Expansion tank question

1daveman

I would appreciate some help with my expansion tank. I hope this is not a dumb question, but I have a radiant floor system that has a 70 gallon water tank that is heated with solar batteries. The water flows from the water tank to a propane tankless. If the solar batteries do not heat the water enough, the propane tankless heats it up. The water then flows through a valve that is opened / closed by the controller based upon the thermostat. (the controller opens the valve before turning on the pump, then closes it when the pump turns off). The water then goes through a manifold, through the floor loops, and back. It then goes through an air eliminator that has an expansion tank installed on it, then through the pump and back to the water tank. See attached schematic.

Question. The pressure increases in the hot water tank as the temperature increases, unless the valve that is controlled by the controller is open. If that valve is closed, the pressure increases beyond the pressure relief valve. If I open that valve manually, the pressure equalizes and does not increase with higher temperatures. I cannot just keep the valve open because the controller shuts it when the pump is off…

Should we install the expansion tank elsewhere? I would appreciate any suggestions.

Dave

PC7060

The closed valve and the pump IFC are blocking the heated boiler water from reaching the expansion tank.
Reconfigure to pumping away configuration; ie propane water heater → expansion tank with air eliminator → pump → floor loops.
Not sure why you have the valve on a single loops. The pump tied to thermostat call for heat is all you need for temperature control.

1daveman

Thanks - that is easy enough and I really appreciate the direction.

Before I ask my plumber to do that, I would really like to understand why that is a better design. Would you be able to point me to a reference or something that would help me understand the reasoning behind it? If I understand it, I will be better able to handle ongoing issues if they arise.

Thanks again!

Dave

EBEBRATT-Ed

I agree with @PC7060 that you probably have a check valve in your pump (most newer pumps have them. When the valve closes and the pump shuts down water is trapped between the valve and the expansion tank. If residual heat or the propane tankless or solar batteries heat this water while the pump is off and the valve is closed the pressure will rise.

Don't see why you need a valve either unless it is to prevent gravity flow.

Before doing anything I would make the thermostat turn the pump on and leave the valve in the open position and see if that solves the problem.

What controls the heat in the loop? Does the solar and propane work independently from the thermostat and maintain temp all the time?

GroundUp

It's better because as it is, the expanding volume of water is disconnected from the expansion tank by the closed valve and the IFC. The expansion tank can not "see" the expansion because it's separated from it. As for the valve, it serves no purpose. The circulator alone should be switched on with a heat call and initiate flow. When the heat call stops, the circulator and therefore the flow also stop. It'd be simple enough just to manually lock the valve open and leave it like that, then everything should function appropriately without cutting anything apart.

1daveman

EBEBRATT - excellent thanks. I did not realize the pump would have a check valve, so with that then I see the expansion tank would not do anything if the pump is off and the valve closed.

The only reason the valve is there is because I could not figure out how to wire my Taco controller to turn the pump on when the thermostat indicated without opening a valve first, so we added the valve. It does not do anything else.

The solar heats the water in the tank to a set temperature when the battery capacity allows for it, independent of the thermostat. The thermostat turns on the pump when the room temperature is below the set point. If the water in the tank is hotter than 120F, the tankless allows the water to flow but does not heat anything. If the water in the tank is less than 120F, the tankless heats it up to 120… That part all works really well.

Thanks again

1daveman

Groundup - thanks so much. That makes sense and I should be able to just lock that valve open. Your explanation really helps.

hot_rod

Do you have solar thermal collectors heating the tank? Or an electric element running off a PV array?

When the tankless runs, you will heat the 70 gallon tank to the return water temperature from the radiant. So you limit some solar potential by warming the tank that way.

To maximize the two heat inputs you could run off the solar tank until it reaches a low temperature, say 90°. Then bypass the tank and heat with a tankless. Although a tankless is not an ideal radiant heat source.

Although unless you have a very small space, 70 gallons will not supply a lot of heat energy.

1daveman

Hot_Rod

I have electrical elements that heat the water when the batteries are fully charged, and an Aquastat that turns that off / opens a relay, when the water gets to a set temperature. I plan to set the aquastat to about 150 - 160 degrees. I understand that is not a lot of heat capacity, but wanted to use the extra PV capacity since I have it. While we are at 5000 ft elevation we live in Arizona so the cold temps are not too severe. We have about 1500 sq feet o the ground floor. I will look into your suggestion of bypassing the tank when it gets down to 90 or so. Thanks!

PC7060

https://forum.heatinghelp.com/discussion/comment/1829184#Comment_1829184

agree with @EBEBRATT-Ed and @GroundUp - no need to reconfigure since you already have pump configured for pumping away from the expansion tanks. Air removal would perform better at highest / hottest point in system but not worth the bother; just lock the valve open.

hot_rod

can your pv system heat your domestic hot water also ? That is a good load for solar as you use hot water most days

Sun Bandit is sn example of PV direct to an elements in a tank. No need to involve you battery bank this way

https://sunbandit.us/how-it-works/

GroundUp

Assuming you have a Taco ZVC controller with 4 terminals per zone valve, I believe you can run a jumper between 3 & 4 (as a 2 wire valve) which would turn the circ on when the thermostat calls. Or swap it out for a switching relay such as an SR501.