Hydro-Air system blowing cold air after thermostat is satisfied...

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Old 12-13-11, 03:45 PM
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Hydro-Air system blowing cold air after thermostat is satisfied...

I have a two story house split by two hydro-air systems with hot water coils fed from an oil fired boiler. The upstairs unit is in the attic and the downstairs unit is in the basement.

It appears the downstairs unit runs for a long time when the thermostat calls for heat. The unit turns on, blows hot air, then you can hear the thermostat satisfied and while the fan still on, it starts blowing cold air. I've timed it and it continues to blow cold air for 5-10 minutes. I was told the unit only turns on if the thermostat calls for heat and when the HW supply line mounted thermostat close to the unit has a satisfied temperature.

The upstairs unit is similar (but smaller). It appears to be working properly and shuts off about 30 seconds after the thermostat is satisfied.

How can I eliminate the "blowing of cold air"
 
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Old 12-13-11, 04:59 PM
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Has it always done this? Do you have history with this unit, or just recently moved in?

I was told the [blower] unit only turns on if the thermostat calls for heat and when the HW supply line mounted thermostat close to the unit has a satisfied temperature.
That would be correct, presuming it's properly installed.

What's the make/model of the AHs ?
 
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Old 12-14-11, 04:44 PM
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I had the unit moved by a HVAC tech about 10 feet in order to install a chimney. The HVAC tech doesn't do plumbing so I replumbed the hot water lines from the original location to the new location.

The unit is a "First Company" MBXB-HW Series Fan Coil. It has a supply line mounted Honeywell L6006C Aquastat.

Does adjusting the dial on the aquastat shut the unit off sooner? Do I lower or raise?
 
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Old 12-14-11, 05:48 PM
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So what you are saying is that it worked properly before being moved, and not afterward, correct?

I believe this is the manuals for your unit:

http://www.firstco.com/firstcoimages...structions.pdf

http://www.firstco.com/firstcoimages/pdf/mbxb-hw.pdf

It has a supply line mounted Honeywell L6006C Aquastat.
Does adjusting the dial on the aquastat shut the unit off sooner? Do I lower or raise?
Here is a PDF file for that aquastat:

http://customer.honeywell.com/techli...0s/60-2104.pdf

Note that some models of the 6006C feature an ADJUSTABLE DIFFERENTIAL. You should check for this on your aquastat. Refer to the PDF file for details on how to do that.

I believe that a decent setting on your unit would be 140 on the main dial, and 5 on the differential. If the differential was accidentally increased to 30 then that would explain why the blower continues to run on after the coil cools.

Does the other AH have the same control on it? Compare the settings on that one to the other.

If you can get the FULL model number of the 6006 I can tell you if it has the FIXED or ADJUSTABLE differential setting... (but you can tell by looking also).

OK, that should keep ya busy for a while.
 
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Old 12-14-11, 06:30 PM
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NJ,

Thanks for the find. After reading the manual, looking at the wiring schematic, and doing a little thinking, I think I solved the problem!

I never looked in detail at the setup before moving. I think what was happening can be described by:

1. Room mounted t-stat calls for heat.
2. This turns the zone on at the boiler and circulates water
3. Supply line mounted Aquastat (set at 120) is then satisfied with 120 degree water and relays power to the fan motor. Eventually 180 degree water is flowing through supply line.
4. A.H.'s internal delay starts fan motor.
5. Hot air circulates and satisfies t-stat
6. Circulator shuts down.
7. Aquastat is at 180 and takes a LONG time to reach it's differential temperature. Coil becomes cold and begins blowing cold air.
8. Because the Aquastat is mounted on the supply line and the water is not circulating, the ambient temperature slowly cools the supply line to satisfy the aquastat and shut the A.H. down. (not to mention, I have 1" thick armaflex on the HWS/R lines!)

By relocating the aquastat to the return line, the A.H.'s fan begins when the return water is 120 and shuts off at 100 (20 degree differential). Problem solved!!!! No cold air blowing after the heat cycle. After moving the unit, HVAC tech or myself must have placed the aquastat from the return to the supply.
 
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Old 12-14-11, 07:16 PM
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It has a supply line mounted Honeywell L6006C Aquastat.
I see where you have now properly moved the aquastat but for anyone else reading the aquastat ALWAYS goes on the return piping in this kind of installation.
 
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Old 12-14-11, 07:46 PM
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That's what I get for reading too fast! Furd is 100% correct!

Although, I think 20 diff might still be a bit much. I wouldn't think you would get much warm air with 100 water, but if yer happy, then I'm happy!

Good on ya mate!
 
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