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Thread: water to air heat exchanger

  1. #1

    Red face water to air heat exchanger

    I have recently installed an outdoor wood boiler and I am in the process of retrofitting my gas furnace to install the heat exchanger in the plenum to utilize my furnaces blower and ductwork to move the heat in my home. What is the thoughts on installing this exchanger in my existing furnace? It seams like every person in the industry has a different opinion on if this can or cannot be done. My gas furnace is a 90% efficient unit and has an AC A-coil in it. Some say I cannot put this wood heat exchanger in and move enough air with the other coils in the same plenum. They say I need to install a seperate air handler cabinet and tie into my duct work? While others have told me they have seen "hundreds" of this same setup done with no problems. Any thoughts or experience with this arrangement?

  2. #2
    Retired Defense Industry Engineer jadnashua's Avatar
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    I have a similar heat exchanger in my air handler along with the a/c coil. Air flow is fine, but the coil I have is fairly small (32K BTU). The key is to have a large enough exchanger and duct so that the back pressure stays within specs. Not sure if this is a spec that is measured for them. What you'd want to look for is the amount of backpressure at a certain airflow, then see if it would be within the furnace's specs.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  3. #3

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    how do you measure this back pressure and air flow?

  4. #4
    Retired Defense Industry Engineer jadnashua's Avatar
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    The spec sheet for the furnace should either state what the fan speed and air flow is, or should be. Note, many of them can be adjusted, either with different taps on the motor, or electronically, or by changing the pully size(s) if it has a belt, so it may take some sleuthing to figure out how yours is currently setup. WIth that, and the duct size, you should be able to figure out the cuft/min. The specs on the heat exchanger are likley to state the restriction of the heat exchanger and that should help determine whether it would work. If the restriction is too great, you could probably go for a bigger one - and expand then compress the size of the duct so there would be less velocity across the heat exchanger - this might also extract more heat from the exchanger since it would go across it slower, but it would be small. You need a pro or good specs to figure this out. Unless the manufacturer has already done the tests, you won't know unless you can install it then test. That can get expensive! A call to the manufacturer might supply some help.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  5. #5
    Master Plumber nhmaster's Avatar
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    Adding a second restrictive coil on top on another one is never a good idea. The furnace\AC set up should have been designed for the proper air flow when it was installed. As in these units are usually matched to each other so that the proper flow over the heat exchanger and the ac coil in proper for efficient operation. Adding a second exchanger will reduce the air flow over both the exchanger and the ac coil. Possible (and likely) problems will be, premature failure of the heat exchanger due to poor air circulation. Icing of the ac coil in very humid weather. High head pressure on the ac compressor due to low air flow. Sometimes it is possible to increase blower speed but this opens a whole new can of worms, noise and excess air flow at the ac coil to start with. The only right way to do this (and not void the warranty) is to install a separate air handler and motorized zone valves to isolate the systems from each other.

  6. #6
    Retired Defense Industry Engineer jadnashua's Avatar
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    In my case, instead of adding the resistive heat coils, I substituted a small hydronic heat exchanger, so the difference in air flow through the whole thing isn't great. In your case, since the furnace is already designed for the a/c coil and the air-air heat exchanger, adding a third could be a problem. If you can make the outlet duct size large enough, and use a larger heat exchanger than might normally be needed, the restrictions would be lower than if you kept the duct size the same, so it might work. You could bring the duct size back to original after a nice transition distance. This would be less expensive than adding the other, but you'd be assured of it working if you put in a dedicated air handler. I look at this sort of like adding a HEPA filter to the system, some can handle the restriction, some can't...you won't know until you talk to the manufacturer of the system you have. If yours can work with one of those, find the cuft/min and size and make the heat exchanger similar, and it should work.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  7. #7

    Default Furnace Finned Coil Heat Exchanger

    The easiest way to size is by measuring your furnace plenum and then choosing heat exchanger that takes the most part of it. For example, if plenum size is 17"x19" or 17"x21", then heat exchanger with finned area 16"x18" will do the best work.

    http://www.brazetek.com/

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