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Help Needed ... turn on circulation pump when furnace in AC mode?

2250 Views 12 Replies 3 Participants Last post by  Yoyizit
Short version: I have a pump that simply plugs in to an outlet and is on non-stop. It draws ~120w. I need it to only be on when the furnace is on, and in 'AC' mode. How can I make this happen?

Long version: We've recently finished building our home in the SF Bay Area (temperate climate) and rather than build a full AC system in (which is fairly uncommon around here), we ran two copper coils into the ground and have created a "poor mans" ground loop cooling system. We've got a water-to-air radiator in the outlet of the furnace before it runs into the ducts, and a pump that pumps the water in a loop throughout the system. Obviously we only want the pump running when the furnace is in AC mode...

(Don't ask how well it works yet ... we just finished plumbing it last night and honestly its not hot enough around here to do any good right now...)
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This works with airflow so you'd need to pull plugs or throw switches twice/yr when changing from A/C to heating and vice versa.
If you want it fully automated post a photo of the schematic of your HVAC unit. Probably one additional relay is all that is needed.
The schematic is probably pasted on the inside of the one of the removable furnace panels.

Making and testing a small logic circuit that does this function requires that you can follow a simple schematic and do soldering, and you may need to borrow a DVM for testing.
It might take you a morning. Parts can be had from Radio Shack or Probably $10 would cover it, with shipping extra.

If the pump is supposed to run when the compressor runs this should be very simple to implement; a step-down 'former to power a relay with Normally Open contacts, or just a 24vac relay that is controlled by your t'stat.
The relay contacts need to be be able to switch a 1A motor load.
Correct me if I'm wrong... should there be a power source from the furnace (one wire, or two?) that sends a small signal voltage (24v?) to signal a standard AC unit to turn on?

If thats the case, I need to have something that can read that voltage and flip a relay that then powers the pump... this is the part I'm not sure how to do though.
Yes, and your added relay [~$2] should have a coil impedance high enough that the t'stat doesn't notice the extra current draw.
If you can get numbers and symbols off the compressor relay we will know it's coil resistance and so we can choose an appropriate add-on relay. This info might be on your HVAC parts list.
Otherwise we just pick off 120v from the compressor cooling fan or 240v from the compressor and step it down with a 'former [~$3] to power almost any relay. You could even use a doorbell 'former and we pick the relay coil voltage to match.

This is the kind of circuit that works on the first try.
The Y and C terminals probably supply 24vac when cooling is selected.
With 40VA 'formers you have 1.7A available, some portion of which is used the existing equipment. Let's say all we want to draw is 1.7/10 = 170 mA for the relay coil.
Hosfelt 45-640 has a coil resistance (or impedance?) of 160 ohms, so it would draw 24/160 = 150 mA or less. Sounds like that would work; otherwise you could look at All Electronics or Digi-key or Mouser or Jameco for a higher impedance coil. Some of their websites are a bear to search, so you might want to call.

You know, some guy back in the 70s did this and he got pretty good cooling. He used a car radiator from a junk yard. HVAC people came by to look at it because they didn't believe it.

While I have your attention, could I get from you your house sq. footage including basement and your heating gas/oil therms for, let's say, last December? I'm doing statistics on heat loss and I'm short about 20 datapoints.

Here's that stuff for my old house. Your new, low infiltration, well-insulated, house should do much better.

My house, 838 therms of NG in 90 days = 9.3 therms/day
3100 sq. ft. including basement gives 9.3/3100 = 300 BTU/sq.ft.
2658 HDD/90 = 30 HDD in one day
300/30 = 10 BTU/day/sq.ft./HDD
Multiply by 0.8 to allow for efficiency of gas/oil = 8 BTU/sq. ft./HDD

Say hi to your EE friend. I don't recommend it as a career field!
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December 1st -> 31st:
70.2 Therms used in 30 days = 2.34 therms/day
2700 sq. ft
(two floors, first floor slab foundation, all R19 and R30 insulation)
(two furnace systems, 75k BTU downstairs, 45K BTU upstairs)
furnaces are both 95% AFUE NG furnaces with electric fans
Thanks; I get 540 HDD for that period at your location which gives you 4.7 BTU/sq. ft./HDD


Your heat loss looks to be about average.
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