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Question: How can I wire my fan ON/OFF to be controlled by the thermostat heat call instead of using an aquastat or fan-limit switch?

Background info:
I have a hydro-air system with an aquastat controlling the fan for heating, thermostat controlling the fan for cooling. Problem has been that the fan was staying on anywhere from 5-12min following a satisfied heat-call despite adjustments to the aquastat dial. I think this partly is due to the aquastat being upstream about 24inches from the heat exchanger. I also don't think an aquastat is the best way of determining when the fan should be turned off.

So...I partially solved the problem by disconnecting the aquastat and replacing with a Honeywell Fan-Limit Switch. The challenge with this is that because the heat builds up slowly in the plenum, I can't set the On-Limit low enough to have the fan turn on quickly, because I am then forced to choose a very low Off-Limit that keeps the fan on long after the heat call. As of right now, it's about 4-5min after heat call until the Fan turns on, and about 3min after heat call is satisfied for the Fan to turn off.

It's better than what I had, but still not optimal as I'm wasting energy over-heating the water to initiate the Fan On, and still blowing cold air for 2min after the heat call is satisfied. Not to mention, I have to change the limits on the switch during the summer because the heat in the attic causes the fan to switch on and run constantly.
 

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You can put a delay on make control and use a relay to cycle the fan on 30 seconds after a call for heat begins, will shut off right away when the call for heat ends.

There may be fan controls that could do it for you or a time delay relay.
 

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Why is it using an aquastat? Does it have a zone pump controlled from a stat? Why not run the fan off the same stat?

I'm going to throw a wild guess and say that the aquastat/fan limit is controlling the fan by breaking 120v directly. Use a relay or contactor with a 24vac coil. Connect the coil to w and c. (the pump is probably already connected there, possibly with it's own relay, the new relay will be in parallel.)

Cheers!
 

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Discussion Starter · #4 · (Edited)
You can put a delay on make control and use a relay to cycle the fan on 30 seconds after a call for heat begins, will shut off right away when the call for heat ends.

There may be fan controls that could do it for you or a time delay relay.
Is this something that can be completed by a novice? What I know about my system (or HVAC in general), has been learned from hours spent on Google last year.

Why is it using an aquastat?
Does it have a zone pump controlled from a stat?
Why not run the fan off the same stat?

I'm going to throw a wild guess and say that the aquastat/fan limit is controlling the fan by breaking 120v directly. Use a relay or contactor with a 24vac coil. Connect the coil to w and c. (the pump is probably already connected there, possibly with it's own relay, the new relay will be in parallel.)

Cheers!
1) Not a clue...maybe to add a degree of difficulty to my life?
2) Yes (I think)...When the thermostat calls for heat, one of the 2 circulators in the basement kicks-on.
3) From a layman's perspective, this would seem to be the most logical approach. I'm just not clear on the wiring and location of these components.
 

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It sounds like you're comfortable with electrical, so you could probably handle the wiring.

Coming into the air handler, you have the wires R, W, G, possibly Y if u have a/c, and a c terminal or wire. (u may not have a terminal strip)

R and C are voltage from the transformer.

W is energized in heat mode.

Anything connected between W and C will be energized on a call for heat.

Connect any 24vac fan control or relay, get rid of the fan limit switch and it will cycle with a call for heat.
 

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Do a google search for time delay fan relays to see what's available.

There are many ways to do this; you can get the t-stat to call for fan during a call for heat, then use some type of time delay off.
 

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if you do as bob is suggesting, check the pump's relay current ratings. the pump may only pull < 100 watts, the fan may very well be 400+
 

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Nothing wrong with trying to improve a system but the average old school gas furnace or oil furnace years ago would run the fan the same way as HC has it now. Doubt there is much that can be done to improve it past using a fan control instead of aquastat.
 

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Nothing wrong with trying to improve a system but the average old school gas furnace or oil furnace years ago would run the fan the same way as HC has it now. Doubt there is much that can be done to improve it past using a fan control instead of aquastat.
Might be able to use a stat from a toe kick heater.
 

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Doubt there is much that can be done to improve it past using a fan control instead of aquastat.
Not true at all. The fan continuing after temp is reached bothers the OP. Not hard to fix at all. Run the fan from the pimp circuit , or from the thermostat which ever is more convenient. If you don't like the cold air for a minute or so at start up then that's no problem either. A delay relay of about a minute will fix that right up.

There are always options and ways to improve.
 
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