Unwanted Flow

[jebatty]

Not quite ghost flow, but looking for a solution. I'm using a plate hx to charge a DHW tank, plumbed as shown. The DHW side circ is a 15-58 IFC. What's happening is that when there is DHW demand and the circs are not "on," I'm getting an unwanted cold water flow through the domestic side of the hx which is letting cold water into the hot water supply from the DHW heater, thus cooling down the hot water supply output. I assume that the positive pressure on the cold side of the hx combined with the negative pressure on the hot side of the hx is sufficient to overcome the IFC and allow the flow. I thought about adding a spring check valve but am concerned about the amount of head it would add that would reduce the flow through the hx when the circs are "on." Also thought about adding a zone valve to block flow except when the circs are "on."

Other ideas:
1) input to the hx from the mix line and output to the cold water supply to the DHW heater? This might also allow flow through the hx, but at least that flow would be at the mix temp, not at the 50F cold water supply temp, and would be back into the tank and not into the hot water supply.
2) ?

Thanks.

 

[Clarkbug]

Jim,

What about moving the inlet connection to the Hx from the cold water line to the drain on the bottom of the hot water heater? Then you are still pulling from the bottom of your tank, but you wont pull any 50 degree water since your hot water tank temps will be higher than that.

 

[jebatty]

Excellent idea. Now I wonder why that didn't occur to me. Thanks.

 

[Medman]

Jim, what size plate exchanger are you using for this setup? I am thinking of replacing my sidearm with a flat plate HX and wondered about sizing. I have a 40 gallon DHW tank, incoming water is 150-190 degrees from boiler.

Thanks,

Ryan

 

[Clarkbug]

While you are at it, you could also connect the flat plate to the T&P Valve opening, and decouple your Hx from the distribution side entirely. That would avoid any of the pressure changes that would cause flow. I dont know if it will give you any other gains after moving the cold water side, but its a thought.

 

[in hot water]

I like c-bug's idea, use the DHW tank as a hydroseparator. If you do remove the T&P put a brass, or stainless tee there an get an extended probe T&P to replace it. You really want the T&P element into the tank.

If you do need a check let me know. The spring checks we offer are a low pop (1/2 psi) and a soft seat to prevent any noise. Basically the Neoperyl that are used inside the circ pumps.

hr

 

[Fred61]

Just be sure your fittings and checks are lead free if using them on the potable side.