Taco SR503-exp and 2 zones

JaymeHart

I have a single pipe steam boiler heating most of my house, with a hot water loop for the kitchen (just a pump, no heat exchanger or expansion tank).  

I have the boiler wired via an SR503-exp.  Right now, everything works off a single thermostat, and both zones energize.  This doesn't work well for heating the kitchen.  

I would like to have the steam come up and heat the house, then shut off the boiler, and just run the pump for the hot water loop to satisfy a thermostat in the kitchen.  

Can I wire the Taco for priority on the steam line, so that the Priority X X connect actually triggers the boiler, and leave the normal X X connection unconnected?  That way, if I understand the instructions sheet, my pump doesn't run when the Priority is called (steam), once it is satisfied, the kitchen thermo will trigger the pump without firing the boiler.

Does this make sense?

HVACNUT

So you'll wire the steam thermostat to the priority zone, then wire XX priority to the burner circuit. 
The kitchen will get a new thermostat and wired as a zone, but will not close the burner circuit. 
So when when steam is cranking, the kitchen cannot run, but when the steam thermostat satisfies, the circulator will run but not fire the boiler.
There's no aquastat on the boiler?

JaymeHart

Thanks.  I dont think there is a hydrostat.  Just a steam pressuretrol to cut out when steam pressure gets high.  I have it set to cut out around 2.5

The house was originally just steam.  The hot water loop was added when we redid our kitchen.  

The boiler does seem to run better when the hot water loop pump is not running (so it has to heat less water).  

This change to priority setup should help the steam run efficiently, then allow the hot water loop to run for however long needed to heat the kitchen.  

109A_5

Hello @JaymeHart,
I would do it something like this. Depending on how much water your boiler holds and the heat loss of the kitchen and the radiation load of the hot water loop, the aquastat will keep the boiler from steaming (shoulder days) when heating the kitchen and/or the boiler from going too cold (design days), XX Main End Switch to fire the Boiler. The rest of the house would be steam XX Priority End Switch.

JaymeHart

Thanks 109A_5.  That helps.  I am considering separating the hot water loop via a heat exchanger and expansion tank.  I will likely add the aquastat at that point.  

109A_5

Hello @JaymeHart,
To me when you add the second thermostat for the kitchen you need the aquastat too, but not necessarily the other stuff you mentioned. If a call for heat in the kitchen allows the boiler to steam you may be heating the rest of the house too, since the BTUs needed for the kitchen only may be a fraction of the boilers capacity. Your comfort may still suffer. Running the boiler temperature lower (at an appropriate temperature) during hot water calls may help with circulator life too.

On colder days if the kitchen loop can't keep up because you don't fire the boiler when needed to heat the kitchen your comfort may also suffer. And the boiler will take longer to recover to the point it starts to steam again for a call for the rest of the house. Since you cooled it down significantly trying to heat the kitchen like in your original perposal.

I suppose you can experiment with it.

Joe Mattiello

Yes, the priority feature will allow steam to heat home,  if your priority is heating home, and leave hot water to be interrupted. That’s not preferred, but it will work. Call Taco tech support at 401-942-8000, and ask for tech support. Repost the outcome, I’m curious how this plays out. 

JaymeHart

Thanks Joe. Yes, this will be an experiment . I agree I need to test it out and perhaps tweak it during the course of the winter depending on how cold it is outside.  I will post back once it gets cold enough to run.  

JaymeHart

Reporting back.  We haven't had sub freezing temps here in NY yet, but so far this setup with Steam as priority and the hot water secondary loop thermo only controlling the loop pump and only running after Priority is done is working well. I also adjusted the thermo to run the boiler a min of 12 min when it fires.  

The kitchen is now always up to temp every morning.  

The other benefit I have noted is that there is now no risk of flash boil (my boiler is in the basement, with the kitchen radiator upstairs). When the hot water loop kicks in, it is always at its hottest point.  I can set the mixing point to be 170 and not worry (since I dont have an aquastat).  

109A_5

Hello @JaymeHart,

It will be interesting to see how well it works when it gets significantly colder outside.

For best equipment longevity, you don't want the water too hot going into the circulator and you don't want the boiler water getting too cool.