Heating Wiring Issue?

[Festerfly]

Apologies for not having posted for a while and then my first post being a request!

Basically i have been looking at an issue which has left me stumped and i would appreciate some advice if possible.

My customer had a new indirect cylinder fitted to their Y-plan heating system around June last year. The system is the sort which uses a 2 port valve as a safety valve to stop continuous heating of the water cylinder.

I carried out some electrical work to the kitchen in October but nowhere near the Boiler or any of its wiring (at least 3/4 metres away - Bungalow!). The only time we 'disturbed' the boiler was when the power was shut off to the boiler as it was on the same circuit that we added 2x 2G sockets too. Shortly after this the customer called there plumber as there was a clicking noise coming from the 2-port valve which the plumber replaced as faulty. a fresh problem appeared shortly afterwards which meant that every time that both the CH & HW were switched on the system shut down. The plumber then proceeded to change the head on the 2 port valve (again), & a replacement programmer. This solved the issue for a time but then the High Limit Stat on the cylinder would kick in as the cylinder would get too hot - this in turn causes the system to shut off for safety issues until the reset switch on the high limit stat is pressed (once the cylinder has cooled obviously).

This is where i come in. The plumber is sure that its an electrical fault somewhere as after replacing nearly everything, (inc 3port head, dual cylinder stat). I have gone over everything and checked the wiring layout against both the STD honeywell diagram and the Gledwell Diagram (manufacturers of the cylinder) and cannot find any errors or omissions.

I then went through the system one stage at a time.
1)HW only, 2P valve opens correctly, then by turning the cylinder stat down, the 2P valve closes. As per its design.
2)CH only, 2P valve remains closed.
3)Both HW & CH, 3P valve opens fully, 2P valve opens, and everything is hunky dory....

BUT

After a couple of hours when the system has been running both HW & CH together and the cylinder is nice and warm, the system fails to shut the 2P Valve and then the high limit kicks in causing the system to perform a safety shutdown and the cylinders temp has risen too high.... SOLVED i think. Must be the Cyl demand stat. The Stat when works correct and when the relay clicks it operates the valve properly however the sensor probe is not reading correctly therefore not operating the relay within the stat.

I advise the plumber of this and today he installs a replacement Cylinder Stat. Same problem as before! only this time you now can no longer call for both HW & CH as the system automatically shuts down. The customer has been left with CH from the boiler and HW from the Immersion for tonight until we can go back again and fix the issue.

This is now making both of us (plumber and myself) look completely inept as we have been unable to solve the problem. Although the parts are all under warranty so no cost to either us or the customer, the labour is starting to rack up and it looks like its coming out of our pockets!

The system consists of the following:-

Worcester Bosch 30Cdi Condensing Boiler
Grundfoss Heating Pump
Honeywell 3 Port Head
Honeywell 2 Port Head
Gledhill Indirect Cylinder
Reliance Water Services Dual Cylinder Stat
Standard Junction Box
Honeywell ST9400 Programmer


Sorry for the long drawn out explanation.

I look forward to your advice!

Cheers

 

[zeljuga]

Check Cylinder temperature sensor is properly in its pocket . or possible faulty,cable damaged, in wrong position(clipped to the cylinder flow inlet) . Seem to be obvious culprit .

 

[Festerfly]

That was my thoughts, it had a new (third in 6 months) Thermostat this after noon which was fitted by the plumber.

The only thought that has just sprung to mind (which i will check tomorrow) is that the wire link between the High Limit & User Temp Stat is on the wrong terminals causing the High Limit to activate when the cylinder reaches std temperature? is this Plausible would you think? (the reliance water systems Temp stat is the same a the Danfoss unit. but comes with the link pre-wired)

 

[Spark1979]

Why would you have a 3 port valve then a 2 port valve???? No need for two valves like this the 3 port valve crosses over to the heating side when the hot water is satisfied, Id say this set up is wrong as if the two port valve fails the pump will drive pressure against a dead end as the 3 port valve will side open. It has to be a wiring fault to be causing this problem

 

[Festerfly]

The two port valve is there as a safety precaution as its an unvented cylinder. As per the manufacturer requirement.

All sorted now too. Wiring was fine all along, as we're the electrical parts turns out the system was so full of crud & such that although the valves were turning as per the demand, they were not always able to close or open fully allowing a bypass of hot water to the cylinder (causing the high limit stat to kick in because the CH doesn't run through the cyl stat when on y plan ch demand only).

The system had never been cleaned or flushed.

All sorted though thankfully!

 

[Spark1979]

Never read that a cylinder has wanted a 2 port valve before - learn something new everyday!

 

[Festerfly]

I was a bit stunned when I first saw it too.

Thanks for your advice though.

 

[Spark1979]

Seems stupid doesn't it as cylinder have more safety features than a car to tell you wire in a 2 port valve too! Pleased you sorted it and ill remember this one next time I get problems with valves