No Hot Water even though Central Heating OK

[Flinstone]

We have an indirect heating system fitted with the following:-
an Ideal Mexico gas boiler, a Honeywell V4044 diverter, a Grundfoss Super Selectric pump, and a Randal 103 electro-mechanical timer.

The water in the cylinder doesn't heat up however the boiler will fire up from cold and the pipework to the feed point on the cylinder is hot, but the return pipe is always cold! After around 5-10mins the boiler shuts off, presumably due to lack of circulation (??)

If I turn the cylinder thermostat down and the room thermostat up the radiators heat up as per normal.

The cylinder return pipe has an automatic vent which has been opened to vent any air and the pump & diverter recently changed to eliminate them.

No prior warning symptoms occurred prior to the fault and no work was done beforehand. Is it simply a case of a likely blockage in the return somewhere, just from the hot water cylinder or what? pipework is 3/4" I think.

Any advice is greatly appreciated.

 

[Flinstone]

Sorry, forgot to mention that I disconnected the pipework from the HW cylinder and checked there was no blockage in the coil itself (if it helps).

 

[croydoncorgi]

If the pump's on, the diverter valve open and the coil input (top) connection on the cylinder hot, it seems impossible for the bottom (out) coil connection to be cold! There must be no flow round the primary part of the system?

How long is the pipe between the body of the diverter valve and the cylinder? Short enough to fool you (with conduction) that hot water is actually flowing into the coil?

Have you checked that the motorised valve is actually working?
The 4044 is not the common 3-port type found on domestic systems: it's motor-open, spring close but does NOT control the boiler. So you could be in a situation where the hot water appears to be ON, the thermostat is calling for heat but the valve is sending water to the radiators because it is broken and not switching the flow to the cylinder.

 

[Flinstone]

Thanks croydoncorgi.

Just to make life a little more complicated, in this installation I believe the coil input to be at the bottom. Incorrect I know but it seems to have been that way for the past 25-odd years.

The pipe between the valve (downstairs kitchen) and the cylinder (upstairs bathroom) is approx 18ft in total.

The way I checked the valve was to turn down the thermostat on the cylinder. Only then did the radiators heat up. At the same time the coil input pipe cooled a lot to begin with perhaps due to the redistribution of the existing hot water. Is this method ok to confirm the valve operation?

Regards

 

[croydoncorgi]

If the Flow to the cylinder is heating up and the pump is running but still no hot water, this is seriously confusing!
However...
If the return from the cylinder stays cold obviously(?) there's no flow through the coil and the boiler starting then shutting down tends to confirm this.
Plumbing to the cylinder coil being upside down may be contributing.

The cylinder return pipe has an automatic vent which has been opened to vent any air and the pump & diverter recently changed to eliminate them

Did any air actually come out? Auto air vents frequently block. With the return at the top of the coil, there could easily be an airlock.

With the pump OFF, try again to vent the system at the top of the coil. Which type of vent is it? (Honeywell Beckmann type - red plastic dustcap? - can be isolated locally and cleaned.)

Has the system worked properly since the pump and valve were changed?

 

[Flinstone]

Yes, plenty of air came out of the vent until water was present.

I initially unscrewed it to ensure any air was expelled at that point (the vent is a new one bought from B&Q, I hope I haven't misled you on this - its the type with the wadding that swells when in contact with water).

No, the system still hasn't worked properly since the pump & valve was changed.

I'll try venting with the pump turned off again.

Do you think it could be an airlock and would it be worth trying a drain blaster on the feed tank?

Thanks again for your help & suggestions.
-

 

[Flinstone]

Well I'm pretty much convinced that the system has been vented ok at all rads and the HW tank.

The current situation is that after leaving on HW only for around 2hrs+ the return pipe from the cylinder eventually gets warm, but not as hot as the feed pipe.

I guess there must be a restriction somewhere in the return pipe between the cylinder and the boiler.

Is it now left to go as far as trying to replace sections of the pipe?

 

[Slogger]

i have similar prob new gas valve overheat stat thermocouple now no hw but ch is fine when i go to hw on control panel nothing yet when i select ch it is ok i tried to move divertervalve by the manual slider it is slack
so i take top of and try with screwdriver and it is very hard but is does move then goes back ? no matter if ch or hw is selected

ideal classic rs 40 wall hung balande flue 20 yrs young ?? any help would be nice