High volt vs low volt thermostat

Bigdaddrock

We have a thirty year old high volt (110) thermostat that controls our oil hot burner (we have baseboard heat).
We are having Central A/C installed with a Low volt thermostat that can be controlled via phone/internet.
We'd like to have the new thermostat control both heat and A/C. What would have to be done to enable the new thermostat to control both A/C and oil burner? Is it even possible?

Jamie Hall

Step one is to find out whether your high volt thermostat is really controlling a high voltage circuit. Some do, some don't. They don't have to. If it is, then you will need a relay controlled by the low voltage thermostat to switch the 110. If you are playing with 110, unless you really know what you are doing you should have an electrician do the work.

EBEBRATT-Ed

If your AC thermostat is a "Heating & Cooling" thermostat then you can use it to control the AC & the boiler.

As @Jamie Hall said you have to "convert" the control of the boiler from 120 volt to 24 volt. Not an expensive or big job you need three things:

Someone who is familer with the controls & working wit 120 volts.

a relay & a 120/24 volt transformer (which can be had in 1 relay package)

24 volt thermostats will give you better (more accurate) control than 120 volt.

120 volt stats haven't been used much in the last 50 years. I'm guessing this job is 1950s or earlier

HVACNUT

I think I have a MED relay somewhere in my van.

Bigdaddrock

I am not about to tackle this myself, I will get an electrician, as long as it is possible. The house was built in the Northeast in 1978.
I just looked at the oil burner, and there seems to be, what I believe is a transformer, on a junction box next to boiler, with other exiting cable leading to the burner. Might this indicate that the burner only requires low voltage, and an electrician can "remove" the transformer, thus allowing the newer low voltage thermostat to be used? If so, will it require new wiring?<

Jamie Hall

That other transformer on the burner is much more likely to be an ignition transformer, or otherwise part of the burner control. Not to be fussed with. An electrician can set you right on this.

JUGHNE

Perhaps an oil burner tech who is good at controls would be a better choice. Actually it is a HVAC/heating control situation.

In my mind it seems that the AC people should have someone on board who is familiar that older set up.

A good electrician could figure the situation out but might have to spin his wheels a little to get tuned in. IMO

EBEBRATT-Ed

I'm an Electrician and I agree with @JUGHNE . Some electricians are not farmalier with controls

HVACNUT

Can you post some pics of the boiler and any controls on or near it? We might be able to tell if you need an electrician, or its already been converted to low voltage.
Man I wish they still made MED relays.

JUGHNE

MED relay???? Never made it to my area. Describe or picture?
I am familiar with RIB relays.

HVACNUT

A MED relay would mount in the gem box of a line volt thermostat. When the contacts closed on your new low volt stat, it would close contacts on the line volt side. No need for a switching relay at the boiler.
Using the old honeywell 822D vertical stat meant no spackle or boo boo plate needed.

Big Ed_4

Do they still sell MED relays ? I have not used one in decades .. Back in the day everyone wanted an clock thermostat ..

Line voltage thermostats was only used on boilers from the '60's to maybe the late '80's .. why.... always the dollar ..

Big Ed_4

Hate to say this , but in that era only the cheapest equipments was being installed.. Garbage can boilers and baseboard .. It was our industry Low...

Bigdaddrock

Okay, fellas. Had a senior tech from my oil company come by today to look into my issue. Bottom line - lots of new wiring to be run, with a cost in the four figures. My wife doesn't need a single heat/AC thermostat that badly.
House was built as spec from a builder back in 1978, so I am sure he used the cheapest and old technology!! I am stuck with it.
Now, another related question. If thermostat is basically an on/off switch, has anyone seen a wifi/radio controlled thermostat that communicates the on/off function directly to a switch on the boiler which would get around the whole wiring issue?
Thanks

Jamie Hall

see my reply on the other thread you started.

EBEBRATT-Ed

Can't discuss price but I think you need a new "senior tech"

HVACNUT

Guess you started a new thread.

Bigdaddrock

Okay, I spoke again with my senior tech. He indicated that a MED relay would work, but they have not been made in years. He wished me luck in finding one.
Any suggestions? With the internet I thought everything was accessible.
Thanks for your continuing patience and assistance.

HVACNUT

They were manufactured by a company in Farmingdale on Long Island, but they haven't been around I guess since low volt wiring became common.
I've got one somewhere.
You have 2 heat zones? If a MED relay would work, then converting to low voltage will work. They probably ran a 3 wire, example; white common, black 1st floor, red 2nd floor.

Bigdaddrock

Yes, I have two zones. If I use the MED relay on the one (in connection with the AC) would I be disabling in any way the other thermostat, which I plan to leave as is?

Bigdaddrock

HVACNUT, I am struggling to find a MED relay. You mentioned you might have one on your truck. Might you be willing to part with it? I am on Long Island.

Bigdaddrock

Just spoke with another electrician about my problem. He indicated that the reason the MED relay went the way of the dinosaur was that it generated heat - right behind the thermostat - creating false readings.
Do you think he was merely telling me this to convince me to allow him to come in and rewire from the thermostat to the boiler? Or was he right, and I should cease trying to find a MED relay.
Thanks again.

HVACNUT

> @Bigdaddrock said:
> HVACNUT, I am struggling to find a MED relay. You mentioned you might have one on your truck. Might you be willing to part with it? I am on Long Island.

>>I am on Long Island, however,
I was generalizing. The stock in my van is in impeccable order. My garage and shed, not so much. It's on my list of things to do. Also, it's well over 20 years old and I would not use it myself, let alone risk giving it to someone and have something bad happen. Sorry.
You have options.
Have separate A/C and heat stats.
Convert heat controls to low volt.
When it's time to replace the boiler, convert to low volt at that time, then merge to 1 heat/cool stat.