Geothermal heat pumps and Radiant, a good match or a poor one?

Discussion in 'Radiant Heating and Cooling' started by eisensms, Jan 19, 2009.

  1. eisensms

    eisensms Member

  2. hardchines

    hardchines Member Forum Leader

    look's like water flow control valves.
  3. eisensms

    eisensms Member

    Valves in system

    I guess they are just check valves ... I was thinking that they could
    be Dole control valves.
  4. kandk920

    kandk920 Member

    Geo and Radiant, a good match or a poor one?

    I had a person that builds a lot of homes in the area that he puts geo heat into that he feels that geo and radiant are not a good match for each other. He said that one would think that they are, but he has found that they are not.

    I would tend to agree(having such a system)that this is true as well. My experience is that when the house needs heat most, after sunset and the whole house starts calling, the radiant water temps drop quickly and geo takes way too long getting them back up to a temp to make the house comfortable.

    Anybody else care to comment? :?
  5. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    They usually work great if the radiant system has a large enough area to transfer the heat into the house. Having a buffer tank helps decreasing response time in a scenario you described. It is a question of designing them correctly.
  6. urthbuoy

    urthbuoy Well-Known Member Industry Professional Forum Leader

    You ask a geo forum whether geo and radiant are good for one another?

    Their perfect for one another :D .

    Makes more sense than producing 160F water and then tempering it back down with cold water - where's the efficiencies in that?

    P.S. If you don't have an outdoor sensor - get one. It may solve the problem you're experiencing.
  7. engineer

    engineer Well-Known Member Industry Professional Forum Leader

    Assuming the radiant system is designed for the lower water temps most efficiently provided by geo, then they can be a good match.

    There should already be a buffer tank - perhaps it is undersized. Outdoor reset allows the system to operate more efficiently in mild weather and gear up a bit in advance of cold weather.

    We need more info, but the system at hand may be undersized or designed for warmer water than what geo efficiently provides.
  8. jrh

    jrh Member

    Great match when designed properly.Like others have mentioned outdoor rest controls,properly sized buffer tanks are a must. Location and spacing of the pipes are also critical. Stapled up pipes dont do well with geo, owing to their need for higher water temps.
  9. kandk920

    kandk920 Member

    The house is new ICF construction, 5T water to water Geocomfort unit, have a 50 gallon buffer tank, lower level floor is concrete, main floor is gypcrete. System was designed for the house or so I was told. There is NO outdoor reset. Floor tubing is in the gypcrete and concrete 12" on center.

    On cloudy days, the house stays comfortable, just on sunny days when the geo does not run all day is when there are comfort issues. The house temp stays above the set points on the thermostats and the water in the system cools to about 82 degrees. The normal set points on the system points are geo on at 92 and off at 98. So when the whole system is calling for heat and the water temp is 82 the geo runs all night before normal temps are regained.

    So, slow to heat thermal mass with slow to heat water gives me uncomfortable house on cold evenings after a sunny day.

    Tell me what should have done differently. PM me and I will give you the email of the HVAC contractor that put the system in and tell him too as he must not know the right way to do it.
  10. engineer

    engineer Well-Known Member Industry Professional Forum Leader

    Thermally massive house with a slow-to-react heating system strikes me as a good combination. I live in and ICF house and one of its many advantages is that its thermal mass spreads loads away from daily outdoor peaks and valleys.

    I wonder if your issue stems from something as simple as poor thermostat placement. If the stat is in a room or on a wall that sees and carries heat from the afternoon sun into the evening it would delay the system's response. Then all that mass works against the system as it struggles to recover.

    I think the buffer tank's role is to reduce compressor short cycling in times of low load. I don't think 50 gallons either way makes much difference in terms of thermal storage capacity, given the thermal mass of the 50-100 tons of concrete likely in your floors and walls.

    Just some thoughts - hope it helps.
  11. zacmobile

    zacmobile Guest

    it sounds as if you may have the thermostats tied directly to the hp, is this true? this would be less than ideal if the thermostats weren't calling all day everything would cool down and have a hard time catching up. the ideal strategy would be to have the heatpump being controlled by an outdoor reset controller (tekmar 256, HBX CPU-500 etc.) on a constant demand with the curve precisely matched to the heatloss of your house. this may not even be the problem, but sounds likely based on your info given.
  12. kandk920

    kandk920 Member

    I don't know if they directly connected to the HP, but I do know this, unless at least one zone is calling, nothing at all is running.

    So how do I convince my installer that he did not control this system correctly and get him to do it right?
  13. teetech

    teetech Member Forum Leader

    Sounds like tank controls water temp and stats control flow.
    I would think telling him you are not comfortable and you want it fixed, unless he is unconcerned with your comfort.

    On a cloudy day does the unit run most of the time?
    What is your comfort set temperature? How far does it drop after a sunny day?
  14. kandk920

    kandk920 Member

    On a cloudy day the unit cycles on for about 20-30 min and off for about 15-20 and we stay very comfy.

    The stat is set at 70, and on a sunny day it can go up to 71-72 on that stat during the day, then will call when temp falls to 69, I have seen it go as low as 67 on very cold days. That is when I see the temp on the water going back to the buffer tank as low as 82 on the control.[attachment=1:1z08nx51]02-22-10_2016.jpg[/attachment:1z08nx51]

    Attached Files:

  15. GeoXNE

    GeoXNE Guest


    Before everyone starts blaming tstats and controls, let's make sure the radiant system is working properly.

    If that circ pump is a ups15-58 than it is not up to the job. The best way to confirm would be to check the delta t between radiant supply and return. With low temp geo systems the tighter the better. I shoot for 10F, 20F would be ok. Some where along the line you have to calculate the head of the system and spec the right sized pump.
  16. Mark Custis

    Mark Custis Not soon. Industry Professional Forum Leader


    That is a good idea, but lets see more and get the exact head pressure and the best curve for a pump. does the system have a differential by-pass valve?

    Does it have expansion?

    What are the loop lengths and tubing diameter.

    I use that style of piping to reduce pump needs all the time. Zone valves are cheaper that pumps. I would use either delta P or delta T pumps and injection pumping every time I could, but those jobs are not the norm.
  17. kandk920

    kandk920 Member

    Definitely not blaming the t-stats, they are very simple ones, on and off with one degree varience. Set at 70 the stat calls when temp drops below 69 until 70 is reached. Pretty simple, only adjustment on the t-stat is to change the varience to 2 degrees.

    I believe the pump is the size you stated. I haven't measured the delta t between supply and return, but just by feeling the pipes would guess that it is no greater than 20, probably closer to 10. I will have to get my meat thermo out and test it and let you know for sure.
  18. Bergy

    Bergy Member Industry Professional Forum Leader


    The Grundfos circulator shown in the above photo needs to be remounted. The shaft is water cooled and needs to be mounted so the shaft is horizontal to the floor. Yours is mounted with the shaft slightly up hill, this will burn out the circ in short order.

  19. zacmobile

    zacmobile Guest

    not to blame the t-stats or anything ;) but just out of curiosity what brand or kind of stats do you have? if they are not designed for the thermal mass of a radiant system they may not be able to anticipate the thermal momentum of the slab cooling down.

    that sound like the problem right there, the ranco aquastat should be keeping that buffer tank satisfied all the time and never powered down, like it is will cause exactly what you're experiencing.
  20. kandk920

    kandk920 Member

    The stats for the radiant are TAC T200 series.

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