Pentair Master Temp 250 Cycling on and off

[Eastern Shore]

Bought a house with an pool and heater. Inlet water temp is about 60 (desire pool temp is set to 80), burner fires up and then cut off. The service heater led light momentarily (sometimes) and the temp read about 100 and begin to reduce rapidly back to 60. A few seconds later the burner fires up again and the cycle begin again. I look on the back of the circuit board and see no fault indicator led on at all.
The pool has not been operational for over a year. The pool technician open the pool couple of weeds ago and tested the heater. It experienced the same problem. He replaced the thermistor. He said the manifold bypass valve was most likely bad. I replace both the thermal regulator and manifold bypass valve today and it still has the same problem.
Any insight or help would greatly be appreciated.

 

[JamesW]

Do you have a picture of the system?

Is the heater plumbed correctly?

Is there an external bypass?

I suspect that the heater is getting pressure but no flow.

 

[ajw22]

Like James I suspect you have insufficient water flow through the heater. I have observed this problem on my MasterTemp heater. With insufficient water flow the heater over heats and the safety shuts it down, when it cools off it restarts, and the cycle continues.

It could be due to an internal problem in the heater, or externally a clogged filter, or a misset valve.

 

[RDspaguy]

Let me be the third to climb on the low flow bandwagon.

 

[Eastern Shore]

I thought that might be the problem last night so this morning I bypass the filter and still the same problems. If low flow then it must be internal to the heat exchanger. Any idea how to flush it. I see the tubes when I remove the manifold, my thought is put a garden hose to the tube and flush. Any idea how to measure flow rate through the heater. Attached are pics as req

 

[Eastern Shore]

What is a misset valve?

 

[ajw22]

What is a misset valve?

miss set valve = valve in the wrong position

 

[ajw22]

Did this heater ever work correctly for you?

Your heater bypass line is installed incorrectly and some of the water flow is bypassing the heater.

 

[JamesW]

The flow to the heater is completely off.

The ball valves are closed. Perpendicular is closed, parallel is open.

Also, your external bypass does not have a valve, which means that you will be bypassing too much water even when the valves going to the heater are open.

 

[JamesW]

Did you try to run the heater with the valves closed?

What pump do you have?

 

[Eastern Shore]

Sorry for the misleading pictures, those were taken when I shut down the heater. I did run the valve in open position when testing heater. Understand the lack of the bypass valve reduce flow to the heater and I will install one in the future. I assume the flow was sufficient without a bypass valve since that was the setup the previous owner had and they said the heater worked fine. Do you think I should check for internal clog before I install the bypass valve. Really not keen on opening up the manifold to access the tubes unless you folks thinks it is needed.

 

[Eastern Shore]

Forgot, pump is an older Hayward 1HP.

 

[JamesW]

The bypass is too big for the flow.

It should be closed when the heater is on.

Don't try to use the heater with the bypass open.

 

[ajw22]

I am not sure how you are going to change the bypass but the best way is remove the ball valves and add a 3 way diverter valve and a check valve. See below...

I assume the flow was sufficient without a bypass valve since that was the setup the previous owner had and they said the heater worked fine.

That is what sellers always say.

Don't run your heater until the bypass is fixed as it is not getting sufficient water flow.

 

[Eastern Shore]

Installed a ball valve in the heater bypass line in order to shut it during pool operations, much cheaper and less work than installing a 2-way valve. The heater now has more flow and works, Thanks everyone. One last question on heater, should keep water flowing through it when not using the heater? Now onto the next pool spa problems (never seem to end) and will be on this site again. Extremely useful site, thanks for everyone advise.