Replacing Gas Furance with Dual Fuel Hybrid Heat Pump System

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Old 10-03-11, 02:16 PM
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Replacing Gas Furance with Dual Fuel Hybrid Heat Pump System

Hi Guys -

This isn't really DIY related as I'm hiring a contractor to do the work but I wanted to get opinions from those who know more than me.

I'm considering replacing my gas only furnace and 3.5 ton AC with a new "hybrid" system that's dual fuel. The heat would come from a heat pump until it's 30-40something degrees outside and then the system would fire up a natural gas furnace inside the house.

What are your opinions on these types of systems? Am I not going to like the heat pump heat if I am used to the gas furnace heat?

I grew up in a house with a heat pump and I remember it blowing sort of warm air but never the hot air that we get now with a gas furnace - they must be better now, right?

My wife is always cold (even in the summer) and I'm easy to please (but prefer it to be cooler than hotter).

I'm in Northern VA where we get all 4 seasons - temps of 100 in the summer, and 10-20 in the winter. It's a typical single family 11 year old 2500 square foot house with builder windows.


Thanks for your advice and opinion!
 
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Old 10-03-11, 05:06 PM
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I have a hybrid (dual fuel) system and love my gas bills! However, if you normally are in a location in your home where the air blows directly on you then you will not like it as much while the heat pump is running. While they have improved the output temps from heat pumps they are still not a gas furnace and will not produce as much heat in cooler weather.
 
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Old 10-04-11, 05:22 AM
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Jfinn

The driving force for sales of dual fuel systems was the ability to leverage less expensive electric rates VS more expensive nat gas rates.

Two things have happened to change this situation.

1. Electric rates are not in a competitive marketplace like nat gas and have continued to climb where nat gas at this time is abundant and prices have remained constant and have actually declined.

2. The introduction of full modulating high efficient furnaces have made the pairing with a HP redundant.

I would ask you two questions.

What is driving your interest in a dual fuel system?

What is your electric rate? What is your nat gas rate? Have you checked with your local electric utility to see if they offer any rate incentive for heat pump heating in a dual fuel system?

You really can not compare today's high eff heat pumps compared to those of yesteryear. As different as nite and day.

If you provide your rates, I will give you a quick fuel comparison analysis between HP heating and nat gas heating.

IMO
 
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Old 10-04-11, 07:16 AM
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They have come around a long ways. My parents had a Trane XL15i put in last fall, and love it! Their is set up to switch to LP gas when it's 20˚ outside.

Get a good thermostat control like Honeywell IAQ.

Some electric company gives you a discount if you do duel fuel. My parents, the electric company charges .05 kWH for the HP where the regular electric rate is .11 kWH.
 
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Old 10-04-11, 09:05 AM
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Thanks for the advice, everyone. There is lots to research here.

I have an email out to my gas company (Washington Gas) to verify my rate, but over the past 12 months my average price for gas was $1.13/therm and I pay $0.12/KWH for electricity (NOVEC).
 
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Old 10-04-11, 10:24 AM
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While you are at it, check with your electric company to see if they offer duel fuel rate like I mention above.

I take it you are out of Washington State?
 
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Old 10-04-11, 10:26 AM
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I sent my electric company an email too to check on a variable rate - thanks for the suggestion - I wouldn't have known about it otherwise. I'll let you know what I hear back.

I'm in Northern Virginia - close to Washington DC.
 
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Old 10-04-11, 01:25 PM
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I just heard back from the electric company. It's the same rate all year around no matter what the electricity is used for. Too bad.
 
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Old 10-04-11, 01:31 PM
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Oh! And they did mention in their email that switching to a heat pump with a gas backup furnace was a *GOOD* idea for efficiency.

I did some calculations though and it looks like:

1,000,000 BTUs of electricity costs me $35.17.
1,000,000 BTUs of natural gas costs me $11.30.

That makes heating with gas 311% cheaper for me. That sounds like an awful lot. Are my numbers screwed up somewhere?
 
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Old 10-04-11, 02:31 PM
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I have had dueal-fuel for nearly 20 years, wouldn't have it any other way. The combo of both bills (gas/elect) seem to be cheaper to me than gas alone. Heat pump down part of last winter is why I know this...gas bill was nearly 50% more than the combo was same time year prior.
 
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Old 10-04-11, 02:31 PM
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In the end a HP will only cost a few hundred more over AC. So if it was mine I'd go with HP
 
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Old 10-04-11, 02:51 PM
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I don't get it. I must be missing something. If my gas costs less than electricity for the same amount of heat, why would I want a heat pump? Did I mess up my math somewhere?

Thanks!
 
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Old 10-04-11, 04:10 PM
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jfinn

your numbers are skewed and incorrect.

Assuming your rates are correct as posted, here is a fuel comparison to be used as a guide only. I used a 2.75 COP for HP efficiency and 95% eff for nat gas.

Cost per 100,000 btu of useable heat

Electric baseboard: $3.16
Heat pump: $1.28
Natural gas: $1.15

Nat gas is cheaper by about 10% on a BTU basis only.

If this was my home and I was considering new HVAC, I would look at a new full modulating furnace 95%+ eff and forget the heat pump.

I can make some suggestions if interested.

IMO
 
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Old 10-04-11, 04:43 PM
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Thanks for setting me straight, TigerDunes.

I'm absolutely interested in suggestions! Thanks very much!
 
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Old 10-05-11, 04:38 AM
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jfinn

as I mentioned, the HVAC market for gas furnaces is moving toward full modulating var speed models as opposed to 2-3 stg models that have been quite successful and a big improvement over sgl stg models.

the beauty of the modulating mdls is you use the BTUs required based on demand. you want to look at the full modulating models that move up and down as needed.

I like the York/Luxaire models and one of their nice features is the fact they don't require a special thermostat for operation. Second choice would be Trane/AmStd models.

I would think an 80K size with a 4 ton rated blower.

BTW, what size/eff is your existing furnace?

post back.

IMO
 
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Old 10-05-11, 05:14 AM
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I don't like the York models because when the call for heat stops the continue firing at a low rate even though there is no demand. Their thought behind this is to just keep the home at a steady temp. But their algorithm they use with a single stage stat is bad IMO. All they are using is basically a timer to adjust the firing rate with a call for heat from a single stage stat.
 
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Old 10-05-11, 05:21 AM
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For previous poster

If not the York mod, what mod if any would you recommend for this homeowner?

Or perhaps, just a good 2-3 stg var speed model?

TD
 
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Old 10-05-11, 05:28 AM
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I really don't have a good opinion of modulating furnaces. The more technology that is in the unit the more there is to break down. However, if I were to have anything to say about them it would be get a communicating model with a communicating stat. These will at least operate and modulate with the demand the thermostat see's. Do your homework on your contractor. The installation is far more important than the brand you buy. As we are talking about brands, my personal choice is Trane, not because I'm a dealer, but because I'm a service tech. They just seem less complicated even on the higher end furnaces than other products out there.
 
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Old 10-05-11, 05:57 AM
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So what is your opinion of the Trane mod with their proprietary communicating stat?
 
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Old 10-05-11, 06:25 AM
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Thanks Guys. This is some really good information.

I have 5 contractors scheduled for estimates - the first one is due here in about an hour. I feel like I can speak intelligently to him now because of the information you've given me.

I'll find out for sure what I have after he verifies it, but I think it's a 3.5 ton AC and I'm really not sure how many BTUs the furnace is.

Most of the dealers in my area either supply Carrier (biggest supplier) or Trane.

Thanks again for the information - I'd be going into this pretty blind without you guys.
 
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Old 10-05-11, 06:28 AM
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Tiger- I thought you had Trane?

I am with HVACtechFW. I'd rather have the mod furnace with a matching mod stat do as what the temp demand is than timing/cycle for the comfort.

I myself have a two stage Trane, and has done very well for me here in Minnesota winter when we get down to -25˚. The only thing I think the down fall about the mod (I could be wrong) is when it's running low side of the stages, the blower/air flow may not be enough to "throw" the air near the floor in our basement living area. Another word, not enough mixing of the air in the room to provide the temp.
 
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Old 10-05-11, 06:40 AM
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Jay

I do have a Trane dual fuel-XV90 paired with XL14i HP.

Because of electric rates, I probably would not go dual fuel today. Little to no economic benefit today.

My changeover is set at 30 degrees-could go lower but I want the furnace to be used some in heating.

Since HVACTech did not like the York mod, I wanted to hear his opinion on Trane's communicating modulating furnace. I believe it modulates in about 1% increments between a 40%-100% range when using Trane's proprietary communicating stat.

IMO
 
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Old 10-05-11, 07:00 PM
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Tiger- Sorry, I miss read what you posted. I read though the post quickly, and thought you switched over to the York Mod.

I think you are right about the Trane's Mod, does the 1% increments.
 
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Old 10-06-11, 05:23 AM
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I like the new communicating stat, but haven't played with it too much. FYI, ALL communicating stats are proprietary to the manufacturer who made them. I dont know why you had to put in your post that they are proprietary.
 
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Old 10-06-11, 09:19 AM
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For HVACTech

I used the word proprietary deliberately.

While furnaces like the mod and two-three stage furnaces will work with third party thermostats, you will lose furnace's capability and functionality without the correct and proprietary thermostat.

That's the reason. And many dealers will substitute the wrong thermostat knowing full well that it's not in the best interest of the furnace's operation and homeowner. See this situation too many
times.

IMO
 
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