Controls for hot water heat loop on steam boiler

jbot81

I purchased a house which has a fairly new (2016 install) Independence Burnham natural gas steam boiler. I am in the process of renovating the basement, which is heated by a hot water loop running off of the boiler (the rest of the house is heated by steam and controlled by a Nest). The hot water loop is fed by a Taco circulator, which is connected to a simple line-voltage thermostat. When the temperature drops below the set point, the thermostat switch is closes, and the water circulates. The problem with the way it is installed is that the water circulates regardless of whether the boiler is on or not, and if the boiler is off (e.g., the main thermostat has not called for heat) the thermostat connected to the Taco has no way of turning the burner on to heat the water. So, in that configuration, it will circulate water all day long, but not heat a thing.

I would like to replace the line voltage thermostat with a typical low voltage thermostat. I believe I can do that fairly easily by adding a Honeywell switching relay to the Taco line. But, I would also like the thermostat to turn on the burner when the Taco is running. I attached a wiring diagram of my system. Any recommendations for how to wire that?

Thanks in advance!

SuperTech

I'm not a steam expert but I would replace the Nest thermostat. Not a good choice steam or hydronic heat.

I think an aquastat would be the only control you would need.

mattmia2

You can either add an aquastat that always keeps the boiler at a lower temp for the hot water loop or switch that aquastat and the pump with the themostat. You could use a relay off the pump to turn on the burner through that aquastat. Make sure the high limit from the steam side stays in series with the aquastat.

jbot81

mattmia2 said:

You can either add an aquastat that always keeps the boiler at a lower temp for the hot water loop or switch that aquastat and the pump with the themostat. You could use a relay off the pump to turn on the burner through that aquastat. Make sure the high limit from the steam side stays in series with the aquastat.

Would something like the Taco SR502-4 2 Zone switching relay work? It looks like if I add that in between the 24V transformer and the thermostat, one zone can be set to turn on just the burner and the other can turn on both the burner and the circulation pump.

Jamie Hall

KISS. Keep it simple... You have a thermostat which switches the circulator. Great. Keep it that way. Now add a strap on aquastat -- Amazon sells a good Honeywell version -- put it on the inlet to the hot water system from the boiler and wire it in parallel with the house thermostat. Set it to turn on if the water going to the hot water system is too cool -- you may have to fiddle with the adjustment. End of project.

On the Nest. Well, they can be made to work moderately well with steam. Defeat all the "smart" features -- particularly any setbacks -- and you will save fuel and money and increase comfort. If you can get far enough into its logic, set it for one cycle per hour...

jbot81

Jamie Hall said:

KISS. Keep it simple... You have a thermostat which switches the circulator. Great. Keep it that way. Now add a strap on aquastat -- Amazon sells a good Honeywell version -- put it on the inlet to the hot water system from the boiler and wire it in parallel with the house thermostat. Set it to turn on if the water going to the hot water system is too cool -- you may have to fiddle with the adjustment. End of project.

On the Nest. Well, they can be made to work moderately well with steam. Defeat all the "smart" features -- particularly any setbacks -- and you will save fuel and money and increase comfort. If you can get far enough into its logic, set it for one cycle per hour...

If I wire the aquastat in parallel to the 24V house thermostat, how do I also have it only turn on the system when the 120v circulator thermostat is also switched on? Maybe I'm not thinking it through correctly.

Jamie Hall

jbot81 said:

Jamie Hall said:

KISS. Keep it simple... You have a thermostat which switches the circulator. Great. Keep it that way. Now add a strap on aquastat -- Amazon sells a good Honeywell version -- put it on the inlet to the hot water system from the boiler and wire it in parallel with the house thermostat. Set it to turn on if the water going to the hot water system is too cool -- you may have to fiddle with the adjustment. End of project.

On the Nest. Well, they can be made to work moderately well with steam. Defeat all the "smart" features -- particularly any setbacks -- and you will save fuel and money and increase comfort. If you can get far enough into its logic, set it for one cycle per hour...

If I wire the aquastat in parallel to the 24V house thermostat, how do I also have it only turn on the system when the 120v circulator thermostat is also switched on? Maybe I'm not thinking it through correctly.

Well... if you want it to not run the boiler unless the circulator is running, that would be what is known as a cold start -- which is alright, so far as it goes, but if the boiler is cold it's going to take a while for the baseboards to do anything. Since the baseboards probably don't really need much more than 140 degree water, the boiler usually will be at that temperature anyway -- and certainly will be if it has been steaming recently -- and you don't want it to run if it's already hot.

However, you can do that if you like -- what you will need is a relay with a 120 volt coil, the coil in parallel with the circulator, and normally open contacts in series with the aquastat.

Adds a mostly unnecessary layer of control, but that will work.

HVACNUT

> @jbot81 said:
> (Quote)
> Would something like the Taco SR502-4 2 Zone switching relay work? It looks like if I add that in between the 24V transformer and the thermostat, one zone can be set to turn on just the burner and the other can turn on both the burner and the circulation pump.


Yes, that will work. Say zone 1 is steam. Wire the Nest to Zone 1 on the SR502.
Wire the circulator and new low volt thermostat to Zone 2.

Run a low volt wire from XX onto the SR502 to Rh and W1 according to your drawing.

The only operating limit is the pressuretrol. There is no aquastat but the hydro zone should never run long enough to cause the boiler to make steam.

mattmia2

It would work, but would be far simpler and less expensive to go with the aquastat or aquastat and relay option unless you really want to rewire that zone for 24v for some reason.

EdTheHeaterMan

You might want to look at this Sequence of operation logic:

The steam boiler operates on Nest thermostat as indicated on the factory diagram.

parallel to the nest thermostat place a second control configuration that will operate the circ pump and the burner on a call for basement heat.




if the boiler reaches the limit temperature set on the aquastat (maybe 160°) the burner stops. No steam is produced at that temperature so no heat to the steam radiators. once the call for basement heat is satisfied, both the circulator and burner stops.

If there is a call for heat from the Nest thermostat, then the water is already hot enough to circulate and heat the basement.

Jamie Hall

Don't overthink it and don't make it any more complex than it needs to be.