Heating and hot water failure

[Alex]

Guys

This could take some time to explain! I have a gas fired central heating and hot water system that is 9 years old. It comprises an Ideal 40k Btu boiler, Honeywell 4073 3 way motorised valve, hot water cylinder and thermostat, room stat, a Horstman Channel XL time control unit for HW and CH, and a Grundfos 15/60 pump.

The problems started early this year with heating failure. My plumber changed the pump and three way valve electronic head (with the wrong item I discovered today). He got the heating running, but if the heating was on with the hot water (both are pumped via the three way motorised valve), the boiler would kettle up in about one minute and click off. So, we compromised and switched the hot water off at the wall controller and hey presto, the heating would run. We have used the immersion all summer for hot water. I tried the heating last weekend, and guess what, with the hot water off, the boiler kettles up in about one minute and there is no heat. The flow from the boiler is scalding, and the return stone cold. So, I figure it must be:

1. Pump failure
2. Three way valve failure
3. A blockage

I replaced the three way valve last weekend, and no change. I took the (seven month old) pump off today and it works perfectly - I ran it with the bleed screw off and it spins without fault. I took it off its mounting to make sure it hadn't seized during the long summer layoff. I changed the Honeywell valve head for the correct one and still no heating or hot water. I lifted the carpet in the dining room and let the system drain through for one hour and the water coming out is CLEAR.

I am absolutely stumped. Basically there is no return flow to the boiler for whatever reason. I can only imagine that there is a physical blockage in the return pipe. What do I do, strip pipes and re-solder new ones in?

There has never been a dreadful sludge build up in my rads - I have taken them off several times for decoration etc and, OK, the water is a bit black, but there has never been excessive deposit.

Does anybody have any suggestions as to:

1. What is wrong, and
2. How I fix it?

I am on the verge of calling the plumber out but even he was baffled earlier in the year.

Thanks

Alex

[Andy Hall]

How far from the boiler does the flow pipe still feel hot?

Are the pump and valve on he flow side?

Can you feel the pump running when in HW mode?

If both CH and HW were affected, then I would say to check the thermostat in the boiler. On a simple standard boiler, the pump over-run thermostat is typically part of the main thermostat. Failure in this area generally leads to the ump not running at all and the boiler short cycling.

If the problem is DHW or CH only, then the microswitches in the motorised valve are often a culprit, but you replaced that already.

This suggests a problem with the controller or perhaps a loose or broken wire. I would look there next. You may have to do end to end continuity checks along the cables. This would apply if the pump doesn't feel as though it's running in DHW mode.

Only after all this checks out and you are sure that the valve is rotating OK and that the pump is running does it make sense to start pulling the system apart.

One other possibility, is if the pipework is badly laid out around the cylinder with no way to vent from the top of the coil, and you have a big airlock in the circuit.

.andy

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[Set Square]

You say that the CH on its own works ok? So CH return flow is getting back to the boiler ok.

You say when the HW is on, there is no flow. I assume that the boiler overheats and trips out, needing to be manually reset. Is this correct? Under these circumstances, is the pump still running?

If you select *both* HW and CH at the same time, does the boiler still trip? I think you said that it does, but we need to be 100% sure of this.

IF having both CH and HW on results in the CH working ok but the hot water not getting hot, there is one possibility which springs to mind. Is there a gate valve anywhere in the HW circuit - between the 3-port valve and hot cylinder - to enable the relative flows in the HW and CH circuits to be balanced? If so, this could accidentally have got closed - maybe by the plumber who fitted the first 3-port valve replacement. This would stop any flow round the HW circuit - and any flow at all when only HW is selected.

[Dave Liquorice]

Any TRVs? Are your rooms cold enough yet for them to open and allow flow. There should be a bypass of some sort, maybe that is closed or an automatic jammed shut.

Baffled pumbers and heating systems is nothing new. Go for a "plumber" that calls him/her self a "heating engineer" or similar.