Straight HP or Dual Fuel?? Need help.

I'm pretty much sold on a dual fuel system, but with gas prices most likely to continue climbing in the coming years, I'm having second thoughts. My question is this:

American Standard system, 3 ton, 1 sty home, well insulated with good windows and doors. East Tennessee, where it gets pretty hot and humid in the summer and can get pretty cold (teens,twenties) in the winter.

Option A: 16 SEER EHP w/electric strip emergency heat

Option B: 14 SEER EHP w/80% gas emergency heat

Option C: 14 SEER EHP w/90% gas emergency heat

The pricing of Options A and C are nearly the same, while Option B is somewhat less than A and C. I've lived with gas heat my whole life and don't have any experience living with a HP. Will the heat pump operate with emergency heat often enough to warrant gas as the emerbency heat source? Advice would be appreciated.

Thanks

The reccomendation from the power company in my area is to install the 80% furnace and the highest SEER rating HP that you can.

From what I have seen the cost difference in 80% and 90% is about $800. If you are just looking at cost - you won't get the additional money for the 90% back in savings (You are talking 10% savings on the couple of days a year that the furnace runs.). Now, in my house they may need to redo the brick fireplace to get the venting right. If they do that the 90% would be a better deal because it vents with PVC pipe.



In general this site might help you http://www.energyright.com/index.htm.

You should probably indicate where you live. A straight heat pump generally will be more efficient when it comes down to dollars and cents and certainly cost less to install. But without knowing where you live, its hard to say.

If it helps, if you live in chicago, go dual Fuel, if you live in Georgia, go with a straight heat pump.

Around here, an all electric house with a good pump is usually cheaper for total bills than a gas/electric home is. If you aren't going to need a gas meter for anything else, you can save $120-150 typically just by not having it. Most utilites have a healthy minimum monthly charge just to have a meter, not counting usage.

In the teens, most heat pumps will need some help. If dual fuel, most of the time you shut the pump off below about 30. So gas would run in the teens & 20s. With all electric, the pump will keep running and the backup strips will cycle as needed. A lot depends upon how warm you keep it.

I'd bet your area would be like ours to the north. All electric would be lowest overall. Check with your electric company, they may be able to model different fuels.

Dual fuel is looking very attractive to me also. I'd like to include them in a few new homes I'm building.

These units don't seem to be on anyone's radar in this area.

As installers/servicers, do you guys find these units pretty reliable?

I live in Knoxville TN. I looked at the average daily temps for the last year. Assuming the balance point of a dual fuel system is around 35 degrees, I counted the number of days of avg daily temp at 35 degrees or less and found 22 days. I then counted cooling days with avg daily temps of 75 degrees or more and found 85 days.

Will a 16SEER HP cost less to run than a 14 SEER. I know it's more efficient, but does that mean it runs less and thus costs less? Some posters on this site seem to say that its more of a comfort issue rather than a cost savings. Given the age of my current system, just about anything is going to give me more comfort than I've got now, so I'm paying more attention to future energy costs over the next 10-20 years and trying to make some kind of rational decision regarding going with gas (80 or 90%) as the backup to a 14 SEER HP or stepping up to the 16 SEER HP and dropping gas. Interestingly, when I asked my contractor for the cost difference to drop gas and go all HP, he said it cost a little more (+-$140 more for all EHP vs 80% gas). From what one of the recent posters said above, a dual fuel system is more expensive than a EHP system. Now, remember, the contractors in my area can't/won't do Manual D and few do Manual J, so I won't even bother to ask them to do an analysis of the annual average daily temps and calculate projected operating costs for gas backup versus electric backup.

Again, any responses would be very much appreciated.

Thanks,

Jim in Knoxville, TN

Yes, the higher the SEER the cheaper is is to run. A 14 SEER unit uses 1/2 the energy as a 7 SEER. With a HP you also want to look at the HSPF (how well it heats).

If you need "emergency heat" you need "emergency heat." If a 14SEER unit will not heat the house, then a 18SEER unit will not either. Now, you may find that some of the higher SEER rated units have lower HSPF ratings then a one step down system. (For example a Trane 19i system has HSPF ratings in the 8's and the 16i system in the 9's).

I have found the cost difference between a 14 SEER and an 17SEER to be about 2,500. I will not get the 2,500 back in operating cost over 10 years (I didn't do numbers for 20 years.)

The link that I put in my post from before to the TVA http://www.energyright.com/index.htm has a calc for the cost of the system vs operating cost and will tell you have long it takes to get your money back.

Personally, if you can get a gas furnace over an electric I would go gas. An electric funace is costly to run. I would defiently not pay extra for it.

Now, I did have a bid to replace my system with a Trane and the contractor gave me a "FREE" 80% furnace. (Don't worry he was still getting his money) That could be why.

Doug

In many cases a 16 SEER system will be two stages while most 14 SEER systems are single stage. The real beinfit is in added comfort. The 16 SEER will run longer but at a lower output and lower input as well. 16 SEER systems generally are little quieter and there are other features and benifits the 14 will not have. You cannot generaly justify the extra cost of the 16 versus the 14 by energy savings. The added cost is justified in the added comfort that goes with it. Only you can decide if it's worth it or not since it is so subjective.

It should be noted that SEER has absolutely nothing to do with heating efficiency and often, there is little difference in the two when it comes to heat. You need to evaluate the HSPF for heating efficiencies.

I installed a duel fuel in my house, 16 seer a/c, and love the savings. I prefer duel fuel for another reason also,if you lose a compressor during the cold winter you immediately have a furnace to keep you warm and don't have to rely on electric heat strips that will cost you dearly to heat your house until the comp. is replaced.

Doug, Doc and Woody:
thanks for three great responses. It seems that there is some disagreement over whether a 16 SEER uses less energy than a 14 SEER to provide me cooling. I would think that the 16 would cost somewhat less to operate than the 14 since the 16 is more efficient. However, whether the added cost is paid back in a reasonable time (10 yrs?) via lower enrgy costs is hard to determine. A friend of mine has a Trane HP and he says that even in the coldest winter temps (15-30 degrees or so), his emergency heat rarely comes on. The Trane is a few years old and has a variable speed blower with electric emergency heat. sEER/HSPF unknown. I know there are a million other variables that factor into the equation (house design, insulation, preferences, etc.), but I was very surprised.

Thanks again

JP

East Tennessee, you say? Cheap electricity, mild climate. Heat pump wins hands down. At your utility rates, though, I would suggest option D: 14 SEER heat pump, electric backup.


Using electricity for backup heat can be expensive in some applications, but this isn't one of them. The heat pump will continue providing heat economically down to zero degrees. The backup only has to cover the shortfall between the heat pump's output and your actual need. So if you look at a 15 kilowatt backup system and do math, it will look very expensive to run that backup, but when you calculate how much it will actually run, it's fine. In this sort of climate you'll be getting more than 90% of your total heating demand from the heat pump (using electric backup). Given the efficiency of the heat pump, especially in mild to moderate cold weather, the overall operating cost is actually lowest.

With gas backup, since the heat pump can't run at the same time as the furnace, you have to shut down the heat pump as soon as it gets below your balance point (often around 35 degrees); then all your heat has to come from the furnace. As a result you can't get as much of your total heating needs out of the heat pump; you have to rely on your backup more.

So no, in this case you will absolutely not use enough auxiliary heat to warrant having gas backup. In the same vein, if you are hell bent on dual fuel you will never burn enough gas to justify the extra expense of a 90% efficient furnace over an 80%. In Michigan, maybe, but not Tennessee.

Don't plan on having a compressor fail in the middle of winter. It's unlikely in any case. Most systems that are installed properly and maintained properly last for 15-20 years. And I consider having ANY source of backup heat a major advantage. Keep in mind how many American houses have just gas for heat. If the furnace is on the fritz, too bad, bundle up. Shivering folks look admiringly at the chance to pay big bucks for a few days of emergency heat while their primary heat source is out of order.

Even if it's the same heating element, I like to maintain a distinction between auxilary heat and emergency heat. The auxilary is just covering the shortfall between your heating needs and the heat pump's actual output in very cold weather. In mild to moderate climates, you don't use much auxilary heat, so the cost of that heat source isn't a big deal. If you're going to rely on that heat source alone for a week straight, though, it's going to show up in your utility bills, and that's when you call it emergency heat (it makes the bill look a little better). It kept the place warm in a jam. If you didn't have a central backup like that you'd probably scrape together all the freestanding electric space heaters you could borrow, just to keep the place from getting super-cold inside, and you'd use just as much juice doing so.

Dual fuel is a great option for cutting heating costs when you have a relatively young furnace already in place, or when your house was set up for gas originally and isn't going to make it easy to get a big 240V circuit run to where the air handler will go (where the furnace used to be). That's why I have dual fuel at home; the gas line went to the same place the ductwork did, and the electrical panel was two floors away, with no access to pull a big cable without ripping out drywall. Otherwise, I'd be all-electric here in Atlanta. The cost of switching to all-electric was going to be high enough that it wasn't worth going to the trouble just for a little savings on monthly operating cost.

[Edited by wyounger on 12-06-2005 at 05:54 PM]

Hmmm..

Wyounger, your comment is much appreciated. It makes me wonder why my contractor didn't tell me the HP would operate economically to such a low temp. If I had known that, I probably would not have been so interested in dual fuel. That's the first time I've heard of anything close to zero in terms of a heat pump still working OK. In fact, I thought that the efficiency started falling at the balance point. Say around 30-35 degrees. Now, you also bring up a good point RE: the electric panel. My panel is maxed out. A sub panel would have to be pulled (do-able, but costly). Then, according to my contractor, I would still have to pay about $140 more to go straight HP versus dual fuel (?). I guess I could get the balance point set a little lower than 30 degrees before the gas backup kick in. May be a moot point at this juncture.

Thanks,


Jim