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  1. #1
    Join Date
    Mar 2008
    Posts
    8

    Variable Speed AHU Questions

    Late last summer I had a contractor install a new American Standard (Trane) variable speed air handler and A/C unit along with a new Trinity condensing boiler and a heat coil in the AHU. I thought the system would be "smarter" than it is. I haven’t used the A/C much since it was the end of the season so my experience and questions so far are mainly about the heat side. The boiler has an outdoor temp sensor and resets the boiler output temp according to changes outdoors. The AHU fan runs at a low speed (compared to the A/C) all the time. The (not so variable) fan speed is what disappoints me. One problem is that when the heat is satisfied and the boiler is off, the same air volume circulates and it’s uncomfortable. At the other end of the scale when the heat comes on in the morning after being set low overnight it takes forever to bring the house up the temp at the low fan speed. It seems to me the AHU fan should drop to very low CFM with heat satisfied and then ramp up to high volume when the room is more than a degree or two below set point. The t-stat is programmable and is capable of sending virtually any signal to the AHU but the installer told me there’s no way to wire the AHU to vary the fan speed the way I hoped and expected. Is he correct? I suspect the answer is no. Please advise what I can really expect of the system and what specifically to tell the contractor I want.

  2. #2
    Join Date
    Mar 2009
    Posts
    4
    they should defenitly be able to wire the system to have differnt running speeds for heat, cooling, and fan only operation. There should be low voltage terminal strip in the air handler with connections marked w, y, and g witch will be for t stat connections. it sounds like the system is not wired correctly as u suspected. U can also run the boiler without the outdoor reset so the boiler will run at the full temp it is set at if u would like. Naturally this will cause it run alittle more in warmer temp but your air will be warmer.

  3. #3
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    68,586
    Tell him you want it to work the way you said.

    He just doesn't have the wiring and or dip switches right.

    He may not know how to wire it right to work the way you want.
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  4. #4
    Join Date
    Mar 2008
    Posts
    8
    Quote Originally Posted by beenthere View Post
    Tell him you want it to work the way you said.

    He just doesn't have the wiring and or dip switches right.

    He may not know how to wire it right to work the way you want.
    I'm sure he doesn't know. That's why I ask what specifically to tell him to do. Is there somewhere I can refer him so he can learn the specifics of this AHU? He's otherwise a very diligent contractor.

    Does the AHU simply do what the t-stat tell it, or is there some automatic ramping of the fan speed internal to the AHU?
    Last edited by tominoh; 03-04-2009 at 12:00 PM. Reason: Add info

  5. #5
    Join Date
    Jan 2009
    Location
    Keokuk, IA
    Posts
    5,520
    Step by step instructions can't be given on here.

    Is this information in the installation manual? I would think it is. Perhaps a pro can e-mail him a link to the manual... then he can refer the installer to the correct page in the manual.

    It's also possible he does "know" but didn't pull enough wires form the thermostat to do what you want.

    The total BTU's delivered form the hot water heat exchanger will be limited by the air flow rate... so you definitely have an operating issue.

  6. #6
    Join Date
    Mar 2008
    Posts
    8

    OK, Thanks

    Thanks, guys. I get that I have a real problem and will challenge the contractor to fix it.

  7. #7
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    34,471
    The lowest speed should only be when the fan on is selected on the stat and the stat is not calling for heat or cool. I think I know the problem and it may take a relay to solve. If he has half a clue about wiring, he shouldn't have trouble figuring it out! Actually I'd use 1 relay and 1 jumper wire. I am assuming you have a hot water coil this air handler is blowing through?

  8. #8
    Join Date
    Mar 2008
    Posts
    8
    Quote Originally Posted by BaldLoonie View Post
    The lowest speed should only be when the fan on is selected on the stat and the stat is not calling for heat or cool. I think I know the problem and it may take a relay to solve. If he has half a clue about wiring, he shouldn't have trouble figuring it out! Actually I'd use 1 relay and 1 jumper wire. I am assuming you have a hot water coil this air handler is blowing through?
    Yes there is a hot water coil. The full system is described in my original post. I know you guys don't want to give specific advice but let's assume my guy has something less than "half a clue". Is giving him the hints mentioned... relay & jumper going to prompt him enough to solve the problem? ...or please give me more.

    ...and while I have your attention... just what is the control logic I should be looking for? Should the fan be ON all the time in heat mode as it is? Of course the stat calls for heat from the boiler when the room is anywhere below setpoint. The temp of the water is set by the outside sensor so no issue there. It's only the fan that's at issue. When room is below setpoint by more than 1-2 degrees it seems to me the fan should kick up a notch... and if more than 4?? degrees below setpoint another notch up? Given what I have now, I think I need to tell my installer exactly what I want. Obviously he needs some guidance. Problem is, I really don't know what I'm suppoed to want. I'm just the homeowner. So if you could answer those questions I would be grateful. By the way I DID question my guy about this and got "what you see is what you get" (but in a nice way). When I go back to him I need to be able to approach him with respect but also from a position of knowing what I can/should expect or he might tell me it can't be done. He already did once.

  9. #9
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    68,586
    Tell him to wire it up as though it had electric heat.
    And the relay is for isolation.

    More then that. We can't say.
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  10. #10
    Join Date
    Jan 2009
    Location
    Keokuk, IA
    Posts
    5,520
    Will the fan speed be limited to the cooling fan speed or can you set a seperate speed for heating?

  11. #11
    Join Date
    Jan 2004
    Location
    Lancaster PA
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    68,586
    Heating and cooling will be different speeds.
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