View Full Version : Temperature recovery time after setback
11-01-2006, 08:40 AM
I recently purchased a home that has a RHEEM 90 furnace (Model RGTA-07EMAES installed in 1997). I like to set the temp back at night and I think it takes the furnace a long to to bring the house up to temp in the A.M. With the outside air temp at 30-35, it takes about 1/2 hour to get a one degree rise. So it takes it 2 hours to go from 63 to 67. I'm concerned because I live in northern Ohio and its not even that cold yet.
I was having some other problems with the furnace and had the guy who installed it out to fix it. I mentioned the long time the furnace ran to bring up the temp and he thought that 2 degrees/hour was not out of the ordinary. Once it's up to temp, it seems to do a OK job of maintaining it.
I'm now having some more problems with the furnace and rather than put more $$ into it, I'm looking at getting a new variable speed furnace. I've had two different guys do estimates and I explained the temp recovery problem to both of them. The first guy gave me an estimate for a TRANE XV90 (74000) BTU. He also increased the blower speed on my RHEEM which did not help with the temp recovery problem. When I explained the problem to the second guy, he immediately said the furnace was too small and that I don't want to increase the fan speed (especially for heating). He gave me an estimate for both an Amana Prestige or an Amana Distinctions (variable speed, 93% efficient, 90000 BTU). He did a heat load calc and it came out around 82000.
I told the second guy I was concerned about getting a furnace too big because of the duct size -- I have a 1800 sq ft 1950's ranch. He said that with the new variable speed furnaces, that this is not a problem.
My basic question is, will getting a bigger furnace help with my temp recovery in the A.M.? Or will I always have this problem because of the age/design of the house. I don't want to buy a new furnace and still have the same problem (I'll just get the RHEEM fixed.)
Also, when the heat load falls in the middle, should you go up to the next size or down to the smaller size?
11-01-2006, 09:01 AM
With a 1950's era home, the first thing that needs looked into is reducing energy loads. Have an energy audit done by a reputable firm (see if your local utilities perform these, as often they are no-cost or low-cost to metered customers). This is designed to look for flaws and fixes that can help reduce your home's energy consumption. A load calculation can be performed in a before and after state to determine how various energy improvements may effect loads.
There are too many variables in a home of any age to offer a qualified answer to your initial question. Perhaps your ductwork has so many leaks that much of the energy is simply lost to outside areas such as attic, crawl space, etc. If the ductwork is in an unconditioned space, perhaps it needs to be insulated well or simply have additional insulation installed. Often we overlook the real problems and make attacks on the wrong front during the battle to improve comfort and lower energy costs.
HVAC manufacturers seldom make the exact size furnace needed, so typically, one needs to round up. As for cooling, keep the equipment size as absolutely close as possible and avoid oversizing.
11-01-2006, 09:10 AM
Thanks for the info. I do know that the attic has about 9" of insulation, the crawl is insulated (on the walls), and the house was 'wrapped' with the 'Tyvek' type insulation and re-sided. I have Pella double pane windows. I don't know the condition of the ducts.
11-01-2006, 09:49 AM
Just based on the sq ft, doesn't seem out of line. You don't want to size the furnace to handle a setback! Sis has the same furnace in a 2300 sq ft 2 story. It struggles at 10 below but otherwise does OK. I'm with Midwest, see what you can do to tighten the house. That will save you bucks year round on your utility costs.
Going from the 75K Rheem to an 80K Trane won't help much. That's less than 1 little electric heater. Even the 90K Amana won't do that much more. And you are right to be concerned about duct sizing. Yes, variable speed motors can push harder against a small duct system but they sure can be NOISY doing it.
Just remember that you are not only warming the air in the house AND making up for what's being lost at the time but you have to warm up EVERYTHING in the house such as drywall, carpet, all the furnishings... As the air warms up, that warmth is drawn to everything cold in the room. We installed a 2 stage furnace for a customer recently, properly sized. He complained that the air did warm up OK but the furnace ran for a long time on low after it got right about temp. Yup, took 2 techs & the salesman to finally convince him (we hope) that the long time is all heating up everything in his house.
Also have the dealer "clock" the meter. Rheem (at least used to) orifice its furnace for gas burning at 1075 BTUs per cubic foot. Most gas is 1000 BTU per CF. He might have to tweak the gas pressure slightly up to get a net of 75,000 BTU input. And if nobody checked, could be under the usual pressure anyway. When I first put Sis's furnace in, our gas was at 910 BTUs. I ran the gas pressure way above normal to get to 75K. Now they've gone to 1000 and I adjusted down.
11-01-2006, 10:50 AM
If I had that furnace in a house I'd bought I would repair. It is a good unit that is still being built. I would also spend the money as suggested on insulation and upgrades to the envelope. Better money spent.
11-01-2006, 11:39 AM
Thanks for the suggestions. I just got done with a third salesman (sold Carrier) who was much more helpful. He thinks this RHEEM should do the job OK and that it is a waste to replace a 10 year old furnace. So they're going to try to figure out what's wrong. I mentioned that the previous owners did some remodeling that involved tearing out some walls (I think the homeowner did it himself). The salesman thinks they probably got rid of a cold air return which may be part of the problem. The house used to have a kitchen, dining, and living room. They tore out a wall so it is now just one big open space. I have three heat registers in this 'room' but no return. At any rate, I feel much better about working with this guy than I did about the other two sales guys. Guess it pays to shop around!
Thanks again for all your help.
11-01-2006, 01:45 PM
I got a t-stat with more set points esp for weekends --
I bring up heat in 1h stages --
play -- decide what works for you
BTW -- return air is a better description than cold air --
In summer, my return air is warm
11-01-2006, 02:33 PM
There could be so many different causes for the problem and probably all acting at once. I will try to list few:
1. Incorrect register location
2. Incorrect duct sizes (poorly designed ductwork)
3. Incorrect equipment size
4. Incorrect load calculations
5. Missing zoning
6. Faulty equipment
7. House envelope (poor insulation):
e. basement below grade walls
f. type of glass in windows/skylights/doors
The list goes on and on. I have a feeling if you don't find something obvious within a day you will not be able to find the real problem. HVAC is the most difficult type of heating/cooling to do it perfectly. That's why I'm in this business.
11-01-2006, 04:35 PM
9" of insulation in the attic is not going to help, You need more to to hold the heat in the home.
BTU size you got sounds right for that home.
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