# Air conditioning and wood/electric furnace



## Beno (Jun 15, 2008)

Hi all,

How can you connect an air conditioner (located outside the house) to the air ducts used by the wood/electric furnace? Do you need a special type of air conditioner?

Thanks,
Beno


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## begreen (Jun 15, 2008)

The A/C system needs an air-handler on the inside of the house that the main plenum is attached to.  It houses the cooling coil and if needed, the electric heating elements. A wood, add-on furnace could be tied into this system. 

For an existing system, a cooling coil box is sometimes added on top of the furnace. This requires a low furnace or high enough ceilings and trunk ductwork to handle the added height of the coil box. 

http://www.trane.com/Residential/SelectSystem/acandfurn.aspx


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## Beno (Jun 15, 2008)

In our case I plan to use the Caddy wood furnace with electric backup (only one furnace, no add-on), I don't see how can I attach the cooling coil to it. Also, on top of the furnace is the hot air plenum.
Thanks,
Beno


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## begreen (Jun 15, 2008)

If the plenum has about a 3 ft height and the trunk ducts come off of this plenum, there may be room for a coil assembly. If not, high efficiency window units or the new mini-split AC systems work well for many situations.


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## Beno (Jun 15, 2008)

There is room, the Caddy is 4 ft height and the ceiling is 9 ft height. That mean I'll need a split system, with the evaporator coil installed above Caddy, in the hot air plenum.


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## Redox (Jun 16, 2008)

Hey Beno, where 'ya located and can you describe the house?

We might be talking two different animals here.  You might have been referring to a package unit that merely connects to the ductwork.  BG is obviously talking about a split system.  The split system uses the furnace's fan to move air through the coil and ductwork.  The most important thing is to make sure the furnace fan is powerful enough and the ductwork is adequate for cooling.  Many older houses don't have very good ductwork for A/C.

It's hard to make a recommendation without knowing more about the house, but I would want to try to keep the "fire" away from the air conditioner.  All that expansion and contraction on the evaporator coil doesn't do them any good and the possibility of condensation in the furnace might shorten its lifespan.  If you really want to cool the house well, consider a separate duct system in the attic with a split system or  maybe a heat pump w/ backup resistance heat, also in the attic.  You really won't believe how much more efficient it is coming from above.  If you are in a northern climate and this seems like overkill, go the window shaker route.  Much cheaper and easier.

Chris


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## Beno (Jun 17, 2008)

Hi Chris,
The location is Ontario, Canada. The wood furnace has a fan of 1/3 HP. The house will be an ICF house very well insulated (still in design). There must be a solution to have a split system and use the same ductwork. Maybe I can somehow remove the evaporator over winter, or protect it?
Thanks, Beno


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## Redox (Jun 17, 2008)

Hmm, Ontario...  Really not much of a heat load up there.  It will be hard to justify putting in a separate duct system just for A/C, but have you considered an air-air heat pump?  They don't have a very good reputation, especially in the northern climates, but are GREAT in the shoulder seasons when it might be too warm to justify firing up the wood furnace.  Since you have already mentioned an electric backup, you might consider a separate system in the attic for a heat pump.  Just a consideration.

The problem I see is that the indoor coils of just about all A/C units have plastic drain pans.  This is an evolutionary step up from steel, as they used to slowly rust out and leak.  They claim to be good to about 180F, but it doesn't take much of a stretch of the imagination to see your wood furnace topping this number in a power failure.  Putting the cooling coil ahead of the furnace would prevent this, but your furnace will rust out from all the condensation that will probably occur.

You could install a package unit (all in one) ducted into the furnace, but the heat losses on these systems is a lot higher and isn't usually done up north.  A mini-split (ductless) is a great option if you have an open floor plan.  These are wall mounted cooling units with a separate condenser outside and are also available as heat pumps.  They are very quiet, simple to install and don't need ductwork.  Sanyo and Mitsubishi are the big names around these parts.  They are a big step up from a window unit, but are kinda pricey.

It sounds like you have the room for a coil and a 1/3 hp blower should be able to move enough air for an A/C unit.  You need 400 CFM per ton of A/C and this is usually rated at 1/2" of water (pressure) across the furnace.  I'm not sure which of the Caddy models you are looking at, but the PF01000 shows a 1300 CFM number that would support 3 tons of A/C.  I don't see a static pressure rating or any indication that it is set up for cooling.  Might be worth an e-mail to find out...

I have never heard of anyone setting up a seasonally removable coil in a duct system, but it is possible.  Most manufacturers make coil boxes so that the coil can be pulled out for servicing.  On most furnaces, you have to remove the stack and may have to contend with refrigerant lines that are too short as well as a drain line, but if you set the coil box sideways or off the back and left a loop of tubing, you could build a shelf next to the furnace to slide it onto.  Replace the coil cover with a blank off plate and fire up the furnace.  It might just work, but make sure the refrigerant lines are supported well and have plenty of slack for the movement.  You don't want to kink them... 

Hope this helps,
Chris


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## Beno (Jun 18, 2008)

We have some very hot and humid summer days in the summer, though, with the concrete walls and floors and super-insulated roof/windows/doors we might very well live with an ERV.
Venmar has few models: Duo 1.9, ERV Constructo 2.0 Quattro which (if I understand correctly) combine an HRV with ERV, giving a good solution year round. And needs less electricity too.
Will this work?
I want to use the existing ducts. The wood furnace is this: http://www.psg-distribution.com/product.aspx?CategoId=16&Id=335&Page=spec

Thanks again,
Beno


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## Redox (Jun 20, 2008)

I am not really up on ERVs for residential use, but they can't really "cool" your house.  They can precool the air coming in, but if the house is already warm and humid, you are going to need something else.  If the place is going to be that tight, a window shaker in the living room might be your best bet.  Pull it out in the fall and no heat loss or ductwork issues.  

Venmar is a commercial equipment manufacturer and their smallest units are 300-400 CFM and probably overkill for your needs.  I'd also be willing to bet they are priced way out of your budget; commercial equipment is easily 2-3X what residential equipment costs.  There are probably more economical pieces out there, but I can't advise.

I can't find anything on PSG's website that even hints at suitability for air conditioning.  This is not to say it won't work, but that the manufacturer hasn't really tested their equipment for it.  You can pretty much stick a coil in any kind of ductwork and call it A/C, but YOU have to be the engineer to make sure it isn't going to cause a problem.  I THINK it would work if you could find an older A-coil that has a metal pan, but I don't know of anyone that makes these anymore.  There were also the rumors about what would happen to a liquid charged expansion valve in an overheat situations, but I haven't been able to find any evidence...

My professional opinion would be to advise against putting A/C on this furnace, unless you develop a workaround like the removable coil idea.  I would want to see the mfr. blower data before doing this, just to make sure it has enough capacity.  If it really puts out 1300 CFM at 0.5" wc. it will support 3 tons of A/C.  I suspect that inserting a cooling coil (approx 0.2" drop) might derate it to 2 tons, but only the blower mfr. knows for sure.  The pressure drop may also restrict the airflow in the winter and lead to furnace overheating on high fire.  Two tons may be adequate for a well insulated home in a northern climate, but you need a load calculation to prove it.  

Sorry to offer more questions when you are looking for answers, but I haven't any experience with A/C on a wood furnace.  I tend to overanalyse these things...

Chris


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## Beno (Jun 20, 2008)

Thanks again for your thoughts. 
Another idea: what if I connect the cooling coil to the return cold air plenum? It's still in the air flow, but doesn't get all the heat directly from the furnace in the winter.
I believe that the recommended connection for an HRV to the wood furnace is in the return cold air plenum, so maybe I can do the same with the coil?

Thanks,
Beno


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## Redox (Jun 21, 2008)

Definitely not recommended.  The A/C will chill the furnace down to 55-60 degrees and will cause condensation in all but the driest of climates.  Even the manufacturer of my condensing gas furnace with SS heat exchangers said this would void the warranty...

Chris


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## Beno (Jun 21, 2008)

I found on the web that an air conditioner works 90% of the time to remove moisture and only 10% to reduce temperature. If I consider that for our house with large thermal mass and super insulated it's more important to reduce moisture than temperature, the logic solution is to choose an Air exchanger. Few examples:
1. Venmar HEPA 3100: Reduces excess humidity, Rate of air flow (cfm)* (low/high) 70-105 cfm, Power 232 watts, 
2. Venmar Duo 1.9: Designed to help retain heat in cold climates and reduce the load on cooling systems in warm climates, Rate of air flow (cfm)* (low/high) 117-180 cfm, Power 250 watts, Type of recovery ERV 

Also, it's important for us to have a low energy consumption.


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## Stove Geek (Jun 23, 2008)

For the A/C on your Caddy, you will have to put it up in your warm air plenum, as high as possible. By what I read, you have plenty of room, so there will not be any problems. The other way of doing it, would be to create a by-pass for the summer, so that the air would flow through your A/C coil. When the by-pass would be closed the heat generated by the Caddy would not 'melt' the pan of the A/C.

If the by-pass is not an option, just install the A/C coil as high as possible.


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## Beno (Jun 23, 2008)

Thanks Stove Geek, I was also thinking about this solution with the by-pass air duct that will connect to the hot air plenum but will be closed over the winter. This way I don't really care about potential excessive heat generated by Caddy. 
Will the new super-Caddy with DHW be ready soon?


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## Redox (Jun 24, 2008)

Has anyone here ever done this?  

According to the manual on PSG's website, you need 6" of clearance from combustibles for the first 6 feet of duct as well as the plenum.  Plastic is combustible and bypass dampers are not foolproof.  I wouldn't do it...

Chris


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