How do I control boiler with 2 different zones and circulators


Old 01-04-20, 09:57 AM
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How do I control boiler with 2 different zones and circulators

Below is a drawing of my setup to the best of my abilities. I have a gas fired boiler for hot water baseboard on the first floor of my rancher and the finished portion of my basement. As the setup is now, the first floor thermostat is tied directly into the boiler and works fine. The basement thermostat only activates the circulator for the basement, it doesn't turn the furnace on. How can I change my setup so the basement thermostat also turns on the boiler like upstairs does? Right now I don't really get heat on the basement circuit unless upstairs has gotten cold for some reason and causes the boiler to run for awhile. Once upstairs gets up to temperature, the basement will stop heating. If it makes any difference, there is a slight chance I would like to add a 3rd zone in the future. We have a nice sunroom on the back of the house that has no heat and I've considered adding it in the future. The way the basement is setup, it would be very easy to run the piping for.
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Old 01-04-20, 11:37 PM
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Let me start by saying I am not an HVAC tech. I am just an engineer in an unrelated field who bought a house with an incorrectly wired HVAC system. Sick of outrageous gas bills, I decided to dedicate a lot of time to learning how it works and rewiring the entire system.

So, based on your pictures and your diagram, it looks like upstairs tstat goes to an isolation relay. That relay then sends signal to boiler thermostat (looks like the honeywell box with the cover on it in your 2nd pic). My guess is upstairs zone's circulator is wired off of the boiler's stat or a transformer from the isolation relay.

As for your second zone, when I bought my house I found I had the same problem. An RA89A and an R854A relay were wired to turn on their zone's circs but not fire the boiler. I decided to swap all that out for a Taco switching relay, like the Taco SR503. This relay will have inputs for each stats (your current two and a third if you add it). The relay will then have a hook up for each zones circulator. Lastly, it will have an output to go to the boiler to signal it to burn (the thing your are missing now). So if a zone calls for heat, say upstairs, its circulator will turn on and the relay board will send a signal to the boiler stat to fire. Same for any other zone on the board. I would recommend doing this. Easy to install and they work well. Here is a link to my previous post that shows a lot of details about my rewiring.

A few caveats. Is your boiler cold start? Does it supply your domestic hot water? This might not change much in terms of solutions for you (the taco relay could be used in either scenario) but would be good to know. Also, take a picture of your boiler controller so we can see exactly how its wired.

good luck and welcome to the forum. I joined about this time last year and have learned quite a lot from the members.
Old 01-05-20, 02:47 PM
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Not sure where your lower wires go from R & C
Easiest to buy a two zone relay for pumps like the Taco SR502. Both thermostats and circulators wire to relay and one thermostat wire to the boiler. Either zone demands and starts the appropriate pump and fires the boiler.
To make the drawing you have work there would have to be wires changed and a different relay used that would have two sets of contacts in it.
If you decide the latter I could supply a factory approved drawing.
Old 01-05-20, 07:10 PM
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The Taco SR503 definitely looks like it would do the trick and allow me to expand the system further if desired in the future. I'm not sure if the boiler is cold start. I think it is but haven't been able to confirm. At one time we use the boiler for hot water but I installed a tankless hot water heater so the boiler only does the heat now.
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