Low Pressure Switch Location

woodshed · September 2015

The combi heating unit does not have a LWCO or a low pressure switch on the heating module. It does have a high temp sensor on the domestic tank but that will only shut down the system until the unit cools and then will re-fire. I would like to install some protection beyond that in the event of no or low make-up water.

I will not be using an auto-fill since the supply water is from a well with a water softener. I'll be using deionized water, no glycol, for the make-up water with no auto-fill.

I was considering using a Honeywell Pressuretrol Controller w/ Auto recycle (2 psi to 15 psi) for low pressure protection that would be tied in series to the TT circuit, so that if there is low pressure the hydronic module would not fire which would also shut off the pumps.

I thought a low pressure switch would be more proactive than a LWCO.

I was going to put the low pressure switch in the Tee below the air eliminator and above the pressure tank.

I'd like to know if this is a correct placement for the pressure switch and my idea in general.

Thanx, Rick

Rich_49 · September 2015

So , you would eliminate parts of a system that would make the liklihood of a low or no water situation on the system side then add controls to shut down the boiler which is protected when it happens ? Woodshed , the only thing you will be preventing is burning up your pump/s on the system side .
The pump on the mass side of the FPHX would not fire with your proposed controls and you could disable the CH pump but the Versa would still operate .

I don't understand the reasoning to protect something that poses no danger with the exception of protecting the circs . Will you have plenty of DI water on hand to get it up and running again ? Seems that not using make up water is counter intuitive . Am I missing something ?

The space heating module does not fire per say , as opposed to pumping water from the tank through the FPHX . The Versa does in fact have a factory installed LWCO on the unit .

woodshed · September 2015

Rich -

I'm trying to protect the system pump and HX pump in the case of low/no water; the LPCO would interrupt the TT dry contacts on the heating module and make a dry contact to an auto-dialer. I would keep a DI-pig as a back-up water source, if needed.

I'm not trying to shut down the Versa; the domestic hot water side would still be operational.

The factory LWCO is on the domestic tank, not on the hydronic module. The hydronic module only has a high limit.

I do not want to use the domestic water source since it's on a well with a water softener. The PEX tubing was quite dirty; I spent two days cleaning and flushing the PEX lines. I'm trying to eliminate/reduce this issue in the future by using DI water.

Thanx, Rick

Rich_49 · September 2015

The water in the Versa's tank is the heat source for the Hydronic module . That module is connected to the tank directly . The HX pump cannot suffer because of anything that happens on the system side . You only need protect the pumps on the System side of the module is my point .

woodshed · September 2015

Rich -

You are correct; thanx for the clarification.

Would you recommend a LPCO switch to protect the system pump in the case of no/low water?

As per your recommendation, I'll be using the VT2218 for the system pump.

Rich_49 · September 2015

I would suggest connecting the DI pig up to the inlet to the space heating module on the left side of that unit . Pressurize the PIG to whatever you want and use a PRV , just as if it were connected to a water source .

If you have zones you will probably be using some kind of Taco or Others relay for valves or circs . Even if it is only one zone (SR501) just let your cutout switch , pressure or low water power that box , if it trips the voltage to the box is terminated . Use the end switch to the TT in the Versa . easier than using the Versa to power the CH pump and going through all the rigamarole you were debating .

Low Pressure Switch Location

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