Extreme Hot water fluctuations in shower. 4 plumbers. No solution. Help!

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Lishaana

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We have 2 Rinnai tankless hw heaters. We are on well water and have a Grundfos constant pressure pump. When our house was built , both tankless heaters were at one end of the house in the basement side opposite our master bath. We decided to finish the basement and it was suggested to move one of the heaters to the other side of the basement(closer to our well pump and master bath) so we can get HW faster in the master bath.

So, 1 tankless was moved and it supplies the master bath on the 2nd floor and the guest bath on the first floor. The other tankless takes care of the rest of the house.

Since the tankless was moved, we noticed that the water coming out of the Moen shower in the Master bath has been cycling scalding hot and lukewarm every 20-30 seconds. We've had 4 plumbers come out and no one has found a solution.

Here is what we do know.

1. With the tankless set at 120. The hw cycles between 113-125 degrees every 20 seconds or so in the main shower with the showerhead(no flow restrictor) and with the showerhead removed completely.

2. Interestingly, in the same shower, there is a Moen handheld shower on its own valve/controller that does not have this problem when it is on by itself. And if you turn it on at the same time as the main shower. The temp fluctuations go away in the main shower.

3. The Moen Positemp cartridge was replaced twice in the main shower with no improvement. They even changed the cartridge in the handheld shower in case cold water was bleeding from that valve to the main shower with no improvement.

4. The problem exists in the first floor guest shower when the low flow shower head was removed or replaced with a shower head with the flow restrictor removed.

5. The jacuzzi tub in the master bath does not have this problem.

6. The sink temps in the bathroom were measured by 3 different plumbers. The one that measured the temps at the hw output line at the tankless heater(some sort of clamp over the metal pipe) didn't find much variation at the sink. The other 2 used thermometers in the actual water flow and seemed to think there was some temp cycling at the sink.

7. If you replace the main shower head(no water restrictor) with one with a restrictor. The cycling is less pronounced, it fluctuates by a few degrees only and in longer cycles. The water pressure from that shower however is not acceptable for my husband. He feels that replacing the showerhead with one that lessened the flow just covers up the problem but does not solve it.

8. The tankless heater has been checked out for error messages problems and is in proper working order.

9. This problem did not exist for 7.5 years when the tankless was in the farther location.

10. The new gas line to the tankless was also checked out and does have adequate pressure/supply to the tankless.

11. The rerouted pipes in the basement were checked it by all the plumbers and no problems were noted.

Any thoughts? One plumber thought it was related to the cycling pressure drops at the constant pressure pump when the flow rate is high. But this does not explain why the problem goes away when the handheld shower is on at the same time as the main shower (seems like it would be even more flow).

Thanks in advance!!

Nancy
 

Valveman

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I agree with the plumber who thinks the problem is with the Grundfos constant pressure pump. I have seen this many times. Those CU301 controllers are notorious for losing communication with the motor. When that happens the system reverts to cycling on and off with 15 PSI difference. It will do this 1000 times before it shows you that it is a fault with loss of communication. So while this is happening the flow through the shower also goes up and down, which makes the WH come on and go off repeatedly. It makes sense that when you open more water the problem goes away because you are using as much as the pump can produce, and it can't reach the higher pressure to shut off, so it doesn't cycle when using larger flows. Check the pressure gauge on the well pump when this is happening and I bet you will see continuous fluctuations up and down by 15 PSI.
 

Lishaana

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Thanks valveman! I actually had gone down myself last night to check the water pressure at the Grundfos while the higher flow shower was on. When the Grundfos was set at 60 psi there is a continous 0-15 psi fluctuation and you can hear the tankless kicking on and off in unison. I bumped the psi to 70 on the Grundfos and the fluctuation is much less pronounced maybe 4-5 psi. I'll call our well company to come check out the pump. Thanks so much for your input.
 

Lishaana

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Update: I had the well guy out. He made some adjustments(speed and maximum pressure is what I remember) to the Grudfos constant pressure pump, replaced the transducer and checked the pressure in the pressure tank. After that, when the psi was set to either 60 or 70, it only fluctuated +/- 2-3 psi, which the well guy said was normal functioning for the pump (The manual says +/- 3). Despite stabilizing the pressure from the pump, we are still getting spikes of very hot water in the showerhead without the flow restrictor every 20--30 seconds or so.. It is improved but still not right.

Letsrunum- I'm not sure what the what a manifold setup is, but I snapped a few pics to see if that helps. My other thought (from my science/physics background)- If it is friction in the pipes that is affecting flow rate, then why is the problem only in the tankless heater that is closer to the well pump and not at the farther tankless(at least 30-40 feet), since I would expect more friction due to more pipe to go through before reaching the 2nd tankless.

The silver tank that is in the bottom left corner, is the sandmaster for our irrigation system which we have not used at all this season, since we have not been running the irrigation as we have been getting enough rain this season.

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I also plan on having the tankless heaters irrigated/annual maintenance since they slightly overdue (because of our basement renovation)- could lime or scale be effecting the flow rate through the tankless?

Thanks everyone for your help!

Nancy
 

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Lishaana

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The pipe coming into the house is 1 1/4", the pipe on the other side of the filter in 3/4". The copper pipe switches to 3/4" PVC, a few inches past the whole house filter.
 

Lishaana

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I currently have no lighting in my basement because of the remodel(They are in the middle of roughing in the electrical). I'll look tomorrow during daylight and let you know.

Thanks for your input!
 
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