Worcester 35 CDI II Boiler HW hot-cold-hot-cold-hot-cold

[AdamWatson]

Worcester 35 CDI II, fitted about 2005, serviced about a year ago according to previous house owners.

We have done a lot of work in our new house. We were moving a some rads. The boiler was switched off and drained down, gas was turned off and left for about 3 months.

Today I refilled the system and turned it on until the rads started to get warm, then turned it off and checked for leaks and left it for half an hour and there was no pressure loss. I then drained down the system from a drain valve on the CH return just under the boiler. The water that came out was quite clean, a little white but really quite clean. I then put 500ml of "No Nonsense" inhibitor and 500ml of leak sealer as it looked to have a little leak out the back of the heat exchanger. I then refilled the system to 2.5bar as stated in the manual and turned it on. The rads heated up quickly and got nice and hot.

The hot water got hot nice and quick but is cycling hot and cold. The demand light is staying on while a hot tap is open but the burner is coming on and off. When the water is off the burner seems to stay on. The lights don't flash to indicate a problem.

From reading this site I think it is either a clogged heat exchanger or a broken thermistor for the hot water. Does this sound reasonable? What should the resistance of the thermistor be?

Any comments or suggestions gratefully accepted

Thanks
Adam

 

[plumbnuts]

Did you vent the pump when you filled up?

 

[AdamWatson]

No, I took the plug off the front of the pump and spun the shaft to make sure it hadn't seized and then put the lug back and filled it up. How do I vent it?

 

[Agile]

You seem to prefer to do your own thing rather than ask for the best advice first.

I would have said use a proper branded inhibitor rather than a cheap unknown brand to just save a few pounds. Do you buy cheap oil for your car?

I would also NOT have recommended putting leak sealer in as that can block important parts of the boiler! Far better to see whats leaking and do a proper repair. Would you really expect a boiler engineer to use leak sealer?

The symptoms are of a blocked secondary heat exchanger.

The sensors are I think on your model the green ones screw in ones and they should probably be 13k cold.

If they are the reddish ones held against a flat plate then they are usually about 25k cold.

Tony

 

[AdamWatson]

Ok so it seems I need to take the heat exchanger off. I didn't buy the inhibitor because it was cheap but because it was what screwfix do and the kit to add it to the system is very cheap. I have no money left after refurbing this house. As for oil, until 3 months ago I restored £1m+ historic race cars so I know a little about oil Yes I use a cheap oil, but its the right cheap oil...

I'll try and consult the forum before I attemp anything else...

 

[AdamWatson]

Right I mesured the resistance of the thermistor and it was 20k (red one) it's pretty chilly so I think that's about right. I tried venting the pump and no air came out. I've got the heat exchanger off and I'll get some descaler in it soon. The water in it was clean so I think it must be scale not sludge. It quite hard water around here. Should the water flow through it pretty freely? Water does go through it but not gushing through. I'm guessing it should flow almost enough to run a tap through it?

Thanks for the help so far.