One Thermostat Controling Two Valves


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Old 10-12-09, 03:05 PM
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One Thermostat Controling Two Valves

I am wanting to have one programmable thermostat control two Taco zone valves. Originally in our church we had one zone for the sanctuary and one for the back offices. When we added on to the building the back offices were taken out and the sanctuary was expanded into that space combining the two zones. We currently have one thermostat controlling the two zone valves, but we would like to replace it with a 7-day programmable thermostat. I was looking at getting the Honeywell RTH7500D thermostat and contacted Honeywell to see if it could handle the current of the heat motors. Here is their response:

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Thank you for contacting Honeywell.

The model RTH7500 operates on maximum 30 volts ac and a maximum of 1.0 amp.

Honeywell Customer Care

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When I tested the AC current by closing the thermostat circuit with my multimeter (where both valves are connected in parallel) I measured something around 1.9 Amps which seems to be double the 0.9 Amps that the Taco valves are rated for. I also got a response from Taco where they referred me to a wiring diagram that might work.

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Taco has responded to the request you submitted on Taco-HVAC.com.

Message: the anticipator is the concern when running one thermostat with two zonevalves. if you do not have an anticipator you can parallel the two valves. If you do have an anticipator follow the attached diagram.
http://www.taco-hvac.com/tmp/onethermostat4valvewiring.pdf

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I am considering getting a Taco Zone Valve Control (ZVC404) unit so that it would be compatible with the thermostat. If I would connect the thermostat in parallel with two zones on one of these devices would the current still be below the 1.0 Amp that the Honeywell RTH7500D thermostat can handle?

We also have two more zone valves for the basement area in the old part of the building (for a total of four). I would also like to replace the old analog round thermostats in the basement with the same model thermostat, but am concerned the current may be too much. I measured 1.1 Amps on my multimeter on both separate thermostats in the basement even though the Taco valves are rated at 0.9 Amps. I would feel more comfortable using a Zone Valve Control unit so the current won't be too much for the thermostat.

The main thing I would like to know is if connecting one thermostat to two zones on the Taco ZVC404 unit would still be under the 1.0 Amp rating of the thermostat. I'm guessing the current rating for thermostat terminals on these devices is much less than directly connecting to the heat motors. As long as the current is under 0.5 Amps for each terminal, I think combining two of them together in parallel should keep it under the 1.0 Amp thermostat rating.

Here are the Taco Zone Valve model numbers in our building:

Taco 573 - Sanctuary
Taco 571 - Sanctuary (old office area)
Taco 572 - Basement Fellowship Hall
Taco 571 - Basement Classroom, Nursery, Restroom
 
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Old 10-15-09, 07:34 AM
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I had contacted Taco again to find out what the AC current is on the thermostat terminals on the Zone Valve Control (ZVC404). The reply I got back said it was the same 0.9 Amp anticipator setting as being directly connected to the zone valves. I thought Zone Valve Controls had universal thermostat compatibility as it says on their site, but I'm not sure what that means exactly.

I was wondering if a Switching Relay could maybe work. Even though it is designed to switch a 120 V AC circuit, could you hook it up to the 24 V AC circuit going from the thermostat terminals on the zone valves? The thermostats themselves would be hooked up to the thermostat terminals on the Switching Relay. When the switching relay is activated it would close the 24 V AC circuit to the zone valves acting like a thermostat closing the circuit. I'm not sure if this would be possible or not. I found this information in the catalog PDF for Single Zone Switching Relay on Taco's website:

"All Switching Relays are relay type DPDT, have a thermostat current of .18, and have a single phase motor rating per zone of 1/3 hp (5 A) @ 120 VAC. All relay contacts are rated for 0-240 Volts."

I am just looking for any way to have a 7-day programmable thermostat control two zone valves without damaging the thermostat. I have been looking into this for over a year or so and haven't found any solutions yet. I would like to get this working before the winter season.
 

Last edited by gwest39; 10-15-09 at 08:15 AM.
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Old 10-15-09, 03:39 PM
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I am just looking for any way to have a 7-day programmable thermostat control two zone valves without damaging the thermostat. I have been looking into this for over a year or so and haven't found any solutions yet.
I think you did get a solution from Taco in that first schematic that you posted.

The way that works is that the thermostat controls the first valve, and the endswitch on the first valve controls the other valves, the current for the other valves going through that endswitch, and not the thermostat.

That diagram shows four valves, but all you need is to wire up the first two as shown.



LATE EDIT FOR BENEFIT OF FUTURE READERS. The diagram above will (or could) cause problems.

The one below should work properly:



If the others are working, don't fix them!
 

Last edited by NJT; 12-04-14 at 04:45 PM.
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Old 10-28-09, 12:28 PM
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One Thermostat Controlling Two Valves

I have one more question concerning the valve wiring. Would it hurt anything to add an extra connection between the two terminals 3 on the two valves? I think this would allow the boiler to start earlier right after the first valve opens and not wait until both valves are open in sequence after the thermostat calls for heat (which could add an extra minute or so). Also, it would probably be more reliable in case the second valve malfunctioned and didn't open then the first valve could still work on its own.

[OP removed diagram]

Or, could another possibility be to just connect the terminal 3 wire to the first valve only and leave the second valve terminal 3 not connected? Once the first valve closes the boiler circuit is broken anyway, so there wouldn't be any benefit where the second valve could keep the boiler running for another minute or so until it closes after the first valve.

[OP removed diagram]

It just seems like this would be more reliable with having less in the chain that could malfunction and then not have any heat from the boiler at all even if the first valve is working properly. I just wanted to make sure this wouldn't cause any problems in the circuitry with the thermostat or the boiler.

I have already connected one of the Honeywell RTH7500D thermostats I purchased to a single zone valve and it seems to be working fine. I was planning on trying out wiring two zone valves to the thermostat like in one of the diagrams above (or from the other posts) within the next few days.
 

Last edited by NJT; 12-04-14 at 02:54 PM.
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Old 10-28-09, 02:24 PM
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At first, I thought, yeah, why not? But then, I looked at it more closely and don't think you want to do that...

You would be creating a 'series circuit' with two transformers, the relay coil in the aquastat (a resistance), the heating element in the 2nd valve (another resistance)... all in series, drawing current...

no, stick with the original plan... if I get time later, I'll draw a diagram that better represents what I'm talking about.
 
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Old 10-28-09, 03:24 PM
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I marked up the drawing to show the path of the possible problem. It is _possible_ that this would work, but the odds are against you, the second drawing will attempt to show why. By ACCIDENT, you would have to wire it such that the transformers were 'phased' as shown in the drawing by the black dot at the bottom wire of the secondary. Even if you did manage to beat the odds, if the two transformers are not EXACTLY the same voltage, you would still have _some_ current flow. If you didn't beat the odds, the valve would be open constantly, and the aquastat would always call for heat.





Also, even if you DID fool around with it until you 'got it right', it's guaranteed that some poor tech in the future is gonna get all bamboozled trying to figure out why it's not working right after he replaces a zone valve or whatever.

So, like I sed, stick to the plan and all will be well...
you won't notice the extra minute, valve A is just as likely to fail as valve B, and there won't be any extra delay in turning off because once ONE of the valves opens, the boiler will shut down.
 
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Old 10-29-09, 08:12 AM
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Thanks for your help with the valve wiring. Also, after looking into it some more I noticed how wiring it like I mentioned above would affect an additional set of valves controlled by one thermostat on the same system (as I was planning) and it would cause a problem there as well. Any time one set of valves would be activated it would also activate the secondary valve in the opposite set. I will try wiring it up the original way you mentioned sometime within the next few days.

[OP removed diagram]
 

Last edited by NJT; 12-04-14 at 03:13 PM.
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Old 11-02-09, 06:29 PM
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I was able to wire up having one thermostat control two valves today. It worked well in starting up and running, however I encountered a problem once the thermostat shut-off. The boiler would shutdown then immediately start back up, then shutoff again, over and over. After trying to figure it out for a while, I realized what was happening. The problem is that even though the first valve is closed, and in turn cutting the circuit for the boiler, the second valve is still open (at least temporarily until it would close on its own) creating a loop for the boiler circuit to travel through the thermostat transformer as shown below. I think the boiler transformer was keeping the heat motor active with the circuit opening and closing every few seconds as the heat motor does, but in turn keeping that valve perpetually open and the boiler turning on and off.

[OP removed diagram]

So, the diagram that Taco gave me won't work. However, after thinking about it for awhile, I was able to make one change to the diagram they gave and I think it would work. Instead of connecting the boiler circuit to terminal 2 on the first valve, I connected it to terminal 3 (in effect not even passing the boiler circuit through the first valve at all). In this way the first valve acts solely as a switch for the thermostat circuit on the second valve, and the second valve acts as the only switch for the boiler circuit as shown below. When the thermostat shuts off and the first valve closes, the potential boiler circuit loop through the second valve's heat motor is broken allowing the second valve to close. The only difference is that the boiler will continue to run until the second valve closes on its own.

[OP removed diagram]

After looking over this, it would work fine if you had only one pair of valves controlled by a single thermostat on the system. I did find one potential problem if you had more than one pair of valves set up like this. If more than one pair of valves are open at the same time and then they both shut down around the same time, it is remotely possible that both secondary valves in each pair could keep each other open if the heat motors in both valves are opening and closing at exactly the same intervals creating a loop of the boiler circuit. This is extremely unlikely, although possible.

[OP removed diagram]

One way to guard against this would be to place each pair of valves on its own thermostat transformer. We already have two 40VA transformers on our system connected in parallel since we have four valves total and each 40VA transformer can only handle up to three valves. If each 40VA transformer is connected to just one pair of valves, it would eliminate the potential loop if that unlikely situation were to happen.

[OP removed diagram]

After all of this, I'm hoping the last diagram above will be able to work unless you can find any other potential problems with it. I was able to connect the thermostats temporarily to just a single valve each for the time being until I can get this worked out. Thanks for your help in figuring out how to get this to work.
 

Last edited by NJT; 12-04-14 at 03:15 PM.
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Old 11-03-09, 12:53 AM
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I think the problems come with the 'phasing' of the transformers that I mentioned earlier.

Unfortunately, most of the aquastats don't indicate the phasing of the transformer because in 99.99% of the cases it just doesn't matter.

If one could know for certain the phasing of all the transformers, it could 'probably' be wired up as in the first example without a problem. I _think_ that if you reversed the connections to the T T terminals of the aquastat in the first example, the problem would 'go away'... but again, a possible nightmare for future techs...

In order to avoid such problems with certainty, you are correct that your final diagram is probably the safest approach.

This would not be a problem at all with zone valves that have an isolated end switch, such as the Honeywell... 4 wires... because there would be no connection between the two 'circuits' of the individual transformers.

Good job analyzing!
 
 

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