geo with gas furnace

Discussion in 'General Discussions' started by openloopfan, Mar 16, 2016.

  1. openloopfan

    openloopfan New Member

    Looking to install an open-loop geo unit (contained) and use the existing gas furnace for back-up heat. Is side by side my only option? Any issues/problems/considerations connecting to the existing ductwork. Thanks
     
  2. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    NO, I have seen systems which a couple dampers to isolate the gas furnace when not in use. But usually not advisable, you maintain a gas furnace you hope you will never use. Electric trip heat is cheaper, cleaner and integrated in the geo unit, and you do not need to maintain it.
     
  3. heatoldhome

    heatoldhome Geo Student Forum Leader

    Easiest way I can think of is to do what I did. I'm on a dual fuel program so lp furnace had to stay. I also use it for aux heat.

    Use a water to water heat pump and run a coil in the furnace. Heat in the winter, cool in the summer.

    And with w2w you can heat your DHW and maybe add some in floor heat.
     
  4. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    Keep in mind that w-w with coil for heating is much less efficient, since you need at least 110F supply temp to get anything meaningful out of a water supplied air coil.

    W-A heat pumps usually run with 70F return air and put out 90F supply air, and lower temps make them more efficient. You have one less heat exchange to go through.

    You might have low energy bills, but most people with w-w feeding an air handler don't know how good it can be.
     
    Mark Custis and heatoldhome like this.
  5. Mark Custis

    Mark Custis Not soon. Industry Professional Forum Leader

    I am not sure those numbers mean anything. We blew 105* air out of A2A stuff.
     
    David Knowlton likes this.
  6. I have the same sorta question on my thread, except undersizing the heat pump to cut instulation cost to keep thing affordable / finance free with a few ideas to keep things upgradable down the line.

    Are you using lp or Nat gas? Keep in mind most Nat gas company's charge a fee for keeping an account open.

    Here's a link to my thred openloopfan I would love to share some opions and ideas with you if you get a chance?

    https://www.geoexchange.org/forum/t...ions-ideas-and-justification.6722/#post-57562
     
  7. Mark Custis

    Mark Custis Not soon. Industry Professional Forum Leader

    When designing I always use the second cheapest fuel for backup heat.

    One does not want to overheat and blow up the heat pump. It matters not to me how we add the heat. I think in both air and water.

    We can talk by phone if you would like to do so.

    Mark
     
  8. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    If any heat pump (A-A, W-A) blows out 105F air, means that at 70F return temp you have 35F delta over your coil, meaning you only have 50-60% of the air flow you should have. Meaning that your refrigerant temps increase, and your heat pump efficiency decreases.
     
  9. Mark Custis

    Mark Custis Not soon. Industry Professional Forum Leader

    I am not sure how you measure my work.

    When did you measure the RA temp?

    Where do you get your numbers? Toilet paper?

    Have you inserted air velocity tools in the duct work you see in your mind?
     
  10. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    Heat Pumps are very defined in their performance and their parameters. There is a certain amount of heat air can absorb through a heat pump coil since the amount of heat the heat pump gets rid off is predetermined. No matter how you bend it, your return air is usually around 70F +/- 2F, unless you are heating a sauna. If your air heats up by 35F you simply are not moving enough air over your coil.

    The capacity is a given at a certain EWT, at a 32F rated EWT the delta T at 400 CFM/ton is pretty much 20F for about every manufacturer. If you claim 35F your are not moving enough air.....:)
    Your heat pump does not magically gain 70% capacity!
     
  11. Mark Custis

    Mark Custis Not soon. Industry Professional Forum Leader

    How do you know?

    Do I give you numbers?

    What makes you correct?

    All systems do not run at your specs. Or do they?
     
  12. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    You gave us 105F LAT, the rest is understanding the relationship between EWT, heat pump capacity, airflow and delta T over the air coil.

    Again, at 400 CFM/ton at rated EWT, every manufacturer has about a 20F delta T over the air coil. If you have 35F you do not have enough air flow. It is the typical symptom of a plugged air filter every HVAC technician should memorize, followed by other ductwork obstruction.
     
    Deuce likes this.

Share This Page