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How many relays does an HVAC / thermostat need?

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blueroomelectronics

Well-Known Member
I've noticed simple*wall thermostats have a single relay with fan & AC/heat done with a pair of switches. So it seems that as few as three relays could automatically control a typical homes AC & furnace.*
On the other hand I've seen high end HVAC thermostats with as many as six relays to run zoned systems and/or heat pumps...
So the question is would a four relay be enough for most people or would a six relay device offer much more flexibility?
Also how much current? All the thermostats I've had a look at use 2A relays is there any advantage to a 5A relay?
Lastly how many SPST relays vs SPDT, I was thinking 4 SPST & 2 SPDT would be about right.
 

smanches

New Member
I would think that just two SPST relays would be enough. One for heat, one for AC.

What would the other ones be for?
 

Mike_2545

Super Moderator
On the other hand I've seen high end HVAC thermostats with as many as six relays to run zoned systems and/or heat pumps...


How can one thermostat control any more than one zone? Maybe a two stage thermostat would need 2 relays on each heat/cool, but unless you put a thermostat on a robot and have it travel from room to room, one thermostat = one zone.
 
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blueroomelectronics

Well-Known Member
One for heat, one for AC.
And one for Fan, Auto/Off/On

How can one thermostat control any more than one zone?
The controller would be similar the grey box controller pictured with an RS485 remote thermostat as pictured below (a TR16 - RS485 Serial Thermostat). The TR16 is a five relay device all SPST.
9204-547.jpg

The Aprilaire 8870 RS422 thermostat has six relays 5 are SPST and one is SPDT
As an added bonus I'm thinking of adding a 433MHz RF receiver as they appear to be compatible with some Oregon Scientific RF thermometers & humidity sensors.
9205-imageloader.asp


On further reading I think it would be a good idea to include an onboard LM34 temperature sensor as a freezing indicator (the centigrade LM35 doesn't go below freezing without modification).
 
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smanches

New Member
So it's a thermostat with a zone control module. My zone control is completely separate from my thermostats and not even from the same manufacturer. I only have hot water heating, so I have really simple thermostats. They have a cool mode, but it's not used for anything.
 
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tcmtech

Banned
Most Helpful Member
As far as switched load the typical 24 VAC system is unlikely to ever see more than an amp or two. The largest transformer I have seen in a residential 24 VAC HVAC system was 100 VA and ran two furnaces and two central air units simultaneously.
I think it was still about double what was needed though. So I would think a 5 amp capacity relay on a 24 VAC system is more than enough.

As far as number of zones thats going to depend entirely on what application the unit is going to be used in. I doubt most systems would ever need more than three. Primary cheap fuel source, Secondary more expensive backup fuel source, and third most expensive last resort backup source.

One fatal flaw is that if the main single controller quits then all sources quit. For that reason when I set up multi fuel heating systems I use an independent thermostat for each system.
They are just programed with the proper time and temperature changes off set from each other. Secondary is set at 5 -10 degrees F lower than primary with the warm up modes starting around 20 -30 minutes later than the primary systems temperature changes if there is a temperature overlap between them.
The third system is the last resort fail safe usually with a old fashioned mechanical thermostat set for its lowest temperature possible. Usually around 45 F.

You may also want to consider that some commercial and industrial applications do use 24 VDC power as well.
 
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