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  1. #1
    Join Date
    Nov 2007
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    Livermore, Ca
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    163

    Heat Pump Evap coil location

    Greetings.

    If I remember correctly, when using a HP, the evap coil is on the return side of the blower? So as to act like a draw-thru?

    If so, why is this? Perhaps so that it doesn't get affected by aux heat?

    Secondly, If someone wanted to just use a HP as only AC, using a gas furnace as the primary heat source, would the evap coil then go on the supply side of the furnace? Disabling the HP function.

    Just researching options

    Thanks, Regis

  2. #2
    Join Date
    Dec 2007
    Location
    Ohio
    Posts
    2,876
    Why would you only want to use a heat pump as A/C only??? Heat pumps can be used with gas furnaces. What exactly are you looking to do??? We are going to need more insight into what you are trying to accomplish here.

  3. #3
    Join Date
    Feb 2003
    Location
    S.W. PA
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    3,298
    coil can go on positive side of system but you can not (on either side) run it at the same time as the gas furnace

  4. #4
    Join Date
    Nov 2007
    Location
    Livermore, Ca
    Posts
    163
    I'm first trying to solve that with a HP the coil is before the blower (?) as compared to forced air systems that seem to have the coil above or after the blower.(?)

    I have a few threads on here about a HP system and ducting that was installed two yrs ago. It has engineering probs, as far as my knowledge takes me. Trying to salvage this with cost effectiveness in mind.

    To recap, it's a 3 ton (1200cfm) with 14" supply and return (99% flex) with 6 six" and 1 four" registers. Everything is wrong from my research in a 1129 sq ft house. 1135 ft/mn of air through 14" ducting is quite noisy and bothersome.

  5. #5
    Join Date
    Nov 2007
    Location
    Livermore, Ca
    Posts
    163
    dhvac. Cool. I wouldn't run the HP as a heat source only cooling.

    But, If there was a way to use the HP AND add a gas furnace I'd then have the option as to what source to use for heat.

    I'm starting to hear and read about dual fuel systems.

    I may be able to salvage my HP as AC Because the airhandler can be tuned down to 1k cfm. The Rheem paperwork puts it as 2.5 ton at 1k or 3 ton at 1.2k cfm's. When researching the properly sized unit for my house, 1129 sq ft, a 2 ton unit comes up. One load calc called for 2.2 ton when using the highest temp recorded. Using gas the furnaces state 850 cfm on heat and 1000 cfm on cool. My 14" flex duct system supports this better than what the 3 ton pushes.

    I can't do a Manual J yet 'cause I'm a Mac.

    My original inquiry is to find out that if it is necessary to have a HP evap coil before the blower, as seems the norm. And why forced air with AC has the evap coil after the blower.

  6. #6
    Join Date
    Mar 2002
    Location
    Concord, CA
    Posts
    2,635
    Your original speculation as to why the HP coil is before the blower sounds right to me. Draw through is more even. And certainly the strip heat must be after the coil. The coil goes after a gas furnace (be it duel-fuel or not) to prevent condensation inside the heat exchager that could result from the cold air hitting the outside of it.

    Duel fuel is a most excellent option for a large swath of the country. I happen to live in one of the few areas that is an exception. When your electric runs 15 cents plus per kwh as ours does; and when a lot of your power plants are driven by natural gas (so that as gas goes up, so does our electric); natural gas heat becomes the only logical option. The only place I see heat pumps locally is when the developer was too cheap to run the gas lines.

    Right-sizing is a worthy goal. However, I don't totally understand why you'd want to replace a bunch of expensive equipment to appease your ducts. Why not just right-size the ducts?
    Last edited by Irascible; 12-16-2007 at 09:03 PM. Reason: details, details

  7. #7
    Join Date
    Dec 2006
    Location
    Middle Tennessee
    Posts
    11,347

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    Quote Originally Posted by regis101 View Post
    is it necessary to have a HP evap coil before the blower, as seems the norm. And why forced air with AC has the evap coil after the blower.
    its not the norm, coil should be on the supply side of the blower (all types of coils)



    .

  8. #8
    Join Date
    Jan 2004
    Posts
    48
    Coil can be located on either side of the blower, blower has to designed for ample air flow and pressure for a properly designed and installed duct system. On dual fuel systems the coil is on the discharge side and is not operated concurrently with the fossil fuel with the exception of the defrost cycle. From your description of your duct system it would appear that it woefully short of minimum.
    Class of 70

  9. #9
    Join Date
    Nov 2007
    Location
    Livermore, Ca
    Posts
    163
    Quote Originally Posted by Irascible View Post
    Your original speculation as to why the HP coil is before the blower sounds right to me. Draw through is more even. And certainly the strip heat must be after the coil. The coil goes after a gas furnace (be it duel-fuel or not) to prevent condensation inside the heat exchager that could result from the cold air hitting the outside of it.

    Duel fuel is a most excellent option for a large swath of the country. I happen to live in one of the few areas that is an exception. When your electric runs 15 cents plus per kwh as ours does; and when a lot of your power plants are driven by natural gas (so that as gas goes up, so does our electric); natural gas heat becomes the only logical option. The only place I see heat pumps locally is when the developer was too cheap to run the gas lines.

    Right-sizing is a worthy goal. However, I don't totally understand why you'd want to replace a bunch of expensive equipment to appease your ducts. Why not just right-size the ducts?
    All good info. As for the last Pp, First, I'm pissed. Rip it all out and do it again. Second, Reducting is the least expensive. I'd still have an oversized system for the size of house.

    This is a 3-1 house. 9 registers is all that's needed. If I cram a tenth one,
    then that'd equate to 120cfm/register. Probably looking at 8" drops. Still an awful lot of air for the house. Need to use flex, works out to 18". Flex returns works out to a 14 with a 12. Wow! A two ton system can have 10 branch runs with 14" trunks. I'm all over the board, here. 6 of one, half dozen of the other.

    I just wish I had the knowledge that I have now. Which may not be much. Would've made better choices. Can't find HP tech's. The shops I do call go into denial because they don't understand them. Can't find decent techs as it is. They can install like there's no tomorrow but wane on the service. I did have a Rheem cont/tech come to look. he was smart. Smart enough to know that I'm, up the creek. I have a gross mismatch. Looking for solutions.

    These are my problems. These are my thoughts as, John Q Public

  10. #10
    Join Date
    Mar 2002
    Location
    Concord, CA
    Posts
    2,635
    I have to assume that you plan on being in the home for a decent amount of time. For my customers at least, I try to look at the complete picture. Cost/benefit is definitely a part of that picture. Even in a down market you’re not likely to get much of a return on capital improvements to HVAC. Few home buyers are informed enough to ask the right questions when it comes to mechanical. Putting out the money for right-sized equipment, in my mind at least, has to be something that you personally want for either your long term benefit or perhaps even for your own personal satisfaction (both very valid reasons). The odds of a future owner of your home being impressed with your HVAC system’s engineering are small.

  11. #11
    Join Date
    Nov 2007
    Location
    Livermore, Ca
    Posts
    163
    Your last sentence is true. But until that time comes, I have to live here. I doubt that they'd be impressed with the tripping outdoor unit due to high head pressures due to small ductwork. I'm not. I got whacked by a hack. Need a way out.

    From the three 0n-line load calc's I performed and looking at rules of thumb, I come up with 2 ton of cooling and 56k of gas heat. Or a 2 ton HP with 10kw heat strips. This will work with the 14" supply and return. Return could be better at 16". The ductwork that the installing contractor put in is the only blessing in disguise I got from his work. Yet even that needs two more registers, but the majority is reusable.

    So If anyone out there in HVAC land needs a Rheem 3 Ton HP, lemme know.

    Sorry to get of topic. It happens sometimes.

  12. #12
    Join Date
    Sep 2004
    Posts
    219
    3 tons for 1100 sq. ft.? No insulation? Around here most houses that size use 1/2 that tonnage, but they are well insulated. Is 3 tons what the load calc called for?

  13. #13
    Join Date
    Dec 2006
    Location
    Middle Tennessee
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    Quote Originally Posted by tuckerin View Post
    Coil can be located on either side of the blower
    why do you think 95% of all systems are designed with the coil on the supply side of the blower?



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