Can Geo tie into radiant and air systems?

Discussion in 'General Discussions' started by centipede16, May 29, 2011.

  1. centipede16

    centipede16 New Member


    My home is 3500 sq. ft. and tightly insulated (closed cell foam). I have an existing oil furnace that supplies hot water for radiant distribution (Warmboard). I also have a separate traditional A/C with ducting for cooling in summer. If I were to add in a Geo system, can it tie into both the water-to-water for heating (while leaving the existing oil furnace as a backup if needed), and also tie into the AC ducts (water-to-air) for cooling (while leaving the AC system as a backup if needed)?

    Thanks for any info and suggestions.
  2. Designer_Mike

    Designer_Mike Member

    You could but controls will be much more complex.

    I think it would make more sense to just have the Geo hooked into the forced air side and leave the hot water side alone. It would be fairly simple to either pull your air handler and drop in the geo heat pump and leave the existing evaporator coil in the ductwork OR your could get a split geo system and put the geo coil into the existing air handler.

    There are probably ten different options that a good contractor could evaluate and make recommendations on.
    I would talk to a couple different guys to "pick their brain" on the most economical and efficient way to do it.
    Personally, I would pull the existing AC system and drop in a standard GEO and leave the oil burner/radiant as aux heat. Odds are the existing radiant heat is designed around 180 deg water temp so the much lower temp heat pump water would be have a hard time getting the BTU into the house. Also, once the oil burner fires up, the geo wouldn't be able to do anything since the boiler would heat the water way above the heat pump capacity so it would have to shut down.
    IF you leave the radiant alone and use the heat pump with forced air, it could keep putting out max BTU while the radiant simply adds a couple BTU to the house on the cold days.

    A good thermostat could easily handle the controls without doing anything crazy.
  3. urthbuoy

    urthbuoy Well-Known Member Industry Professional Forum Leader


    In theory and practice it can.

    But, load and distribution work is in order to assess your options.

    Your radiant spacing may be inadequate for lower temp geo. Your ducting may be inadequate for higher flow geo. But there are multi-use units, ecm blowers, higher temp water units, fan coils, and so on that could all play a part.

    With the right controls, one can consider radiant cooling as well. But humid climates can make that an uphill battle.

    Worth having a good contractor go over these options with you.

    (edit) replied before seeing Mike's comments.
  4. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    Given you house size and insulation status, you should get by with one heatpump. Usually, radiant supply temperatures are below 120, especially with top of the floor applications, so well within the operating range of the heatpump. You can leave the existing oil oiler, but a supplement heater in the air ducts is cheaper than the re-piping, and easier to control and for sure cheaper to maintain. I never heard about a backup a/c.
    Keep it simple, get a hybrid packaged unit, like the synergy 3D from Waterfurnace, or the combination unit from Hydron.
    Get a buffer tank, and you are good to go. We have done quite a bit of those systems, in some we kept the original furnace, but only for needed supplement heat to keep upfront cost down. Again, once you have the strip heater, no need for any other backup. If you decide to keep your oil boiler, you would need a multistage boiler control. If you can part with your old equipment, with one multistage thermostat you can control the hybrid unit, you just need to decide what to prioritize on. Here are some of those systems with the supplement boiler, one without it.

    Temperature and Energy logging by: Web Energy Logger
    Web Energy Logger:
  5. zacmobile

    zacmobile Guest

    hydro air

    I do quite a number of these combo systems using a hydronic air handler and radiant floor both sourced from a single water to water heat pump and I quite prefer this approach to a combined all in one unit like the synergy 3D or a separate water to water and water to air unit in tandem. They tend to be less expensive to install and are more flexible and even simpler in some respects. The controls are not necessarily more complicated but you do need to know what you are doing or things can go wrong in a major way (like anything with geo really).

    Warmboard actually works very well at low water temperatures even though it has 12" spacing because of its good contact with the pipe and heavy aluminum heat exchange surface.

    A scenario I could envision for you would be a water to water heat pump and your existing boiler (if you have one?) both feeding into a purpose built geo buffer tank (Bock makes them) with 2 supply inputs (1 geo, 1 backup), then from there you could feed both a hydronic air handler and your radiant floor in parallel with their own separate pumps. You could do away with the oil altogether as the buffer would have a 4KW element as a second stage backup.

    This would all be managed by a geo hydronic control like an HBX CPU-500 or ECO-1000, the cooling switchover could be a manual switch or activated by a cooling call from your air thermostat. An interlock relay should also be employed to diasble the radiant pump when the system is in cooling mode, unless you set your cooling system supply temperature no more than 2 degrees below the air dewpoint.
  6. centipede16

    centipede16 New Member

    Thanks for your inputs. It is a unique situation I'll admit, where I invested in the conventional boiler and AC, and the rest of the house is ideally designed for a Geo fit, and now I'm looking to "add in" Geo. I can say that heating the house with the radiant (vs. forced hot air) is a given - the water temps are ridiculously low (as Warmboard indicated they would be). I believe the boiler cycles between 130-160, but then the radiant mixing board temps are around 90 degrees or so in the sends, perhaps on a very cold demand day they might go up to 100, but I'm generally seeing 90 degrees or so on the board guage. So, that makes it an easy job for the Geo. I was expecting that the Geo would essentially be "spliced" in so that it feeds the radiant distribution, and then if it weren't working for some reason, then as a backup the boiler could then kick in. I should also mention that the boiler heats an indirect tank for DHW, so I don't know that that role would go away, although a desuperheater would presumably heat the indirect in the summer? I guess I'm not necessarily looking to ditch the conventional boiler/ac systems, since they're already baked into the mortgage, I'm just looking to keep oil usage to a minimum (DHW only). On the cooling side, I could just leave the conventional AC but was more less curious if the Geo could (cost-effectively) tie into the ducts and again, if the Geo was not working for some reason, then let the conventional AC kick in, etc.
  7. zacmobile

    zacmobile Guest

    duct coil

    I guess it all depends on what your idea of cost effective is :D If you wanted to keep the existing boiler, and i'm guessing it is atmospheric (130-160) I would still do the buffer tank thing, you would just keep your mixing valve in between the boiler & the tank, there are rare scenarios where you can get away without a buffer tank with geo like if you had a big warehouse with multiple stages of heat pumps for example, but for pretty much anything else you will need one. However you could do without a whole new air handler and just splice a water coil in your existing duct work fed from the buffer tank. You also could do all your DHW with the geo as well with a Climatemaster TMH, it has separate DHW connections and will ramp up to a higher temperature to supply the indirect tank, but it needs a large heat exchange surface to do this which would probably mean needing a new tank too, but at the moment the TMH's are pretty pricey, in the lower 5 figure range or so. Just some ideas...
  8. centipede16

    centipede16 New Member

    To the cost-effective point, I guess I'd be curious about the cost of electricity to run the Geo for cooling. If it basically costs the same amount of electric to run the conventional AC as it would to run the Geo, then I likely wouldn't try to tie the Geo into the cooling/ducts. So then my primary Geo interest is toward heating, and secondarily the AC, assuming there is an ROI there. I'm kicking myself for not just putting in Geo from the outset, but since all these systems are only a few years old, I don't want to just kick them to the curb either :(

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