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Aquastat controling Unit Heater

I sketched out the piping arrangement for a hydronic unit heater in my basement. I'm wondering if this piping arrangement or temperature settings are right or if I should reconfigure it somehow. If I give it a call from the thermostat, the zone valve kicks the boiler on, but it takes a very long time for the fan to turn on. The boiler will actually cycle on and off a few times before the fan turns on.
If I turn the aquastat down, the fan will stay on too long, sometimes when the zone isnt even calling, so it ends up blowing cold air. This is all within 6 feet of the boiler, so I'm wondering if the proximity of the aquastat to the other zone and boiler piping might be part of the problem.
Beyond that though, is this a normal setup? I feel like the ideal thing would be to have a relay enabling the aquastat so that it will only work when the thermostat is calling, then I could turn it down much lower and not have to worry about it coming on when there is no call. Has anyone ever done this?



  • icesailor
    icesailor Member Posts: 7,265
    If that was connected to steam, the steam would come into the top, and the condensate would come out of the bottom.

    I never connected a steam one.

    All the ones I ever connected, had the hot water coming in the top and coming out the bottom with the 006B control on the bottom. As it was explained to me by my old dead boss, the hot water comes in the top, gives up heat as it flows through the heater, and looses heat through the coils. Once the hot water gets to the bottom, the control starts the fan. The cold goes back to the boilers. (Why unit heaters should be piped as parallel reverse returns) Not until the return gets hot does the fan start. Once the fan starts, the return outlet coil water can drop to a point that the control will shut off the fan. Or, why you usually set them for 110 degrees.

    I think that your drawing is backwards. Because every wall hung unit heater I ever installed had the control on the return, and on the bottom.

    Run it with the fan off. See how hot the return water gets. Turn the fan on to high, Feel how fast the return water drops in temperature. That's the fan extracting BTU's from the water
  • Milkman_Spawn
    Milkman_Spawn Member Posts: 6
    edited February 2015
    Icesailor, I agree with what you said about how the supply should be on the top and return on the bottom. However, the sketch is accurate--the supply comes in the bottom and the return leaves the top. The control is actually on the return then.

    I guess I could try moving the aquastat further from the other return piping. I feel like there is some flow being induced even with that zone off because i'll still hear a whirring noise inside the heater.

    Is the relay idea unheard of?

    Edit: Here is a picture

  • Zman
    Zman Member Posts: 7,493
    I looks like it is piped correctly.
    http://mesteksa.com/fileuploads/Literature/Sterling Gas Products/Indoor Products/HIM-16.pdf

    The problem you are having is that the flow stops when the call for heat stops. The section of pipe the aquastat is on never cools with the fan coil because it is to far away from the coil. You might fix it by moving the aquastat closer to the coil.

    The relay is a better idea.A simple relay with a 24v coil wired in parallel with the zone valve would do it..

    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • billtwocase
    billtwocase Member Posts: 2,385
    I always instal the aquastat on the supply. Big difference in operation
  • billtwocase
    billtwocase Member Posts: 2,385
    also, I always feed the bottom, return and vent on top
  • billtwocase
    billtwocase Member Posts: 2,385
    you can also lower your control setting to about 110-120
  • Milkman_Spawn
    Milkman_Spawn Member Posts: 6
    Interesting, Zman. I didnt realize the supply is supposed to enter on the bottom for hot water.

    I'll try moving it closer. Failing that, I'll put a RIB relay in there like you said.
  • billtwocase
    billtwocase Member Posts: 2,385
    relays are OK, but keep in mind that you will start out with cool or cold air. I prefer a black "T" at the inlet with a well installed. I am not much of a fan for strap on controls
  • SWEI
    SWEI Member Posts: 7,356

    I prefer a black "T" at the inlet with a well installed. I am not much of a fan for strap on controls

    Especially without any insulation!

  • icesailor
    icesailor Member Posts: 7,265
    Is there an air removal device on the top/return of that piping?

    If you put your hand on the supply and return, and the other main zone isn't running, and the boiler is hot, does the water rip into the heater and come out rapidly and back to the boiler return?

    I only see one zone valve. Where is the other one? Or do you only have one?

    The heater isn't piped exactly as you show. Are you treating the unit heater as a separate and single zone separate from the above floor? It acts like there isn't enough resistance against the other zone to make the water flow properly through the unit heater. If there are two zone valves, and that is a second zone, I have never seen one like that unit heater not work. If you don't have a second zone valve to control the other zone, and the resistance through the coil is greater than what the system makes, it will be like Mark Etherton's lazy Brother In Law. Rowing a boat with one oar.

    Without an air vent on the top, how do you get the air out of the coil?

    Water coils are counterintuitive to steam coils where the hot steam comes in the top and the condensed steam water goes out the bottom. I always had to RTFM to be reminded that with hot water coils, they want the lighter and rising hot water to "push" the cold water out of the coil from the bottom to the top. Nothing on the Modine web site showed any piping. Like the Sterling one does.
  • Milkman_Spawn
    Milkman_Spawn Member Posts: 6
    Its hard to see in the picture, but there is a second valve behind the makeup. Its an older Taco w/ a green head to the right of the new one.
  • Harvey Ramer
    Harvey Ramer Member Posts: 2,239
    The symptoms you are describing lay the blame at the feet of the strap on aquastat. They respond pathetically slow and are less then ideal for your application.

    Here is what you need, http://www.etcsupply.com/

    Install this control and strap the sensor to the pipe close to the unit. Make sure you insulate it.

    I find them very reliable, less expensive than an aquastat and more features and ranges. It'll also give you a digital temp readout, which is nice. They can be used as high limit or low limit and sport a 1-30 degree differential setting.