nycgooner

I suppose you have three choices as far as heating conventional systems.

Straight high eff HP system, LP gas system, and finally dual fuel-HP with LP backup furnace.

I always like to start with fuel costs.

What is your electric rate? Does utility offer any special winter rate for HP heating? What is your LP cost/gallon?

Post back.

IMO

See attached calculator...

Here is a link that might be useful: Fuel Comparison Calculator

Propane is a byproduct of natural gas production and, to some extent, oil production. Propane is, and will always be, one of the most expensive fuels to heat a home.

I figure that the cost for propane heating is the same for straight electric heating.

I would go heat pump for sure, as heat pumps produce cheap heat. Your back-up heat can be electric strips or propane, whichever is cheaper to run. Note that the purchase price for a propane back-up system will be more $$ because you are buying a furnace in addition to the heat pump and air handler.

FYI: a heat pump has a lower life expectancy than straight a/c because it runs in both summer and winter. Heat pumps put out large volumes of warm air (90 degrees when its 30 outside) as opposed to furnaces that put out 120-degree air.

Good luck.

for Gary

One statement I question from above post about supply output temps on furnaces. Today's modern condensing furnaces (those with secondary heat exchangers)have a lower supply temp than the traditional non condensing models. True or not?

And for nycgooner

Today's HPs are as different as night and day in comfort to your old/broken HP.

TD

TD:

I am a heat pump home-owner, semi-ignorant to gas furnaces so I was guesstimating their output temps and comparing them to heat pump temps.

What would be the output temps of a typical 80% efficient gas furnace compared to a 95% efficient condensing furnace of the same btu rating?

Why would there be a difference in output temps between the 2 gas furnaces assuming the blower fan speed is the same? The only difference should be in gas consumption and exhaust temperatures released out of the vent stack.

The btus produced by the furnaces are either going into the house as heat or up the stack as waste heat. A 95% efficient furnace wastes only 5% of the gas, the rest is used to heat the house. The exhaust gases of the 95% are much cooler, hence the use of PVC exhaust pipe, but the heat produced, and the output temps, should be the same for furnaces rated with similar btu output.

Take care.

"Why would there be a difference in output temps between the 2 gas furnaces assuming the blower fan speed is the same? The only difference should be in gas consumption and exhaust temperatures released out of the vent stack."

Gary,

If you take a look at the rating plates or install manuals you will see a suggested temperature rise or "delta t"(across heat exch) for which the manufacturer suggests using as a guide for setting the blower speeds. Aside from all the other variables, input pressure/capacity, manifold pressure, duct static pressure, external accessories, etc. these temp rise guidelines need to be followed for mainly two reasons:

1. Avoid overfiring and stressing heat exchanger and avoid excessive cycling of gas train and safeties
2. Exceed dewpoint (especially on 80% to avoid ht ex deterioration.

All manufacturers models are different but as a base typically you will find that a standard 80% has a 45 to 90 degree rise variable allowed versus the standard 90% will be in the 30-65 degree range. Thus the discharge air temp of a 90%+ is lower and efficiencies are met due to the secondary ht ex is extracting the additional heat from the flue temps and it's condensation.

ZL:

The pros on HVAC-Talk believe that the output air temps between the 2 furnaces are based on fan speed and are basically equivalent.

Link below.

Thanx.

Here is a link that might be useful: HVAC-Talk thread

They certainly could be if you hit the fan speed selections right and with a little luck since they overlap. The key though if to have the lowest temp rise (higher fan setting speed) in order to gain highest efficiency from the appliance.

Of course people will always select a fan speed based on perceived comfort which effects the discharge temp at the grilles again due to many variables (crawlspaces, uninsulated or sealed duct, long runs, etc.)

The measureable difference would be at the flue temps which is hard for most since they are at different velocities. However with some equipment, a combustion efficiency test can be easily performed.

"The air temp leaving the furnace is a function of temp rise through the heat exchanger. The temp rise is a function of the CFM of the air moving across the heat exchanger. We know this from the formula CFM = output/1.08 X temp rise. This has nothing to do with furnace efficiency, only the output. Also the lower you are in temp rise the more efficient the furnace will be operating (more CFM through heat exchanger removing more heat)."

gary,

I read the HCAC-Talk thread and really felt the responses were more opinion than fact,all but the one above which I copied. I was told by Trane dealer that the high eff gas furnaces including the two stg var speed condensing models put out lower supply temps than the conventional non condensing traditional models. And these temps were not that much higher than today's modern high eff heat pumps.

Not attemping to be argumentative, just making an observation about these 93-95% AFUE high end gas furnace models and getting some knowledge. Without taking measurements and just providing a subjective opinion, I would say the supply temp from my XV90 is definitely lower than the old Armstrong sgl stg conventional model that I replaced. You just don't get that hot blast of air like the oldies provided.

Best Regards,
TD

"And these temps were not that much higher than today's modern high eff heat pumps."

An interesting experiment would be for you to measure the temperature output of the heat pump (at say 40 degrees when the heat pump should put out 97 degree air) and compare this to the temperature output of your new furnace. I would think that your furnace should put out at least 120 degree air.

Delta T = BTU / (1.08 x CFM)
so if the fan speeds, btu ratings, and return air temps are the same for both types of furnaces, the output temp has to be the same.

The point of my original post was that heat pumps put out large volumes of warm air (warm is a relative term - my Goodman manual says 100 degrees at 47 outside and 70 return air). Furnaces put out hot air (again, hot is a relative term but it has to be much hotter than a heat pump).

This was a good post and a good learning experience for me.

Thanx and take care.

Thanks for the responses folks, especially garyg and tigerdunes. As for my electric rate, I am unsure since I have offpeak KWH and onpeak KWH listed on my bill. I'd have to look at it more closely. I will call the utility company tomorrow and see if they have a special rate for HP. As for modern HPs, are there any HPs worth getting (Trane, Lennox, etc.). Once again, thanks!

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"I would say the supply temp from my XV90 is definitely lower than the old Armstrong sgl stg conventional model that I replaced."

Assuming an 80,000 btu furnace output and 1200 cfm fan:

Delta T = BTU/ (1.08 x CFM)
= 80,000/1296
= 62 degrees

So if you have 70 degree supply air, you should have 132 degree air coming out of the furnace.

You can plug in your actual numbers and see what you get. Your new furnace may have less output btus and the v-s air handler may be preventing the initial air blast.

Take care.

Nycgooner, if you have a peak/off-peak rate plan, you have more ways to save. Find out your peak and off-peak rates (the "total" rate -- energy charge, plus delivery charge, etc.) and post back, if you can. The heat pump incentives are good to know as well, but usually those are lump sum payments, so they figure more into your initial installation cost than running cost.

With the peak/off-peak rate, you have several different energy prices to consider:

-cost per BTU when running on-peak as a heat pump
-cost per BTU when running off-peak as a heat pump
-cost per BTU when running on-peak in aux/backup heat
-cost per BTU when running off-peak in aux/backup heat
-cost per BTU of propane

My guess is that everything but electric on-peak backup heat will be cheaper than propane, but it would be interesting to find out. Plus, one other thing in your favor is the off-peak rates usually occur at night when it's colder, working to your advantage, and also on weekends, when you're home more and want the house to be warmer.

I have an air source heat pump and these cold Indiana winters are costing me. I want to put in a propane heater to help the heat pump. I know I can install a forced air gas furnace to do the job but, can I buy a wall mounted propane heater that will have the 24 V controls. I want to allow the propane to come on automatically when the heat pump thermostat calls for 'auxillary' heat.