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Old 01-12-2009, 10:23 AM   #1
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Second Furnace for dual fuel


So I've been thinking about the feasibility of adding an LP or fuel oil or even a boiler only furnace to my existing HVAC system to act as part of the dual fuel system instead of the electric resistance heat in the winter. I live out in a rural area and there is no access to NG.

Is this a bad idea? I would put a damper between the output and supply duct of both units to ensure there is no backflow of supply air through the return of the unit that is off. My current system is a 5 ton heat pump.

I attached a sketch of what I am thinking.

Sure, it probably would have been better at the time of installing this new system to have gone with a dual fuel air/handler that supported what I wanted to do, but I didnt.
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File Type: pdf dual-fuel.pdf (36.2 KB, 65 views)

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Last edited by AndrewF; 01-12-2009 at 11:26 AM. Reason: added size of heat pump
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Old 01-12-2009, 11:06 AM   #2
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Second Furnace for dual fuel


What is your cooling (outdoor) unit? AC or Heat Pump?

Are you wanting to get completely away from the resistance heat in your air handler, or are you wanting the option of either fuels sources (LP/Elec)?

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Old 01-12-2009, 11:25 AM   #3
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Second Furnace for dual fuel


It is a 5 ton heat pump, which works well, except for those extremely cold days that we don't get often. My thought was if I could pick up a used unit, I could set it to run based on the existing outdoor t-stat.

Instead of it turning on the resistive heat, it'd turn on this second unit. Otherwise, it'd sit idle all the time.

Note: I removed a 75k but oil burner when we put the HP in.

Right now, we just start a fire in the fireplace to take the chill off when it is cold out.

To reiterate, this was just a thought I had yesterday and I wanted to see if it was abnormal to do something like this or not.

Quote:
Originally Posted by Home Air Direct View Post
What is your cooling (outdoor) unit? AC or Heat Pump?

Are you wanting to get completely away from the resistance heat in your air handler, or are you wanting the option of either fuels sources (LP/Elec)?
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Old 01-12-2009, 11:55 AM   #4
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Second Furnace for dual fuel


I would just replace the current AH with a 90+ gas furnace and call it done. Either add a dual fuel kit or change out your Tstat to a Honeywell 8000 series and use that as your kit.

To me, I would not want two sets of everything in my basement. Just more stuff to break. But, from a NASA perspective, maybe not so bad (two of everything).

As far as design, let others weigh in. Not my strong suite.

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Old 01-12-2009, 12:02 PM   #5
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Second Furnace for dual fuel


You might consider a duct furnace mounted in a bypass duct. Would require a motorized damper on the bypass leg - pretty straight forward though, I'd try to run it off the outdoor temp, so when it falls below 15*F (adjustable) the duct furnace kicks on, bypass opens, a fraction of the supply air (downstream of the HP discharge) diverts thru the furnace.

You don't need a lot of burner to accomplish what you want to do. Your 5 ton HP probably has 2000 cfm. It's probably discharging about 85 degrees in heating. You'd probably appriciate 100 or 105 degree discharge instead. So you have a 20 degree temp rise.

CFM * Temp rise * 1.085 = BTU, 44 MBH is really all you need.
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Old 01-12-2009, 12:24 PM   #6
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Originally Posted by Garasaki View Post
You don't need a lot of burner to accomplish what you want to do. Your 5 ton HP probably has 2000 cfm. It's probably discharging about 85 degrees in heating. You'd probably appriciate 100 or 105 degree discharge instead. So you have a 20 degree temp rise.

CFM * Temp rise * 1.085 = BTU, 44 MBH is really all you need.
Yes, it is a 2000 CFM. We are in the midst of upgrading the duct size now as the old trunks werent big enough.

It generally outputs 104 degree air with the old duct design, it'll be interesting to see how much it changes with duct work rated for 2000 cfm.

The flat line in the attached chart is when the fireplace was going lastnight, it makes a big difference, especially when I toss in a few lumps of coal.
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Second Furnace for dual fuel-hvac-temp-graph.png  
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Old 01-12-2009, 12:28 PM   #7
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Quote:
Originally Posted by Home Air Direct View Post
I would just replace the current AH with a 90+ gas furnace and call it done. Either add a dual fuel kit or change out your Tstat to a Honeywell 8000 series and use that as your kit.
I do not want to rely on LP to heat my house as the COI is much higher than a HP.

I use a t-stat with almost identical features of the HW 8000.

A Venstar T1900. I picked it as I can interface it with a home monitoring setup quite easily.
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Old 01-12-2009, 12:32 PM   #8
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Second Furnace for dual fuel


Wow monitoring software - most commerical clients I work with aren't even that sophisticated.

While your replacing ductwork, now would be a good time to add a duct furnace!

Assuming you understand the sensible heat equation I posted earlier, you'll see exactly how changing the airflow will affect the discharge temp. However, the amount of heat your unit puts out will vary a little bit with airflow, so it's not perfectly linear.

EDIT what is your motivation for going dual fuel? Is your current heating system not providing comfortable conditions? In your studies so far, including the 104*F discharge temp statement, was that with the electric resistance on?

Last edited by Garasaki; 01-12-2009 at 12:35 PM.
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Old 01-12-2009, 01:00 PM   #9
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Second Furnace for dual fuel


I should clarify. The output air-temp is around 94 without resistive heat. I have that staged in 5kw increments and it wont come on until 30 minutes of runtime have passed and no temp change. The suction line temp at the AH is 100-104. (depending on outside temps)

I like the idea of a duct furnace...I could add one pretty easily down the road in the return drop box 20"x25"x75" with little modification. My AH sits in the middle of my basement and then there are trunks going to the left/right of it. The return drop is the only single point all air flows through.

The motivation was to find something cheaper than resistive heat. I got a $430/electric bill for Nov23-Dec23rd. (Monitoring system shows a lot of resistive heat usage). The utilities out here also raised the rates 6% Jan 1. I was expecting a $230-250 electric bill.

I did a Manual J on the house and confirmed my original suspicions of improper duct size. The upgrade project should be completed by this weekend.

I may not do it, I just want to know my options should I need or desire to do something for the future.

----
See this thread on my monitoring setup.
HVAC Monitoring
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Old 01-12-2009, 03:04 PM   #10
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Ok I got ya.

The duct furnace would be on the supply side. Won't really work on the return side - you'll just decrease the effectiveness of the HP which obviously is not what you'll want to do.

Remember you'll have to have a PVC vent line and maybe an intake too.
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Old 01-12-2009, 03:33 PM   #11
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Quote:
Originally Posted by Garasaki View Post
Ok I got ya.

The duct furnace would be on the supply side. Won't really work on the return side - you'll just decrease the effectiveness of the HP which obviously is not what you'll want to do.

Remember you'll have to have a PVC vent line and maybe an intake too.
Why couldn't it be on the return side?

I guess I don't understand why it makes a difference where it is in the loop (return vs supply) as in both cases it would still be in the loop and the airflow would be impacted one way or another for the AH.

Running PVC vent/intake wouldn't be a problem, if I went that route.
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Old 01-12-2009, 03:48 PM   #12
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Second Furnace for dual fuel


Ze loop, as it were, would not be supply-return but supply-duct furnace-supply.

You would raise the temperature of a fraction of the supply air, from 90ish to 130 or 140ish degrees, which would then mix back in with the rest of the supply air, and the house would "see" an air temperature of 105ish.

Your heat pump warms the air by passing it over a hot coil. You've actually told us, based on the suction temp, what the approximate temperature of that coil would be (I think). Suction temp = 100 to 104.

Heat transfer effectiveness is directly related to temperature difference. Larger temperature difference = better heat transfer.

Your bringing air back to the system at (arbitrary numbers alert) 65*F. Passing it over the 104*F coil. It then leaves the system at 94*F.

If you heat the return air, especially with a gas furnace, you will be bringing back warm return air (in fact it will probably enter the unit at 100*F or more).

So now you are passing 100*F air over a coil thats 104*F. Again, heat transfer effectiveness is DIRECTLY related to temp difference. You have changed your temp difference from 40*F to 4*F. (In fact you may even reach a point where your heat pump is COOLING the return air).

You want to add your heat downstream of the heat pump coil.
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Old 01-12-2009, 03:58 PM   #13
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Second Furnace for dual fuel


Not worht the headache.
Unless you have power outages, and are planning on installing a generator.

Untill you get your duct work straightened out, your not going to get 2000CFM out of that air handler.
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Old 01-12-2009, 04:27 PM   #14
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Quote:
Originally Posted by Garasaki View Post
Ze loop, as it were, would not be supply-return but supply-duct furnace-supply.

You would raise the temperature of a fraction of the supply air, from 90ish to 130 or 140ish degrees, which would then mix back in with the rest of the supply air, and the house would "see" an air temperature of 105ish.

Your heat pump warms the air by passing it over a hot coil. You've actually told us, based on the suction temp, what the approximate temperature of that coil would be (I think). Suction temp = 100 to 104.

Heat transfer effectiveness is directly related to temperature difference. Larger temperature difference = better heat transfer.

Your bringing air back to the system at (arbitrary numbers alert) 65*F. Passing it over the 104*F coil. It then leaves the system at 94*F.

If you heat the return air, especially with a gas furnace, you will be bringing back warm return air (in fact it will probably enter the unit at 100*F or more).

So now you are passing 100*F air over a coil thats 104*F. Again, heat transfer effectiveness is DIRECTLY related to temp difference. You have changed your temp difference from 40*F to 4*F. (In fact you may even reach a point where your heat pump is COOLING the return air).

You want to add your heat downstream of the heat pump coil.
I follow the logic, however, I meant dual fuel in that the HP would be off...producing 0 heat gain and the alternate unit would be carrying 100% of the load.
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Old 01-12-2009, 04:30 PM   #15
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Not worht the headache.
Unless you have power outages, and are planning on installing a generator.

Untill you get your duct work straightened out, your not going to get 2000CFM out of that air handler.
Well since you brought it up....the second reasoning behind this logic would be that our current generator is 6kw and cannot run the HP.

It could however, run a furnace blower and provide backup heat.

We have had more power outages at this house than we have at any other place.

The ductwork upgrade is in progress.

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