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Control Strategy?

I went there today at about 1:30 pm. Cloudy, windy, ODT 67.3*. The heat was disabled for the last two days. All 9 indoor stats read 69-70 and all are set for 70. None were calling for cool. Basement 69*. The slab has been off for a week at least and I swear you can still feel the heat coming out of it when you walk down there. The ODT the night before had gone down to 49* at my house. So I reenabled the auto mode and set the CW for 68* ODT and the HW for 53*. If the ODT drops below 53* the plant will start producing HW and the floors will start once the loop goes over 90*. I also started breaking the HO in to the idea of maybe needing to take somewhat of an active role during times like this. It would be a bit simpler if it weren't for all the stats. It does look like if it goes below 53* ODT the chances are slim any stat will call for cool. Now we wait and see.

So while I was there working on other things I noticed I had a call for cool on the Taco board and I'm thinking crap, I don't have any chilled water. So I go over to the pump room, lo and behold the plant is running and I DO have chilled water. The ODT had risen to 70*. :)


  • Unknown
    Outdoor Temperature

    You may or may not be on board with the various posts I've made on the new air to water heatpumps I've installed here in CT. They have a feature built in where they will shut down and come back on by outdoor ambient and switch themselves back and forth from heat to cool accordingly. They are 2 pipe so the hot and cold water use the same circuit. I have manual H/C t'stats around the house and just recently set them all to cool. Now, watching the weather here, I'm seeing some pretty wide swings to where, the way they are set now, the heat pumps are most likely running in cool during the day and heat late at night so I'm envisioning a t'stat calling for cool at 3 a.m. and getting heat. What is the lowest heat "on" setpoint ODT you have been able to get away with before the customer starts to notice he's chilly. I'm set for 63* right now and I think that's the threshold for an extended period. Any thoughts? I CAN manually disable the H or C accordingly by season but would rather it were automatic.

    There's more. Mostly FYI so you have the whole picture. ;)

    The slabs and some floors are heated. I've turned the slabs off for the summer and have the floors set so that if the main loop temp goes above 90* they come on. Meaning that, right now, the floors actually WILL heat overnight if the ODT goes below 63*. The floors are controlled by floor temp and set for 77*. I see this as desirable for comfort but not so much for overall efficiency.

    Do you have any other ideas as to how to best approach this using what is installed?
  • hvacfreak
    hvacfreak Member Posts: 439
    I'm not real sure

    what this system consists of( is water the transfer medium , or air ...typical outdoor condensing unit ) ? Well , either way , I might look into a control like Carrier / Bryant " Thermidistat ". I once used these for some air / air hp's with hydronic reheat...( duel fuel option is standard and has balance point and such ). I was able to set these up to use hot water as aux. heat and to change - over completely based on outdoor temp.

    If your looking to operate these systems as " typical residential " , I would check out the Thermidistat. If you looking to maintain water temps , I'm sure Tekmar would have a solution. Perhaps looking at both controls will give you some ideas ( along with some stats and relays to cure the " what if's ". Good luck. -M


    Edit : found install manual for Thermidistat
  • Unknown

    i've set up a few similar systems using ground source heat pumps. my solution (with invaluable help from Hydronics Mike from tekmar) was to use the tekmar tn4 system which has a system wide heating/cooling interlock that will prevent any heating demands when any cooling is happening. and to handle the cooling source control i used a 152 two stage setpoint control: one sensor as an outdoor sensor to enable cooling at a certain OD temp (energize the reversing valve) and to provide cool weather shut down a few degrees above the main system warm weather shutdown point to provide a dead zone buffer. the other sensor is in a chilled water storage (buffer) tank.

    it can be a complicated situation when you have heating and cooling sharing the same piping and source, but this system has worked well for us so far.
  • Unknown
    Thanks guys...

    Yes it does get complicated, as soon as you start thinking about the transiton periods between heat and cool seasons, and an interlock is exactly what the situation calls for. I'll take a look at these controls. A four pipe would have been nice but the additional cost would've just been way too much.

    The heat pumps are air source to water which I then send to some floors and some fan/coils as needed. They use ammonia as the refrigerant.

    The air handlers are my concern. They're controlled by programmables and all set back 4* at night. I'm thinking that on a 63* night the chances of any room going up 4* is practically nil.
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