Circulator pump control on hydronic heater system?

Old 06-05-18, 08:26 PM
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Circulator pump control on hydronic heater system?

Hello DIY Community:

I have an older hydronic heating system in my house with an AO Smith boiler. It is sufficiently old that it uses a millivolt style thermostat.

I am pretty certain that I know how to interface a newer, smart thermostat to it by incorporating a 24 VAC transformer to power the smart thermostat and a 24 VAC SPST relay to provide the contact closure that the dumb thermostat provided to turn on the main burner without introducing any undesired voltages on the burner side.

This system, is old enough that it has no control of the circulator pump. When I turn the heat on in the fall, the circulator pump runs 24x7 until the heating system is turned off in the spring.

Am I correct in thinking that more modern hydronic systems actually turn off the circulator pump when the main burner is not on?

Is there a good reason that I couldn't add a second SPST relay (also controlled by the thermostat) so that when the thermostat calls for heat, one relay turns on the main burner and the second turns on the circulating pump? And then, when the set point is reached, not only would the main burner turn off, but the circulation pump would quit running?

Am I missing something or is my thinking flawed?

Thanks for your consideration,

Old 06-05-18, 08:51 PM
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Welcome to the forums.

Yes.... a 24vac transformer to run the thermostat will work. A 24vac relay will interface the stat to the millivolt gas control. You could use a fan center too. That is a combination transformer/relay that mounts directly to a 4" square box.

Are you sure the pump runs 24/7 ?
Usually a boiler will maintain heat at a certain temperature until the pump runs and then the boiler temp will increase as load demand increases. It could also be a cold start boiler which means it won't heat until the stat calls for heat. The pump should not run continuously with this method either.

Having the pump running continuously doesn't allow for much control of the heat.

It would be helpful if you could post a few pictures of your setup there. Maybe we can come up with a more efficient way of controlling the system.

Old 06-05-18, 10:03 PM
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Thank you for questioning my assertion that the pump was running continuously ... even when the main burner was off. I took a more careful look (and listen) and confirmed that the circulation pump doesn't run until the main burner comes on .... and stops when the main burner shuts off.

While I clearly missed that behavior previously, I thank you for questioning me and for preventing me from making my problem more complex than it originally was.

I think that my 24 VAC transformer and relay will now, without complication, allow me to intergrate a newer, smarter thermostat into my system.

Old 06-06-18, 02:46 AM
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Originally Posted by shott
". . .Am I correct in thinking that more modern hydronic systems actually turn off the circulator pump when the main burner is not on? . . ."
As long as there's a call for heat from the Thermostat, AND the Boiler Fluid is hot enough to warrant circulation (meets the low limit), then the Circulator runs, whether the Burner is running or not.

The Burner runs anytime there's a call for heat from the Thermostat OR the Boiler Fluid's Temperature is below the Controller's Low Limit. If the Boiler Fluid reaches the High Limit, the Burner will shut down; but the Circulator will continue running until the Thermostat has been satisfied.

My Controller is fairly modern, a digital Honeywell L7227U from the early 2000s and the Burner is a newish Beckett AFG from the 1980s; but my old steel John Wood Boiler dates from the late 1940s or early 1950s when they first brought electricity to this area. The rest of the system is comprised of individual plumbing components that have evolved piecemeal over the decades.

You can probably check the operational logic of your Controller to verify that it is close to the same.
Old 06-06-18, 04:01 AM
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To know exactly what type of hot water system you have let me add a few points. Millivolt systems were common when I entered the heating field back in the late 1960's. They were a good idea if you had an old gravity heating system with large delivery pipes and cast iron radiators. You had heat all the time even during power failures which were common then. That type system ran safely with no electrical power input except the thermopile or pilot generator as they are sometimes called. That said, they were slow to respond to temperature changes and cost a little more in fuel. After a circulating pump was added to decrease fuel usage and the heating temperature response time, the systems were dependent upon electricity to operate efficiently. Much has changed since those days and usually the system looks and performs quite differently. On older systems with cast iron radiators the circulating pump would usually run 24/7 as long as there was warm water in the system. the pump was controlled by a reverse acting temperature limit control set at a low temperature. I set them at 110 degrees F. Just thought you may want to know.

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