?? Hot water backing up

[GrahamO]

I have a tricky problem with my plumbing that I could use some advice on.
I have hot water backing up the water heater cold water feed line at the bottom of the pump/accumulator cycle, just before the pump kicks in. At the moment I have neither a check valve not a vacuum break so it's not entirely surprising. Here is the problem; Raritan indicates a vacuum break and Jabsco indicates a check valve in the cold water feed. If I use a vacuum break I'm concerned that the system will just pull air instead of water. If I use a check valve it will isolate the hot water tank from the accumulator tank and risk overpressure during heating.
At the moment my accumulator tank is located close to the pump output and is on about a 1m hose. Marked with a red dot in the image here.
I was wanting to try increasing the cut-in pressure of the pump but that does not appear to be adjustable.
Thanks!

[Lepke]

As water heats it expands. Without a check valve on the incoming cold water, the water heater will push hot water into the cold water plumbing. You don't need a big accumulator tank. You could add a vertical pipe or hose capped on the top, on the hot water side. With the pipe mostly full of air, the expansion pushes water up the pipe, and cooling it goes back down. Mount your pipe somewhere behind cabinetry.
To get better control of cut in and cut out pressure, you'll probably need a separate pressure switch. Lots of pressure switches for sale on ebay, 12v, 120v, etc. I use 2 switches, one normal 40/60 and another that turns on the pump as soon as a tap is opened, so showers, etc., maintain constant pressure. I switch between the two with a relay controlled from the galley.

[chrisr]

the fact that hw is pushing back up the cw line suggests the tank is not installed correctly

your hws must have a pressure relief valve, with an overflow pipe to the bilge or to an expansion tank. this is shown on yr diagram but is it working ?

without this you run the risk of rupturing the hws tank when the water is heated and expands

don't put a non-return valve in the cw line until you've sorted out the pressure relief valve

cheers,

[rslifkin]

Add a check valve on the hot water tank input. If you then start to push water out of the temp/pressure relief valve on the tank when it heats up, you'll want to add a second accumulator in the hot water side of the system to absorb the expansion.

[GrahamO]

Thanks everyone. That is very helpful. To clarify one point. The hot water is not backing up during the heating process. It backs up regularly and briefly when the system pressure is low but still above the cut-in pressure for the pump. It appears that the right solution is to put a non-return valve in the cold water supply line plus a small expansion tank, probably a vertical hose section, between the non-return valve and the tank. The tank does have a pressure relief valve but I'm thinking I should not rely on that for day to day use.