The simple direct questions are: Can I run two hot water heat boilers off the same thermostat? Both boilers are currently ran off their own thermostat, but it wouldn't be hard to run a thermostat jumper wire from one boiler to the other and then just disconnect the 2nd thermostat all together. Is that something I can do and not cause a problem with anything?

The long story is: The house was built as a mirror image, ranch style, side by side duplex in the mid 1950s. Everything was separate between the two sides. Somewhere around 25 years ago it was opened up into one house. One kitchen was removed, and they removed most of the wall between the living rooms of the two sides (where both thermostats are located). The utilities were all merged together, but the heating and plumbing is still largely separated with the exception of one of the water heaters that was removed and the two hot water systems were connected through one water heater. Around 5 years ago a complete new AC system was installed and ran off one thermostat.

Last winter, both of the 1950s dial thermostats were replaced with digital thermostats. Since then we have had inconsistent heat. Either one side was hot and the other was cold, or vice-a-versa, unless it was really cold, then both sides functioned well. This would be if both thermostats are set at the same temp. The thermostats themselves are about 10' apart on the same pretty much open wall. if we bump either stat up one degree, that side is warm and the other side is cold, especially in the more separated rooms. I believe I would be better off is both boilers ran off the same thermostat so both turned on and off together.

Seeking educated opinions please.
Thank you, Gene

Where are the sensors for each located? And are you running TWO thermostats?

Did an internet search and there are a number of hits that describe just what you are trying to to.

We are currently running two thermostats, one for each boiler. One of those two thermostats also operates the whole house AC. There are probably no sensors other then what would be required to fire the boilers, this is all pretty much 1954 tech.
What I would like to do is eliminate one of the two current thermostats.

i will check on line to see what I can find.

It shouldn't be that hard to get them both going under one thermostat. The only issue would be if one system is blowing more heat then the other and that side ends up being warmer because of the thermostat placement. At least with two you could turn one down.

A question of curiosity for Poorboy, have you tried swapping the stats from boiler to boiler?

I'm just wondering if there's a problem with the temperature sensor in one.

I've honestly not been impressed at all by the newer digital/electronic stats. I actually chased down a couple of the old mercury switch wall warts.

As someone that has lived in the electrical industrial world for a very long time and play with HVAC, I would just use an interposing relay. The thermostat runs the relay coil and 2 independent relay contacts run to each boiler.

Now is that the best solution to run both boilers off the same thermoset, I don't know. But this will definitely allow you to run both boilers off the same thermostat with no issues.

Yes, I have actually swapped the two thermostats with each other. I bought thermostats on the same day, off the same hook in the big box store. Both thermostats are on the same wall, about 8'-10' apart. There was originally a wall between them, but it has been gone for many years. We did get a bit of this before I put the new thermostats in service, but it has been really noticeable since. It seems that whichever boiler system kicks in first, it brings the temp up before the other boiler fires up, and it is the only side of the house heating. After two or three heat cycles with the same boiler running, the other side of the house gets a lot colder. Enough so you can feel the temp difference walking through a door way. Its almost like one of the thermostats is running about a 1/2 degree above the other and with both thermostats so close together, I'm thinking we are only getting one boiler running. The cold side has cold registers, like that boiler is turned off, until it get real cold, then both will run.

I was under the impression that all the thermostat did was pull in a relay to start the circulation pump and then boiler would fire as needed. You don't think one thermostat has enough power to pull 2 relays in at the same time?
Where would I get an "interposing relay"? Pretty much all we have around here are big box stores.

As it is now, one side is off and one side is on. I'm willing to adjust the one thermostat up or down to get a desired temp, if we can get a pretty consistent heat on both sides.

I may not be understanding the situation completely, wouldn't be the first time. But why not move the stat that controls the area of the house that is getting cold away from the other stat?

Maybe to a bedroom where the door can be shut to isolate the stat from the area controlled by the first stat?

Kevin

I'm thinking the two boilers have their own ductwork? If so, linking the two boilers to same stat won't help....

The house was built as as side by side duplex. Each boiler is plumbed to heat 1/2 of the house and isn't large enough to heat the entire house. I could move one thermostat to a hall way on one side, but I would rather only have one thermostat if I can. Moving the thermostats in this house would be a royal pain. The central AC is also ran off one of these thermostats. it would need to stay centrally located.

yes an isolation relay to keep from mixing voltages thats how we always do it

Ok, that gives me a better sense of the situation. However, it seems that one boiler is stronger than the other. I'm not sure a single stat will get you what you're after.

Even if both run from the same stat, one side will get warmer faster than the other.

I really think separating the stats is going to be the real fix. Or, one big boiler and tie the sides together.

I would not run two hot water boilers off the one digital thermostat,
although like you I have experienced problems when 1983 vintage mercury switch on/off mechanical thermostats were replaced by “better” year 2007 digital thermostats when I was not present.
I suspect the HVAC employee as “padding the bill” when the real fault was simply a bad bearing on the air handler fan motor.

If you're having problems now with one side being colder then the other because the heat from one is shutting the other off connecting them to one thermostat won't fix that It'll do the same thing. Pretty much all you can do is either crank the other thermostat up or move the one that doesn't control the air conditioning to a better location.

It just occurred to me that my youngest brother has a WiFi enabled portable HVAC thermostat that he likes.
You can move it around so that your “problem room” will more likely be at the temperature you desire at that moment.

Even with dampers in ducts “balancing” flow to each room is still a challenge because hot winter optimum is not cool summer optimum.

Variable speed fans to replace simple grille covers can help.

My next door neighbor went to great expense to completely separate his two story house so his second story had its own isolated HVAC.

I talked to his installer who remarked to me that adding a 12 inch duct with a variable speed and reversible direction electric duct fan is much less expensive and works nearly as good - but does not give you the benefit of half the house staying cool or warm when a HVAC unit is broken.

A royal pain? No basement?

Full basement, 8' ceilings and a smooth cement floor. Nearly the entire basement ceiling has 2 levels of dropped ceiling. There was an original dropped ceiling that has small 12" x 12" tiles supported by a wood framework that is nailed to the bottom of the floor beams. Then at some point, someone decided to do another dropped ceiling 6" below the 1st one. The lower dropped ceiling is pretty easy to deal with, its the "modern" style with the 2' x 4' panels in an aluminum frame work that is suspended by wires. Getting the wires through the floor is the hard part. Most of the interior walls are built on top of the floor.

I'm not sure you guys are grasping my situation. The house has hot water heat. There is no furnace duct work. The house heating system has two complete and separate systems. Currently, each system has its own thermostat. Each system has a boiler that its only function is to heat the water in the closed system when it is needed. From the boiler, the heated water is pushed through the copper tubing by a circulation pump that is turned on and off by the thermostat. When the thermostat calls for heat, the pump starts pushing the water through registers along the floor that are all connected in series. The hot water passes through the register in the kitchen, then on to the front room, then the front bedroom, then the bathroom, then the 2nd bedroom, then returns to the boiler. The circulation pump passes the water through the entire system in about 45 seconds. When the water cools enough, the boiler fires and warms the water. When the front room reaches the set temp on the thermostat, the thermostat shuts off the the pump and the water stops moving until the thermostat tells the pump to run again. Since the house was built as a side by side duplex, both systems are a mirror image of each other with a shared common wall Both systems function correctly. It is a very efficient method of heating a house.

My issue is simple, one side or the other is not turning on because the wall between the two systems has been removed. We have lived here for 20 years, the problem started when the thermostats were changed. I had to replace one of the old mercury switchs because it started reading erratically, I didn't think have a new digital switch and an old mercury switch was going to work. I suspect the settings inside of the two "new" Thermostats are not in sink with each other, there was not a problem before the "new" thermostats. These new replacement thermostats are designated for "boiler heating systems" and are the only option for the mercury replacement thermostats available locally.

The original round dial thermostats are nothing more then a mercury switch. The power for the thermostat comes from the boiler control box (the replacement thermostat has an incoming power wire, an electronic switch, and an out going wire). If you connect both wires together at the thermostat, the circulation pump will start.

Maybe pictures might help.
Pic 1) The wall with the two thermostats. The "window" is the wall that used to separate the two front rooms. There is also a 8' wide opening and another window. That wall around the window and another one just like it is the only thing remaining from the 14' long wall. The west side thermostat is just below the lighthouse, and the east side thermostat is just on the other side of the two coat closet doors.

Pic 2) Kind of hard to see, but that thing along the floor is the heat register. The other rooms have a pretty cove over this, but all the registers are the same except the length of them. Excuse the dirt on the wall, this room is the next one to be updated.

Pics 3 & 4) This is the east side boiler. Notice the height compared to the door knob. The water heater beside it is a 40 gallon heater. Pic 4 is the side view of the same boiler. The circulation pump is the left most "thing" hanging on the copper pipe that delivers the hot water through the system. This is a very simply system. About all that can go wrong is the circulation pump, the boiler heat exchanger, or air getting trapped in the line at the registers. Every year I blead the air.

First thing I would try is turning one stat off or down as low of a setting as possible. Then go to other stat. Turn it up, make heat come on. Does it warm that side of house?
If so, turn temp down on this stat as low as possible. Go to other stat. Turn temp up. Make heat come on. Does it warm that side of house?


Had one where customer complained her bedroom stayed cold. She would shut her bedroom door at night. She also would leave fireplace logs burning. The logs, being a heat source, would warm the room thermostat was mounted in. Thermostat felt the room needed no heat, thus it would not call for heat. System that provided heat to entire house would not come on. Bedroom got cold..... I provide this bit of information as an example of sometimes it's the simple things.


If you want, if both systems work after performing above suggestion, choose the stat you want to control both sides of house. Find the boiler this stat controls. Should have a transformer. If this transformer is rated less than 75va, change it to a 75va. Take wires controlling stat you are removing and connect them to the new transformer, along with the wires from stat you are keeping.

You now have both boilers connected to one stat. I think this will work.

Hope this helps. You have helped me with a ton of information in the past, and I appreciate it.