View Full Version : gas furnace or dual fuel heat pump?
09-15-2005, 12:39 PM
I live in East Tennesse where the winters don't really get cold until the end of November and extend through February at the latest. Cold temperatures usually in the 20-30 degree range, although it has gotten down in the teens and single digits for a week or so most years.
I currently have an 18 year old Rheem gas furnace that will be replaced in the next month or so. I had decided to install a Var Spd Goodman 93% two-stage gas furnace. Now, though, I am reconsidering and thinking about a dual fuel system with heat pump and gas backup. My question is this: Since the gas will only come into play when the weather is really cold (i.e., not all winter), should I get an 80% furnace or a 93% furnace. Will the added cost of the 93% furnace (exact cost unknown as of yet) pay off in terms of substantial gas savings over an 80% unit. Either way, the unit will have variable speed blower. Any help would be greatly appreciated.
09-15-2005, 12:55 PM
Please do a search, this question has been hashed out several times in the last couple of days.
That being said, go the HP system unless your utility rates are completely outrageous.
09-15-2005, 08:58 PM
Might be even better off to go with a heat pump with electric backup. Once you ditch gas a lot of times you'll get a big discount with the electric company.
09-15-2005, 09:18 PM
Most people that switch from gas to heat pump are never happy, unless the goal was to save money and sacrifice comfort. On average gas heat gives 40 to 60 degree temp rise, Heat pumps generally give 25 to 35 degree temp rise. T-stat set at 70 degrees, gas heat 110 to 130 degree supply air, heat pump 95 to 105 degree supply air. Your body is 98.6 so the air from the hp will heat your home but most people feel that the air is cold.
09-16-2005, 12:50 PM
Dual fuel will work fine; if you do it get a nice 80 instead of the 93, though. The heat pump will do so much of the heating that the furnace won't run enough to ever pay back the price difference for the little bit of extra efficiency. It is true that the heat pump's output may feel a little drafty if your ductwork is set up where it blows right on you. Provided you can avoid that condition, it works fine. People who have that problem probably don't have variable speed blowers, and I'd bet a lot of the lower end dual fuel systems may not even have their defrosts tempered by the furnace. Bad dual fuel is bad (I've had it myself in the past, I'll admit), I won't deny; my point is that dual fuel can be done well and it's quite comfortable when it is. The best applications are variable speed, with dual fuel aware thermostats (not fossil fuel kits), and with homeowners instructed to use small setbacks or no setbacks at all during heating season. In a perfect world they should probably have demand defrost systems, too.
When I switched to dual fuel, what I noticed was not the "drafty" issue, I noticed that temperatures in the house got much more even because the system runtimes were so much longer. With the blower running almost constantly on chilly days, a lot of air gets moved and all the rooms tend to have even temperatures. Compared to the previous system, which was a vintage 1981 gas furnace, getting a new dual fuel system with an 80% furnace made my winter electric bills go up $12. My gas consumption during the three winter months is 35% of what it had been (not 35% less, 35% of what it was!). I don't use any gas for space heating at all in the fall and spring.
As for operating cost and which furnace to choose- I have dual fuel, and my furnace ran for less than 170 hours last winter (and that was with the thermostat on 71 degrees). The heating cost is NICE this way, even with an 80% furnace. If you got dual fuel with a 93%, in your climate, you'd never recover the extra cost of the 93% over the 80% furnace. Maybe in Minnesota, but not in Tennessee.
The cheapest of all to operate, though, in your area or mine, will be a heat pump with electric backup. If your wiring is up to snuff and it won't be hard to run a big circuit between your breaker panel and the current furnace location, you might want to consider that and skip the gas completely. Winter electric rates are pretty cheap all over the southeast, and the weather is mild enough that heat pumps can do the vast majority of the heating.
If you do go gas only, then definitely get the 93. It will still cost more to operate than either the dual fuel at 80% or the all-electric heat pump, though.
[Edited by wyounger on 09-16-2005 at 12:55 PM]
09-16-2005, 10:04 PM
If you go with gas, be prepared to pay thruogh the nose. This country is past peak gas and this winter will be the start of the dramatic rise in gas prices.
Being in the business I installed duel fuel, and I chose the 2 stage Goodman, not for the 2 stage, but for the VS blower. I am on LP so a tank of fuel will last a while. How ever I wish I would have just used a AEPT that comes with VS blower and a TXV. A choice you might want to consider.
I am going to install a HP and elect back up for a customer that now has natural gas. His logic was that he would not need any gas in the summer as his water heater is elect, and he did not want to have to pay the minimum though out the summer because he had a gas furnace.
Of Course the down side of that is,, we donít know what will happen the price of Elect in the next 15 years.
But as far as pay back in the difference between a 80 percent and a 90,, it will not happen in the life of the system.
09-17-2005, 03:03 PM
Another consideration for someone looking into dual fuel. The 90+ furnaces have a secondary heat exchanger, similar to a coil in most cases. This is an added restriction that can reduce the efficiency of your heat pump system by possibly a SEER point as well as some efficiency on the heating side of the heat pump. An 80% furnace with variable speed is probably the best match for a dual fuel system. IMHO.
09-18-2005, 12:34 PM
Doc, most manufacturers of 90+ furnaces that utilize a secondary heat exchanger have taken the added restriction into consideration in their design. The effect, whatever that might be, on the seer would not be any more or less on heat pump than on straight air conditioning. For my own benefit, do you have some information that backs up what your indicating about the drop in seer due to the secondary heat exchangers? thanks.
Jax, around here (Louisville) we pay a minimum to the utility company whether or not we use ANY gas. There is a connection fee that is month in and month out regardless of use. I am not familiar with the 'minimum' requirements. Just FYI. Thanks.
Note: I am a very strong proponent of the dual fuel concept. I have it in my own home, so I can speak of it first hand. I have a high efficiency Bryant Evolution system that is dual fuel and have been extremely happy with the operation and the utility costs. Just across the river in So. Indiana, the gas and electric utility companies compete for business. Here with LG & E, they are the one and only for both gas and electric. No competition and no incentive on their part.
[Edited by John Lloyd on 09-18-2005 at 12:37 PM]
09-18-2005, 12:48 PM
Dual fuel 80% all the way
09-19-2005, 11:59 AM
"Dual fuel can be done well and it's quite comfortable when it is. The best applications are variable speed, with dual fuel aware thermostats (not fossil fuel kits),"
Can you explain why dual fuel thermostats are better than a fossil fuel kit?
Also, do the controls work differently?
09-19-2005, 02:06 PM
The original way to do dual fuel was with a two-stage thermostat (your typical heat pump thermostat) inside and an extra "outdoor thermostat". When you call for first stage heating, fuel selection is determined by the state of the outdoor thermostat. If it's above about 35-40 degrees outside, the heat pump runs. If it's colder than that, the furnace runs. If the thermostat calls for second stage heating (meaning the first stage isn't keeping up, or there is a large differential between first and second stages), you always use the furnace. The fossil fuel kit basically includes the outdoor thermostat and a module that makes sure that you can only run the furnace OR the heat pump, not both at the same time (except for defrosting).
This can work ok for people who set one temperature and don't change it. It doesn't make a difference when it's very cold or when it's very warm. Before dual fuel thermostats existed, this was all there was, and that was the best sort of control you could set up with off the shelf components.
The primary drawback is that a heat pump thermostat is set up with different assumptions about what the second heating stage means. With a traditional heat pump, you do try to minimize your use of the second stage (which is electric resistance heat), but there's no drawback to using it in short bursts- a minute here, a minute there. Resistance heat is just as efficient turning on and off every other minute as it is running longer cycles. With a dual fuel setup, though, there is a significant efficiency penalty to switching between heat sources- either because of a temperature change or because you are changing between first stage and second stage heating. If you are running the heat pump, you don't want the system to try to switch to using the furnace for three minutes, then fire up the heat pump again. There's far too much efficiency lost by making the heat pump stop and then start again, and there's too much efficiency lost in making the furnace run for such a short cycle. There can also be a bit of a comfort penalty with unnecessary stopping and starting, as it takes a few minutes for a heat pump to warm up after starting.
A real dual fuel thermostat knows what kind of equipment each heating stage may be- heat pump or a furnace- so it can make more intelligent decisions about how it should operate them. It won't hop back and forth between the heat pump and the furnace willy-nilly. That lets you take better advantage of the efficiency and strengths of each piece of equipment and maintains comfort better by avoiding all the stopping and starting that can happen with a control system that doesn't know that starting and stopping is not a good thing. Some fossil fuel kits try to cut down on the hopping back and forth between fuel sources, but it's still a less elegant solution.
A heat pump thermostat with fossil fuel kit setup can also have trouble figuring out how to recover from a setback period. Put yourself in the shoes of that thermostat. "My boss says he wants it to be 70 degrees at 5:30pm. Yesterday I recovered by four degrees in a half an hour, so I'll start at 5 today." What that poor thermostat doesn't know is that yesterday it was too cold for the heat pump to run, so the furnace did the job, and did it fast. Maybe today it's warm enough for the heat pump to run, but it would have to start at 4 instead of 5 to recover in time. The result is that it will start running on the heat pump at 5, then realize at 5:15 that it's not getting there and switches to second stage (the furnace again). So it's tough for the thermostat to figure out what's going on with recovering from setback periods because it can't tell what heat source may run on any particular call for first-stage heat, and since the heat pump and the furnace probably put out vastly different amounts of heat, it can't do a good job of recovery. In comparison, a dual fuel thermostat knows the outside temperature (they all require an outdoor sensor). Since it knows the outside temperature and it makes the decision as to what heat source to use, it has no trouble figuring out how much heat output it's going to get from the heating system, and so it can tell when is the right time to start recovering from a setback period- and how to do so maximizing the use and efficiency of the heat pump.
So in short, they work very similarly in the big picture, but a dual fuel thermostat does better at getting the details right.
09-20-2005, 05:21 AM
I would go dual fuel too...
Check with your electric company, our charges 3.99 cents KWH as long as your primary heat source HP. Gas backup is no problem.
Thanks wyounger for posting info... That somewhat describes the problem we had on our old house when we replaced thermostat with entry level honeywell HP junk from Lowes... Themostat was never able to figureout recovery properly, flipping stages short cycling etc etc etc. ( I figured just keep it constant temp during the heating season ).
My question to you would be about VisionPro 8320 with external air sensor. Does it utilizes knowledge of outside air to compute recovery strategy and 1,2 stages???
09-20-2005, 05:50 AM
One advantage of furnace backup is that it could be ran on a generator if power failure. If the vent piping is no problem 80% would be logical if pump is correctly sized to around 35 as balance point.
09-21-2005, 12:32 PM
Thanks for the great info in the last few posts. If I do choose to go dual fuel, I will definitely go 80% on the gas furnace. Also, I often see where you tell folks to make sure to use a contractor who will do a Manual J load calc to determine the proper size. I can't seem to get anyone in Knoxville, TN to do the Man J AND show it to me. I have a 2.5T Rheem gas furnace installed in 1987. My home is 1,450 sf with poor insulation and new insulated windows. After remodeling, though, I will have added 500 SF, for a total of 1,950 sf on one level. the attic insulation will increase from R-6 or 8 to at least R-38 and maybe adding a little more. The added room containing the 500 new SF has finished, heated space over it that has a separate heating system. My question is this: If the old 2.5T unit adequately heated and cooled the 1,450 sf that was poorly insulated, wouldn't you think a 3T unit would be more than adequate to handle 1,950 sf that is much better insulated than before? My HVAC guy wants to bump up to a 3.5T unit and I'm just not sure it needs it. Without a Manual J calculation showing me the indicated unit needed, I'm unsure how to proceed. Any help would again be very appreciated.
Jim in Knoxville
heck as many shops as you have around their I would not think you would have that much problem getting a heat loss unless your trying to get it for free. Why not click on the bullseye on the top of this pager and buy don's version of heat loss and gain program for the homeowner. You can do it yourself and its not that hard. As far as what you are paying for electricity around their the duel fuel is definately the way to go and dont let them talk you out of it by saying the heat pump will be drafty. With V.S. speed furnace you wont have that problem. I sure miss that part of the country.....lived in Morristown a ways back for a short period of time. lucky you
09-21-2005, 09:00 PM
I also am replacing my HVAC system this fall, and I am so bewildered about what to do. I'm not experienced in these things but am gathering all the info I can get before I make a decision. The old system is propane gas Ruud Deluxe 90 plus 75,000 thing that is 15 yrs old. I made a poor choice of installer back then and they put the furnace vent under the floor thru crawlspace and it exits the crawl on the north side of the house. The furnace started having trouble a year or two after installation and things have gone downhill since then. I should have replaced it years ago but I paid so much for it I was determined to get my money's worth. This is a log home with stress skin panel roof system that I think is fairly well insulated. I have to use propane gas and I live in an area with REMC electric company which is more expensive than average. Also have a wood stove which is used most evenings in winter and banked at night.
I have had 3 local HVAC companies in for estimate. First one recommended Rheem modulating gas with 3 ton 13 SEER AC unit. (currently have 2.5 ton but planning to increase house size from ~1700 sq ft to ~2100 in next couple of years.)
Second estimate was the Lennox guy with 70,000 BTU with 2 stage 94/1% propane gas with 3 ton 10 SEER AC.
Today the Carrier salesman visited and we discussed propane gas and electric heat pumps. He left a graph which shows annual heating costs of heat pump (12SEER) being about 1/2 what propane gas is. It doesn't mention the type of backup heat but I assume it is electric. He said life expectancy of gas furnace is 20 yrs vs 15 on heat pump.
Now I'm looking at info online and everyone is promoting geothermal, which I haven't discussed with anyone and would be increased installation cost. Also I notice on this website some remarks about using the '80' energy efficiency rating furnaces with heatpumps. Winter is cold and wet here, I assume I would use the backup quite a bit, but I don't really know. Also, the carrier saleman recommended that GE blower (multistage?) and to keep it set to run on low all the time for increased comfort and even heating all over the house. He said little increased cost running it all the time. ??
If go with gas I am going to have to change the vent route and make a channel above a built in bookcase in next room and drill a couple of new holes in the wall, which is not a problem. However, would not have to do this with electric. (No one likes the under the floor furnace vent.??)
So here is my delimna, go with gas furnace and electric AC, or heat pump with gas furnace backup, or heat pump with electric backup. How much would the backup heat run in my area, how many $ spent on the backup heating in the winter? I've looked at efficience rating on gas furnaces, do electric furnaces have similar rating?
Thanks in advance for your help, Lynnie
09-22-2005, 08:51 AM
Jinpy, I'd suggest picking whichever contractor you like best, and put a provision in the contract that you get to see the Manual J once the contract is signed. Then they aren't giving you their calculation work for free, just to have someone else undercut them and get the job using their work.
Lynnie, you will get much better response if you post a new thread with your message in it, rather than attaching to this one.
09-22-2005, 11:37 PM
Thanks for your reply and I took your advice and opened a new thread. What is the Manual J? Is that a regional sort of thing? Thanks again, Lynnie
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