I could burn money in my fireplace and save on the gas bill at this point... so anything will be an improvement, I'm just trying to keep from overpaying for to large of a furnace, or finding myself chilly because the 80,000 isn't keeping up.

Thanks again for the replies... you've both given me some things to think about.

03-02-2007, 09:11 PM
Oversizing a furnace is not a great thing to do, nor is oversizing an a/c unit. They work best when running full-time. A 200K unit would be turning on and off way too much, and never reach its optimum when the system only needed 60K or so. WHile the 80K unit would be running 3/4 of the time at the design temp, a 200K unit would only be running around 3/10 of the time. As said, for both heat and a/c, you will be more comfortable running the appliance at as near 100% as you can. Also, the on/off cycles tend to shorten the life of the unit - and, it may not get hot enough to get rid of all condensation, which can mess things up quickly.

TedL
03-03-2007, 04:13 AM
currently have about a 15-20 year old single stage gas furnace... about 65% efficient. I could burn money in my fireplace and save on the gas bill at this point... so anything will be an improvement, I'm just trying to keep from overpaying for to large of a furnace, or finding myself chilly because the 80,000 isn't keeping up.

Thanks again for the replies... you've both given me some things to think about.

The heat delivered by 65% efficiency on 100,000 btu input (you actually didn't answer current input) almost exactly equals 80% efficiency on 80,000 btu input. How big and how well does current system heat?

Last fall I put an 80,000 input 96% efficiency unit into 2700 sq ft, plus 1000 sq ft fin basement. Works fine in a -10 design temp area.

jimbo
03-03-2007, 07:25 AM
I would support the guy who made at least some attempt at a manual J.

If the furnace is too large, you will more quickly heat up the area so the thermostat "feels good" and turns off. Howver, you no longer have the circulation going to move the air around and really evenly heat up the whole space. The short cycling will result in uneven heating, hot and cool spots, and is not good for the unit.

An appropriately sized unit will have a longer run time , producing a more evenly heated space. The unit installation manual should have some guides on run times.

taylor46952
03-03-2007, 07:52 AM
Yeah, you're right I didn't answer the input question... sorry. Current furnace is actually 125,000 BTU, so compared to the quoted 80 and 100 it's somewhere in the middle for output. It does o.k. with keeping the house warm. We run it a little cool just to keep the bills lower, and there is an addition that has pretty poor airflow (something that was included in the quotes as just needing an additional vent or two. None of the quotes thought that would be a problem).

I've called the guy who did the load calc. to get some more info from his viewpoint, but probably won't here back until Monday.

Thanks for the help everyone. This is a great forum, that I just sort of stumbled across looking for something else.

TedL
03-03-2007, 12:57 PM
Have them put a performance clause in the contract ("System shall maintain 70 degrees in outside temperatures of xx degrees.") Replace xx with your local design temp.

T_Hartigan
03-03-2007, 04:28 PM
I would seriously look at a higher efficiency unit. 80% seems low.

Have them run some numbers for fuel cost for 80% and something in the 90's.

Tim

FredC
03-03-2007, 06:05 PM
I had all the same questions a few years ago when AC & furnace went K-put after addinga an addition. I had a 2200 sq ft, 2 story with a 600 sq ft addition and 2 new vent legs off the trunk and a finished basement in Tenn. Originally had a 125K BTU, low efficiency gas furnace. Although my AC failed, when we got the quotes to replace all we got 2 - 80K BTU, 2 100K & 1 125K BTU. ALl were 85 - 90% efficiency. We stayed in the middle with the 100K and had no issues. 125,000 to 80,000 is a 36% swing, which is quite a lot. Even with a 20 - 25% efficiency gain you are not quite breaking even with what you had and after it ages it loses effiociency. I would stay in the middle.

taylor46952
03-03-2007, 06:47 PM
That's a good idea TedL, I will have to get that included... I actually need to see if the company offers a "comfort" guarantee. One of the other companies offered one, similar to your thinking.

Since I'm combining the system with a heat pump the upfront cost becomes an issue with jumping from 80% to 90%. Where I live, the heat pump will actually do a majority of the necessary heat throughout the winter months. Just didn't want to end up cold when temps do dip.

03-03-2007, 09:39 PM
If the calculation was correct, the house won't magically get a lot colder if the outside temperature gets below that if you have at least that amount of BTU's available. As you approach the design temperature, the furnace will run longer and longer until it runs constantly. If the outside temperature continues to drop below that design temperature, the house will gradually cool off. At any outside temperature once it has dropped below the max output, it will stabilize somewhere.

I disagree about the furnace becoming more and more inefficient as it ages. If it is cleaned and adjusted, it should remain at or near its design efficiency until the heat exchanger or some other major piece dies, then you'd need to replace it.

taylor46952
03-04-2007, 05:28 PM
Thanks again for the help everyone. I received a message back from the guy who gave me the load calc. He used -5 as a design temp, which I think Central Indiana did hit maybe twice this winter... so It's rare. He also put in that we have very little attic insulation, even though we have roughly 13 inches, so he built in some wiggle room. Also, stated he would be more than happy to put it in writing that the system will keep the house at 70 at the design temp. So, I'm happy, sounds like I will be getting a new 80,000 BTU furnace and some significantly lower heating bills.

falcon
04-24-2007, 01:18 PM
I have a 100,000 BTU Lennox Pulse that is 21 years old. My house is 1700' with a 900' walkout basement that is finished and heated. I live at 7400' altitude in the Denver foothills. This furnace does a good job at heating the house evenly. I keep it set at 72 degrees during the day.
I am looking to buy a Rheem RGRL 95% 2-stage furnace. The maximum BTUs of this model is 90,000. To get 105,000BTUs I could step down to a Rheem RGRK 92% 2-stage.
Winters here are usually mild. Most nights are 10-20 degrees but we can get temps of -10 to -20.

I'd prefer the 95% RGRL but am afraid it won't have enough BTUs for the severe cold. Can anyone offer some insight? I don't like to be cold!
Thanks

04-24-2007, 04:39 PM
The size of the house is not a direct input to the size of the furnace that is required. WIthout a heat load analysis, it is impossible to say what you need. It was not very uncommon to have builders just put a bigger furnace in that was actually required just to make sure. Energy wasn't too expensive back then, and people didn't really care. A smaller furnace might be more than enough. You know that the 100K job is big enough.

At the coldest temp, an ideally sized furnace is running constantly. You obtain the greatest comfort when it is on all the time, thus, no ups and downs in the indoor temperature.

A good heat load analysis requires knowing the size, quantity, and type of windows, doors; the quantity of insulation, and the anticipated temperatures, the length of the exterior walls plus probably a few other things.

Only when you get this done (and you can probably find a spreadsheet on-line that will help you calculate it) will you know how big of a furnace your house really needs.

Note, once the furnace can't maintain the set temperature (i.e., it is running all of the time), the house doesn't magically become a refrigerator - it drops a little with each degree the outside temp drops. Eventually, it reaches an equibrium - putting in as much heat as is leaking out. That is why is it required to analyze the house - leaking windows, insulation, etc. Once you know this, you can intelligently choose the best size for your conditions.

chas22
05-07-2007, 08:31 PM
If it was my house and I were to just ballpark it. I would look at something about 92% 80,000 BTU furnace with a 5 ton drive blower, then add a 13 seer 4 ton condensing unit, and use an expansion valve.

Becka
02-17-2010, 03:38 PM
In general, longer run times are more efficient.

What's your location and the design temp?
What do you have now? BTU input, estimated efficiency and/or age?

Curently have a 18-20 year old furnace . Replace heatpump with new Carrier 3-1/2 ton (bigger than what was there), 15 Seer last year. I "thank God" got the maintenance contract on both the furnance and Heatpump. The service people have been to my house 7 plus times for NO HEAT calls since Christmas Day! Everything from door swtich, control panel, ignitor, etc. etc.

I have Propane GAS backup (when temperature drops below 40 degreees).

My house is a little over 2,500 sq. feet, 275 yards from the Chesapeake Bay (on sand of course) no basement, crawl space with all pipe underneath the house.

The current furnace is 100,000 BTU and I am replacing it with the Carrier Infinity Gas furnace. Which should I get 100,000 or 80,000 BTU? The salesman told me either one would work.

I have has for a load survey.

What do you think!

Thanks so much
Becka;)

Becka
02-18-2010, 05:55 AM
Not sure what the design temp was, but I feel confident that the guy knew what he was doing. I'm in Central Indiana, so I doubt it was to low. We have pretty moderate winters. Rarely dipping below zero. All three quotes were for variable speed blowers. It sounds like from you guys that, assuming he ran the calculations right, an 80,000 BTU furnace should be able to keep up 0.K.

I guess I'm just unsure of whether the difference in BTUs is going to make so much of a difference that it should be a major issue other than up front cost?... if that makes sense. Would 20,000 BTUs to much cause a furnace to shutdown prematurely enough to see a significantly higher gas bill from a properly sized furnace?

Indiana energy cost are pretty inexpensive... not sure why but electricity is dirt cheap. Probably the easy access to coal from southern Indiana and Kentucky... anyway, with our moderate winters the heat pump will do most of the heating, until the temp lowers enough to kick on the gas furnace. The price difference between 90% and 80% would just take to long to make up for what the more efficient run time savings would add up to.

Thanks for the responses.

Thanks for the response. The price will be the same whether or not I get the 80,000 or 100,000 BTU Carrier Gas Infinity furnance.
Which BTU should I choose?

Dana
02-18-2010, 08:15 AM
Thanks for the response. The price will be the same whether or not I get the 80,000 or 100,000 BTU Carrier Gas Infinity furnance.
Which BTU should I choose?

When in doubt go smaller. An 80K/80% is 64K out, which is literally twice what I need on a similar-sized house with even less attic insulation at -5F. Knowing the ratings of your existing furnace it's possible to use it's fuel use correlated against heating-degree-day data to come up with a fairly precise estimate (more precise than any heat-loss calc based on construction, since it's a measurement not an estimate.) Most heat loss calcs use "typical" air infiltraion and sloppy R-value/U-value estimates and pad it to protect against undersizing discomfort, which ends up spitting out a number 25-35% over reality, then the contractor goes the next size up from that "for good measure". Odds are your real design-day heat load is more like 40K than 64K, which would make the 100K/80% literally 2x oversized, and the 80K/80% unit 1.6x oversized, which is close to the oversizing assumption in an AFUE test, so it'll actually DELIVER 80% efficiency, whereas the 100K/80% unit will be delivering between 75-78% as-used efficiency. The 80K unit WILL be more efficient, and more comfortable, but not as comfortable (and far less effiient) as a 2-stage condensing furnace.

Condensing furnaces just aren't all that expensive, and even without subsidy the payback is good. A 95% AFUE furnace, perfectly sized to the heat load will deliver slightly better than 95%. At 1.7x oversizing it'll use 18% less fuel than an 80% unit per year. If you're currently burning through 1500 therms/year with the 65% beast you'd burn 1220 therms/year with the 80K/80% furnace, but you'd burn ~1000 therms with a condensing furnace. At a buck a therm (it may be more, it may be less where you are) the annual difference in operating cost would be \$220, or a 5 year cost difference (assuming no fuel price increases) of \$1100. If you look at the difference in up front cost, and the after-tax return on that difference it's usually quite favorable compared to other places you might have invested the money. Don't forget to factor in any state/federal/utilility subsidy for going with the higher efficiency unit too.

Bear in mind that if you lator add insulation to the attic or make other envelope upgrades your heat load goes down, which makes your oversizing factor go up, reducing net efficiency. But with a properly sized 2-stage condensing unit the efficiency extends much better than with a single stage unit, and you'll be taking fuller advantage of the insulation upgrades. (In fact some 2-stage condensers run slighly MORE efficiently when oversized 2x, since the higher stage burner runs at higher temp, reducing the condensing factor, but oversizing does result in reduced comfort.)

But the short answer: Go with the 80KBTU unit, if those are your only choices.