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Vitodens - No DHW Priority

The Vitodens will have one temp DHW production (either high fire or 20° higher than tank temp depending on coding).

Most hydronic AHU's are rated for potable water. One solution is to run the AHU's out of the Vitocell and use a mixing valve for House Hot.

Alternatively, you can cut in a NC relay to the 24V solenoid inside the Vitodens to kill the fans when DHW is called.

We have done both.


  • larry
    larry Member Posts: 91

    We have a Vitodens (model 8-32) installed with an 80 gallon Vitocell 300 on the DHW circuit and two low temp oriented hydro-air handlers on the space heating side (and maybe soon some panel radiators to supplement a sun room). The system is setup with DHW priority set ON. The installing contractor (who has since moved to a different part of the country) recommended the 80 gal tank because of a whirlpool tub. That aside, our normal DHW needs are much more modest and even use of the whirlpool will still leave the tank with reserve.

    For as wonderful as our air handler are, they are dumb when the Vitodens starts servicing the DHW circuit and will continue to blow until hot water returns and the need is satisfied. I know there are probably a number of solutions to this problem. As a possible easy solution, I'm curious what behavior the Vitodens takes if the system is set so DHW priority is turned OFF. Does it then wait until the heating circuit is satisfied before servicing the DHW circuit or does it try to do both at the same time? If it does wait for the heating circuit to be satisfied, what happens if it's in the middle of producing DHW when the space heating side calls for heat? Does it drop the DHW side and switch back over?
  • Steve Ebels_3
    Steve Ebels_3 Member Posts: 1,291

    Install an aquastat that will keep the fans off until the piping feeding them is up to temp. A simple strap-on Honeywell will do this by installing it in series with other fan control devices on the AHU's.
  • Good one

    Simple, smart, effective.

    Good plan.

    Love that

  • larry
    larry Member Posts: 91

    Thank you for the excellent suggestions. Here is some more info about the setup that complicates things a little bit...

    Yes, these AHU are rated for potable water, but it's already piped and working with a shallow reset curve between about 105 to 140F on the boiler side. Even on a design day (O degrees) it works fine firing to only 140F supply temp - not too bad for hydro-air. If the AHUs get put off the Vitocell, wouldn't I be giving up some operating efficiency as DHW production won't let the Vitodens condense as much as I'm getting now? Don't have any idea if I'd be giving up a lot or a little efficiency by doing this. Normally we have the DHW set to 122F, so this approach would probably mean setting it up around 140F as well..

    When the contractor did the install, I had him put a flow switch on the DHW side so I could know when it was running (with the thought of solving this problem). He didn't know you could have just tapped into the Vitodens and gotten the same information. My original intent was to use a relay and fake out the call for heat line (white wire) going to the AHU control board so the AHU would think the tstat was satisfied. Would you view this as an acceptable solution?

    The AHU control board is semi smart in that it looks at time to satisfy setpoint and varies the blower speed based on recent history (it intends to run the blower as slow as reasonably possible). Even after setpoint is reached it continues to trickle warm air out at a very slow blower speed for an additional 15 minutes. The goal is nearly constant gentle heat even though it's air based. If you cut the blower, the control board would think setpoint wasn't being reached and it would dramatically increase blower speed as it's response. Even though a relay cutting the blower would stop the blower, the control logic would think it's either gotten VERY cold or your in some setback recovery mode. Because of this, when the hot water returned, the blower would be going at a very high speed and subsequent cycles would also start off fast because it keeps track of recent history.

    The AHU logic board already looks for pipes being hot before starting up the blower. The problem is once it's started it doesn't look for the pipes going cold (i.e. DHW priority kicking in) in the middle of a cycle. I've heard the company has updated the control board to solve this problem, but it would be a very high cost (~$600 for 2 control boards) to replace them.
  • Your 1st idea

    In the Vitodens there is solenoid which directs water either to DHW tank or the heating circuit. There are 2 small wires with small spade connectors that connect to this solenoid. One of the connectors has "30" marked on it. This solenoid is activated (or de-activated...don't remember which one) via a 24V signal. A 24v coil relay connected here will cut out the white wire to the ahus

    By the way, this is true ONLY in the 6-24 or 8-32 model. The 11-44 or 15-60 does not have this set up.

    This is how you could "fake out the AHU's to think that the call for heat has been satisfied.

    Working with the sophisticated electronics I'll have to think on a little bit.

    Whose fancoils are these? I am most familiar with ESP out of Edmonton, Alberta.
  • larry
    larry Member Posts: 91

    Thank you for confirming this approach as good.

    The units are from Ecologix in Cambridge, Ontario.

    Website - http://www.ecologix.ca

    Other than this solvable issue, it's worked well in our house. Website isn't much to look at.
  • Howie_6
    Howie_6 Member Posts: 8

    I also used this approach... Works well.
This discussion has been closed.