Cutover for HP vs Nat. Gas on Dual Fuel

[Jon30044]

First of all, thanks for all the information I've gotten while lurking on this forum for the past few weeks. I will be having a Lennox G71MPP furnace installed next week, paired with my existing Trane XL1200 to make up a dual fuel system. The furnace is replacing a Trane XE80 to take advantage of two-stage variable speed features and better efficiency. I'll also get a Comfortsense CS-7000 w/thermometer to control the system.

I've been thinking about the appropriate cutover temperature to switch from the HP to the gas furnace, which I believe can be programmed in the thermostat. My electric rate is .069 cents/kwh and the gas rate is somewhere between 1.139/therm and 1.606/therm, depending on whether you include the gas co's base charges, which I am going to be hit with anyway. This is from the bills covering January, 2009.

Based on these numbers and my calculations, it would always be cheaper to use the heat pump than the nat gas backup, assuming a 3.0 COP. So what am I missing -- the current system switches to the back up around 32 degrees.

FYI - I'm on a grandfathered time of use rate that also charges higher prices in the summer. I also think that with the new energy legislation contemplated by congress that coal generated electric, which is the norm here, will skyrocket, hence the bet that a 95% furnace is a good hedge vs an 80% furnace with the same variable capacity/two stage features.

Any other advice for things to look out for during the install would be appreciated.

[jstjohnz]

The problem is that the heat output from your heat pump drops as the outside temperature drops, and you will reach a point where the heat pump doesn't put out enough heat to heat your house. That's the temp where you should set your stat to switch to the gas.

The disadvantage of the dual fuel setup is as soon as you turn on the gas heat you have to turn the heat pump off. With electric strip backup heat both the heat pumps and the electric strip can run at the same time.

[Jon30044]

The problem is that the heat output from your heat pump drops as the outside temperature drops, and you will reach a point where the heat pump doesn't put out enough heat to heat your house. That's the temp where you should set your stat to switch to the gas.


Is this something I should expect the local contractor to know, or is it a trial and error type of thing?

Thanks

[BaldLoonie]

Most contractors aren't smart enough to figure this. But with your good electric rates and high gas rates, run the pump as cold as you can. It's cheaper colder than it can keep up.

[beenthere]

Wish we had that cheap electric rate.

We get a 30 to 40% rate hike in jan 2010. We're already at roughly $0.105 for electric.

Run that heat pump until it can't maintain temp.
Its cheaper then your $1.139 gas, down to about a 1.65 COP.

[Jon30044]

Most contractors aren't smart enough to figure this.

I hope that my contractor is "better than most." (Thanks Gary Koch for the quote.) Another Q - the XL1200 has a two speed fan, which to the best of my knowledge wasn't taken advantage of with the old system. There's also a TXV valve installed. Is this something that should be programmed into the new system to make it more efficient?

Thanks again.

[HVAC9900]

FYI - I'm on a grandfathered time of use rate that also charges higher prices in the summer. I also think that with the new energy legislation contemplated by congress that coal generated electric, which is the norm here, will skyrocket, hence the bet that a 95% furnace is a good hedge vs an 80% furnace with the same variable capacity/two stage features.

Any other advice for things to look out for during the install would be appreciated.[/QUOTE]

I too am waiting to see what happens with the rates ,am wanting to put in two heat pumps with strip back up and get rid of my gas.

I don't think the electric will go up dramatically, especially since the remc's are building a new"green" coal plant in washington county(ga),supposed to start within a year or two.

Can't hurt to go with that 95% no matter what happens.

[Jon30044]

I don't think the electric will go up dramatically, especially since the remc's are building a new"green" coal plant in washington county(ga),supposed to start within a year or two.

Interesting analysis from the Heritage Foundation on the latest version of the cap and tax bill:


Compared to the baseline, residential natural gas consumers will see their inflation-adjusted price rise by 55 percent. Because of its reliance on coal, the cost of electricity will rise by 90 percent — again after adjusting for inflation and in addition to what the price would have been anyway in 2035.

If that happens, 95% efficiency on the gas portion of the dual fuel will have been a smart investment. Also I talked to two folks with local EMCs that I know last week--seems that there are some second thoughts on doing this.

[HVAC9900]

Interesting analysis from the Heritage Foundation on the latest version of the cap and tax bill:



If that happens, 95% efficiency on the gas portion of the dual fuel will have been a smart investment. Also I talked to two folks with local EMCs that I know last week--seems that there are some second thoughts on doing this.

With those kinds of increases, I guess our friends in government want to ensure we don't have to worry about paying for silly things like food.....
Wow!

The coal plant did have two big remc's pull out, but the remaining group said they are still building it. That plus the nuke plant additions at Vogtle will hopefully keep the rates down.

Maybe I'll just get a bunch of wood for heat, and some huge ice blocks for ac,and solar collectors for power.

[beachtech]

is as easy as finding the balance point...