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Steve G.
Steve G., Electrical Engineer
Category: Electrical
Satisfied Customers: 17987
Experience:  Spent 20 years as an Electrical Engineer, designing electrical plans and specifications.
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I am hooking up a thermostat to drive a pump on my boiler.

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I am hooking up a thermostat to drive a pump on my boiler. I am using a relay hooked to a transformer (25v) and switched by the thermostat. The second side of the relay controls power flow to my pump. The problem I am having is the relay will not pull in on the low voltage side. It will hold in if I manually close the contacts............or if I hookit up to the normally closed side of the relay switch the relay chatters...........I have tried a second relay and it does the same thing...........any thoughts?
Hello there and welcome to Just Answer. My name is XXXXX XXXXX I will be assisting you today. I am an engineer with over 30 years of electrical and electronic training, repair and installation experience. I will try and answer your question accurately and precisely so that we can get you on your way.

Now..what is the coil voltage of your relay? It will be written on it somewhere - note that I need the coil voltage NOT the contact rating.

Please let me know so that we can continue.

Customer: replied 4 years ago.

I believe it is AC 24v 60HZ

Ok, well that is your standard household HVAC voltage typically seen at thermostats.

Can you bypass the thermostat and see if 24V AC will hold in the coil?

Please let me know so that we can continue.

Customer: replied 4 years ago.

I have bypassed the thermostat totally during testing and the relay will pull in normally when current is run directly from the transformer to the coil on the relay. As soon as I try and connect in the thermostat or make the relay work through a switch it either won't pull in on the NO side or it chatters on the NC side.............I have an identical relay running the thermostats for another building and it seems to work fine? Could it be amperage related as there is no load on the circuit per say?

Yup. I would suspect the amperage of the coil is too much for the thermostat contacts although I have never run into this before.

Can you get me the make and model of the stat?
Customer: replied 4 years ago.

It is a very basic thermostat - Honeywell CT50 series.

Ok, I do not see a contact rating for that stat but I think that is where your problem lies.

I can opt out if you want and you can ask other HVAC guys.

Hold on.
Customer: replied 4 years ago.

I have no problem picking up another stat. Is there a contact rating I should be looking for?

Actually, it's the relay you want to change. You want something that only draws 30-50mA like >>>THIS ONE<<<

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