Ever a good reason to use desuperheater as radiant heat source?

Discussion in 'Radiant Heating and Cooling' started by kajinka, Oct 3, 2018.

  1. kajinka

    kajinka New Member

    Hi, I know that the desuperheater is generally not a good source for radiant heating. However, I have a situation where it seems to make sense and I'd love to get thoughts on it.
    This is a 1978 house in Maryland that was retrofitted with a 6 ton Waterfurnace Synergy 3D heat pump five years ago with a desuperheater piped to a buffer tank in front of an electric water heater. It has been running only in forced air mode so far. As part of a complete overhaul of the house, radiant heat tubing was installed in all rooms except the unfinished basement (majority is in tiled floors but basement rooms had to get the tubing into walls in front of R20 insulation since the ceiling clearance was too low and just barely fit 1 inch of R5 insulation). We are finally ready to connect the radiant to the Synergy and here is the logic for the desuperheater to be redirected to the radiant tanks (paired 80+50 gallons Boiler Buddies).
    1. Hybrid water heater will replace the old one next week since it is at end of life and the rebates make the hybrid one cheaper than a straight electric (no gas lines in our neighborhood and I don't want any tank on the property). It will also help with dehumidification of the unfinished basement in our very humid summers.

    2. The Synergy was turning on the auxiliary heat for forced air for a fair amount of time during the past winters, so even the extra 10% or so of output from the desuperheater would be good to have in the radiant tanks.
    3. In the summer, the basement rooms have all registers closed because they get uncomfortably cold if they don't. We end up having to run dehumidifiers in every room all summer in our very humid area. Hence, we would like to actually use low amount of radiant heating in those rooms sourced by the desuperheater while the Synergy is in cooling mode. That would allow us to open the registers for A/C and ideally get rid of at least some of the dehumidifiers. The desuperheater would be heating the 50 gallon radiant tank while the 80 gallon one would be shut off in the summer. I know the desuperheater would not heat the radiant tank on demand, but I'm hoping it would be enough for this purpose. I don't know if it would actually make more sense to leave both the 50 and 80 gallon tanks open for the desuperheater to heat in the summer or limiting it to 50 is a better idea.

      This makes sense to me but that could be just me. I'd love to hear others' view.

      Thanks a lot,
  2. ChrisJ

    ChrisJ Active Member Forum Leader

    Seems there should be a more efficient way to reduce humidity then heating up the rooms to be able to cool them off.

    Heat pump hot water heater. "It will also help with dehumidification of the unfinished basement in our very humid summers."

    Ok so maybe the insulating was only done in finished rooms and more of the basement is unfinished.

    I guess I would look into reducing the humidity by air sealing and insulating the unfinished area.

    Could the Synergy unit switch from air cooling to radiant heating in the summer?
  3. kajinka

    kajinka New Member

    I don't know how frequently would the Synergy be able to switch between air cooling and radiant heating in the summer. I had asked the rep about it a while back and it sounded as if switching a few times back and forward during the shoulder season is fine, but switching multiple times every day all summer wouldn't be a good idea.

    The basement rooms are already too cold with the registers closed. We want to be able to heat those rooms selectively in the summer even if we still have to deal with the humidity through dehumidifiers. We just ran into big trouble this past summer when grandpa stayed for a month in the guest room down there and kept the window open the whole time since he likes a toasty temperature. He also dislikes the noise a dehumidifier makes, so that was a really bad combo in our humid climate...

    Reducing the flow of moisture into the unfinished basement is definitely one of our top priorities by next summer. We just happen to have a plumber friend coming to stay with us for a couple of weeks starting next week to do some sightseeing. In exchange he offered to help, so we are going to get the hybrid water heater swapped in and if we don't hear fatal flaw comments, we will just give this experiment with redirecting the desuperheater a try to see what happens. We definitely don't want to break anything, but I'm hoping that wouldn't be the case. If this just doesn't work well for what we hope, we'll switch it back to the typical setup. Redirecting the pipes is within our skill set.
  4. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    The DSH will take up to 10% capacity away from the unit.

    However, if the water is warm already, the contribution is not very high due to the low delta T. Plus the understanding is that you use the Synergy's hydronic part for your radiant floors, correct?

    You are getting a air source heat pump water heater, correct? You have radiant with 2 buffer tanks, correct? Why do you need a desuperheater for radiant. It does not add much, if anything. Does not make much sense, and just adds complexity.

    For your humidity, fix the infiltration , and get a dehumidifier if needed.
  5. ChrisJ

    ChrisJ Active Member Forum Leader

    I read a thread you started 6 yrs ago. https://www.geoexchange.org/forum/threads/loop-and-duct-sizing.5092/

    So a 6 ton unit was installed not a 5 ton. In first stage it runs at all of 4 tons. Do you get long run times in the summer?

    Did you have it zoned as mentioned in the above thread?

    "The Synergy was turning on the auxiliary heat for forced air for a fair amount of time during the past winters,"

    Aux heat coming on a fair amount could be a control/thermostat issue, 6 tons is a large unit.

    "so even the extra 10% or so of output from the desuperheater would be good to have in the radiant tanks."

    Like Doc said the Desuperheater takes away capacity, it is not extra 10%. I would leave it connected to a non-powered electric tank ahead of the heat pump hot water heater.

    Docjenser has experience with the synergy units, he would probably know if it's ok to switch back and forth from Cooling to water heating.
  6. kajinka

    kajinka New Member

    You're awesome for looking up the old thread! We ended up with a 6 ton unit because we decided to add 22 skylights to the house to get some more daylight in given that we are surrounded by poplar trees. The installer sounded very convincing back then that a 6 ton unit was needed with that. To deal with the overheating upstairs, we ended up running new bigger ducts up there without creating a separate zone for it. After a bit of tweaking of the manual dampers it worked out great.
    Thanks a ton for the explanation on desuperheater not actually adding to the unit total output and on the hint that the auxiliary heating could be a control issue. That all helps.
    I did not watch out the run times in the summer. I will during the next summer. We will hopefully have the unfinished basement insulated by then, so we will know better where we stand with humidity. In general, we probably don't keep the house as cool in the summer as others might, so we may not be giving the A/C enough time to dehumidify (plus having the 6t unit makes this worse). I wonder whether there is a anything in the setup of the unit that can help increase the humidification since I understand that slower flow is better (stages, blower speed...or anything else I don't fully understand)

    On the note of a setup, does anyone have a recommendation for a good installer in the Washington DC Metro area by any chance? We'll do the piping for the radiant, but I'd love to have someone who really understands the unit do the connection to the Synergy and the setup of the controls. The original installer we went with is a big company in the area, but when we invited them back a couple years after the initial install to bid on the radiant part it turned out they really only had one designer for geo and he had since left. It also turned out that they had provided an undersized buffer tank for the radiant and were not willing to correct it. They weren't even able to submit a bid for the radiant, so I don't have a lot of trust in them. They didn't care when I told them that we see a flash of light reflecting through the ducts on the main floor when the unit sometimes comes on..... Hence, any help finding a better company would be greatly appreciated.
  7. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    The lesser you have a unit do, the better. I do have a synergy in my house, doing radiant heat, 100% of the domestic hot water, and summer A/C via the forced air portion.

    Working well for 8 years right now, but I would put in a couple w-w, both there for hydronic heat, but one also for summer A/C, and one also for 100% DHW.

    Overall, we moved away from the SYNERGY, it is not very resilient to have one unit do too many things. The reversing valves are not too reliable, and also the pressurizing- depressurizing seems to create issues long term.

    For better dehumidification, simply slow down your blower.....check the dip switch setting. You can also force the unit to come on in first stage only, which is roughly 4.5 tons.
    For piping, make sure you follow the direct to load principle, it is really important for the efficiency, and puts lesser stress on the unit. Buffer tank size is really not that important, what is yours?

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