Water getting hot and cold with pump activation

[dartmouth01]

Hi all, I have a 2015 FR Sandstorm 250SLC (toyhauler) that came with a Shurflo 4008 Revolution By-Pass Pump. This pump acts like an accumulator is in the system, so it doesn't pump immediately unless you turn the water all the way on. Here is the problem:

This past weekend I had the water heater on (electric side) and was running using the water pump. I found the water would get warm-hot when the water pump would cycle off, and then get cool again when it cycled on. This would happen with ONLY the cold water side running, so its not as if the ratio of cold and hot water was fluctuating. Only cold water should have been coming out. I could get the water to stay a constant temp if I had enough water flow to keep the water pump on constantly. Any ideas on what is wrong with the system? I read through the manual for the pump and it does not indicate this as a possible problem in the Troubleshooting section. Thanks!

[dartmouth01]

So I checked everything and found that the shower heads are all turned off at the handles, and there is no water heater bypass that I can see (I can see the tube that goes from the water pump, to a 3 way junction, where 1 hose goes to the water heater cold inlet, and no valve anywhere along there). So I'm at a loss, where can the hot water be mixing with the cold?

I have noticed the hot water mixing with the cold on the kitchen sink, as well the shower. It was worse with the shower, it would get scalding hot when the water pump kicked off, and then get back to comfortable temps when the water pump kicked on.

Any other ideas? Thanks!

[Old-Biscuit]

That pump uses an internal 'bypass' to control flow. Idea it for it to not short cycle On/Off when demand is low.

Pump is coming on/running but bypass is closed at first then as pressure builds up the bypass starts opening (at 40#) and then is fully open before reaching pressure set point (55#)

Like posted earlier......cold/hot have to be mixing for water temp fluctuations to occur out of COLD side of faucet

Is it just out of one faucet or does this happen at other faucets?

If not water heater bypass or showers and only at one faucet then that faucet is mixing cold/hot internally.
If other faucets involved then WH bypass or showers.

[dartmouth01]

This pump is different than the traditional ones I've used before. It works like an accumulator, building up pressure somehow so that even when you turn the tap on, the pump doesn't turn on until after some amount of water comes out (enough to flush the toilet). So I thought it might be an issue with the pump, but good idea, I'll check the WH bypass and the outdoor shower first, to see if they're not closed/leaking.

Thanks!

Tin Pusher wrote:
Pumps typically cut off when pressure is built up in the system, so that sounds normal.

Sounds like hot and cold mixing. WH bypass open?

[Tin_Pusher]

Pumps typically cut off when pressure is built up in the system, so that sounds normal.

Sounds like hot and cold mixing. WH bypass open?

[Old-Biscuit]

Water heater bypass valve open/leaking thru
Shower (inside/outside) not turned off using cold/hot knobs (just with button on handle)

those are the two ways cold/hot can mix other than faulty faucet internals