CH problem

16 Dec 2007
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United Kingdom
Let me set the scene:

Boiler = Radiation FuelSaver 12
Control = HoneyWell Sundial Y-Plan
Pump = Grundfos 15-50
Radiators = 7 singles with double entry valves
Pipes = Microbore

The entire system is many years old, it was here when I moved in over 2 years ago.

Not had any problem from the system from before but when I switched it on this winter the CH has problems.

So what's the problem?

If I have just the CH on, after about 20 mins of running, the system switches itself off!!!!

The radiators are at this point only luke warm. The room thermostat is no where near it's desired temperature.

After about 30 mins it will come back on, run for 1-2 mins and switch off again.

HW is fine, and when switched on will keep the system running until the tank is at temp.

If I switch on the HW and CH at the same time, I get better (but not perfect) results. The radiators reach a warmer temperature, but the pipework around the pump and diverter valve get very hot. Much warmer than the radiators.

The pump seems fine (but I dont know for sure).

The 3 way divertervalve is set to automatic, but I've also tried it in manual.

When the CH switches itself off, the boiler and pump stop, and the 3 way diverter valve changes position (you can hear the motor moving it). Perhaps 2-3 mins after it switches off, there is a noise like running water, the pump is not running.

The system has not had any treatment (anti-rust etc) in the 3 years I've been here. To be honest I'd never heard of it before reading this site.

So what could it be?
  • Boiler : I dont think so, water is heating up nicely.
  • Pump : Maybe, but I can hear it going round and the HW is fine.
  • 3 Way Valve : Maybe, but why would that shut the heating off?
  • Sludge/Rust : Quite probably, the radiators (when they do work) are slightly colder at the bottom to the top, but I'd put that down to hot water rising.
  • CH Thermostat : Possibly, the symptoms fit a broken room thermostat, but I can here it click on and off at the correct ambient temperatures when I rotate the dial.
Anyone any suggestions as to what it might be?

I was a little worried about the sound of running water after the CH (including pump) had switched itself off.

There is no pressure gauge anywhere in my system, I think that is normal for a closed system?

There is a manual valve just above the pump (Bypass valve?) and this is set somewhere between fully open and closed
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i would be looking at diverter valve head, for the switching off problem

the water rushing thing could well be due to a build up of sludge, especially around the cold feed joining system area
i would be looking at diverter valve head, for the switching off problem
I dont understand how the diverter valve can tell the boiler/pump to turn off. I thought only the thermostat/timer could do that?
The room stat 'talks' to the valve, the valve 'talks' to the boiler.
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The room stat 'talks' to the valve, the valve 'talks' to the boiler.
I dont understand what the valve would say to the boiler, it only has three positions (HW, CH, HW+CH).

I can understand the timer telling the valve which position (in this case CH) and I can understand the room stat telling the boiler and pump to go because it's not warm enough, but I dont think a faulty valve could result in the system shutting off before everything is hot??

Can it?

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