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Old 01-21-2017, 03:26 PM  
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Switching Thermostats from Line Voltage to 24V Units


Hi

I'm in need of some advice.

I'm trying to figure out how to replace my old line voltage thermostats with some newer 24V ones.

I have hot water boiler / oil burner controlled by a Honeywell L4081B Aquastat, with two pumps/zones and two line voltage thermostats.

It's a very simple system, but the tricky part comes from the fact that the Aquastat and the existing line voltage thermostats are wired in series, such that the thermostats are only getting power if the Aquastat says they should - which in my configuration means that a low-boiler-temp condition would close the circuit and hence cut the power to the thermostats, preventing any chance of the pumps activating if the boiler is cold.

So ... this is what it looks like:


LINE -----> Aquastat (switch) ---> Thermo 1 ----> Pump 1
|
----> Thermo 2 ----> Pump 1
(i'm leaving the line from the Aquastat to the burner out of this, since it's not relevant).

My problem is my power source to the new thermostats will be conditional on the Aquastat switch being open ... which it may not be, if I run out of oil or just shut the system down. If there's no power to the thermostats, they'll go dark since they require constant 24V.

Question is how do I wire all of this so that the Pumps will come on only if BOTH i) the Aquastat switch is open and ii) my new thermostats demand heat. Put differently, I don't want the operation to run in series any more.

The ideal apparatus here would be some kind of a switch that takes two (120V) line inputs, senses whether BOTH lines are "hot", and allows current to pass only under this condition (... think of it as an "AND" gate for line voltage applications). This way I'd be able to power the thermostats completely independently of the Aquastat - so they're always on - and then give power to the pumps if BOTH the Aquastat AND the thermostats independently allow it.

I don't know whether this is possible (and if so, how) - or whether I need to think about another solution here.

PLEASE> I do not want to buy (new) battery operated thermostats. I know there are models out there that might do the trick ... but ultimately I really want this to work using a CONSTANT 24V supply to the thermostats.

Wiring/re-wiring is not a problem. I can run any wires that are necessary.


If anyone has come across this before, would love to hear your suggestions.

THANKS !!!

----


Additional info:

- The Aquastat is a Honeywell L4081B
- The existing thermostats are LUX ELV1001 units. They're basically nothing more than switches that open and close the line voltage circuits using battery powered relays
- My new thermostats are GE IS-ZW-TSTAT-100 ... they require a 24V C line
- I'm adding Aube RC840T-120 relays to step the line voltage down to 24V for the new thermostats ... but again the problem is the INPUT into the relays is itself switched by the Aquastat. I need to get around that somehow.
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Old 01-21-2017, 03:47 PM  
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Re: Switching Thermostats from Line Voltage to 24V Units


Not my area of knowledge, but I have a question...

Why do you have a problem with the pumps not being activated if the boiler is cold?

Presumably everything is wired so that the burner can cycle and keep the boiler hot without a call for heat -> something i don't agree with since it wastes energy.

If the boiler is cold running the pump is of no use.

----------
Anyhow, with respect to your wiring dilemma,

24v stats have battery backup, a lot of them are 100% battery powered so you don't have to worry about that.

Now, the best way to approach this would be to leave the existing high voltage wiring that's in the boiler but install a 24v transformer and relays.

Have the 24v go through each thermostat and back to the relay in place of the existing line voltage thermostats.
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I am not in the business of any trade I give advice on. I have non-professional hvac experience + good knowledge of theory. Attempt repairs at your own risk. Never jump out safeties - especially pressure switches - on a furnace for testing with fuel supply on; use a meter. Do not troubleshoot with live line voltage present unless there's no alternative.
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Old 01-22-2017, 05:39 AM  
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Re: Switching Thermostats from Line Voltage to 24V Units


Use a zone panel. It will have its own 24 volt transformer for you to connect to. The zone panel gets its own 120 power source, and only opens and closes the contacts to L4081B's low limit.
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Old 01-22-2017, 09:31 PM  
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Re: Switching Thermostats from Line Voltage to 24V Units


Quote:
Originally Posted by beenthere View Post
Use a zone panel. It will have its own 24 volt transformer for you to connect to. The zone panel gets its own 120 power source, and only opens and closes the contacts to L4081B's low limit.
I agree. It'll keep the boiler wiring simple, and reduce the chance of messing that up.

Cheers!
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