Vokera Compact 28 Hot then cold then hot water......

[ChrisR]

HW flow:

I don't see any reason to assume "The flow through the DHW side seems to be correct ", especially if it's less than it was! Simply measure it! You should expect about 10 litres per minute from a hot tap.
If it isn't, though the little O ring referred to above may look OK, but they sometimes swell up, surprisingly. If you let them dry out they shrink again!
If you take it out (advised) you might find you have to reduce flow at a tap say for a bath in winter to get your HW hot enough, but I doubt it'll be a problem.

Even so, reducing HW flow shouldn't make it go hot and cold!

Onoffonoffon:
Traditionally sludge in the primary side of a heat exchanger, though the 4 pipe types are less sensitive, I think. Can also be faulty sensor(s) , or, sludge built up around a sensor. This insulates it from seeing a temperature change quickly enough. Best to put new sensors in and poke about the hole checking for sludge. (This assumes it's a "wet" sensor, not in a dry pocket).

You could try treating the system chemically to help clear the h/ex. Inject cathartic elixir, run for a week, drain, flush, refill with Inhibitor.

 

[paulrushton]

Agile

Do i just put a multimeter across the different thermistors attached the heat exchange. What should the typical reading be when cold?

 

[Trance Nation]

Id go for the thermistor first anyday. No offence but i think some of you are looking into it a bit far ! I find even a minimal amount of scale on the hw thermistor causes it to fail, in some cases giving the on/off prob the op has.

FAO op: Theres nowt much to these boilers. Dont worry about sludge etc till the basics have been looked at. The hw thermistor is located at the rhs of the boiler, the pipe closest to the front, rubber cover thingy with thin wires coming out of it. Isolate electricity, pull back small cover, turn off water to boiler, unscrew thermistor, replace and re-assemble. Cost of thermistor id say is around the £5-£10max region. Cost of de-sludging/replacing heat ex.....£££££££

If in doubt get a CORGI Engineer to check out.

Good luck.

 

[weareleeds]

Why dont you just take out the restrictor & sling it set water flow rate with black knob @ bottom rhs of boiler

 

[paulrushton]

Thanks thats helped a lot i'll check it tommorrow night seems the simplest approach yet. If it isn't that then i'll start with the more advanced things. Your all out there probably saying that the things your suggesting aren't that complex but a complete novice like me they are!!!

Thanks though for all your advice it really is appreciated (Please dont think i'm not grateful, i really are)

Paul

 

[paulrushton]

Well i whipped out the flow governor and tried the boiler which made no difference. So then whipped out the secondary thermistor. This was scaled up a little. I cleaned this off and re-tried but still the same. Therefore just ordered another one for £9 from a website i found. I'll try that and take it from there.

I have only checked the seconday thermistor (located on the right hand side of the boiler) I assume this is the only one that deals with the hot water. Do i need to remove the two on the left hand side of the heat exchanger and replace these.

Also how do i check the resistance to see if there working. I assume the resistance across will drop with temperature increase so do i just put a multimeter on the two terminals and measure the resistance. Also what values should i expect.

Think thats it all for now!

 

[paulrushton]

It was the secondary thermister. It was showing an infinity reading on the multimeter so not creating a circuit. ordered a new one and replaced it and it works great now!

 

[Agile]

I am glad you got it sorted out at last although its almost a month since I asked you to check the sensors!

Because they are so easy to check it would be routine for a boiler engineer to check them first before looking at any sludge issues which are much more time consuming. The symptoms of primary sludge and failed sensors can be somewhat similar.

Tony

 

[paulrushton]

I sorted it a few days ago and had a few hicups on the way. I ordered the new thermister 2 weeks ago it came and i tightened it up.

They'd sent the wrong one so it stripped the thread on the copper pipe. I managed to put the old one back in to make do but then had to order a new pipe and the correct thermister! Luckily got it trade from a supplier in Brighouse. UK Gas Limited. Excellant customer service and very very friendly and knowledgable.

Thanks for everyones help it has been appreciated!

 

[Chris46664]

I've got a Vokera Compact 28 boiler and I found that when I had a shower the temperature of the water was going hot and cold. The symptoms got worse when I had the central heating on. Eventually it got to the stage that if I put the hot water on at all it knocked off the boiler completely (including the central heating). I tried to use the fault finding guide which for an average guy like me was difficult to follow. Further more most of the repair actions instructed you to change the printed circuit board! Which is not cheap!

I figured that it had to be something that was unique to the hot water side and not something that was common across both. The Flow switch was fine which I tested by putting a voltmeter across the switch and I could see it switching from 0 volts when the water was on to 5 volts when there was no flow.

I measured the resistance of the Thermistor which gave me a reading but I wasn't sure what it should read in the first place. I did a quick search in the internet and read about other people having a similar problem and they replace the the thermistor. I did the same and it cost me £11. It has worked fine since then.

 

[DP]

Start a new thread instead of extending a thread that is over a year old.

 

[beebee]

I had a problem with hot-cold-hot-cold and also hot water which then cuts out completely. I have a vokera compact 24. Turned out a plumber who had serviced my boiler didnt know what he was doing when checking/ setting gas burner pressures. He set the minimum pressure at very high so the boiler was constantly overheating the water and thermostat was therefore causing it to cut out. Also - possibly the thermostat wasnt sensing properly because was covered in debris. Once the minimum gas pressure was set correctly, the hot water temp remained far more constant. Unfortunatley, having the gas pressures set so high, the water in the heat exchanger had been boiled and made to deposit it's limescale in the exchanger. DHW pressure is now lower. Am not sure how to go about descaling DHW system. Any ideas? Central heating water system I guess I could put a system descaler into the system for a few days then wash it out. I suspect the exchanger can be cleaned better if taken out. Would the plumbers in the forum advise that too?