Hot water cycling hot and cold

[AdamWatson]

I have a Worcester bocsh 35 CDI II. Last year the hot was cycling and after advice from here I replaced the secondary heat exchanger and all was well. Now it's doing it again. I have had the exchanger off again and connected a hose to it and run water through it. The water ran through well and not a lot of crud came out. Refitted it and it hasn't made any real difference. How blocked do they have to be to cause the cycling? When i ran the water through it, it didnt seem blocked at all. Could something else be causing it? The heating seems fine. While the hot tap is on the DHW demand light stays on but the flame turns off.

Thanks

Adam

 

[AdamWatson]

Anyone?

 

[Agile]

Thermal blockage can be distributed over the plates and not just loose bits.

The correct diagnosis is to measure the temperatures on all four ports of the plate HE.

Tony

 

[petertheplumber12]

Their are 2 pathways for water to flow in the secondary heat exchanger, one is the boiler water that is used as a source of heat, 2nd is rising main the becomes hot water. the one to check is the pathway for the boiler water that heats up the rising main thro' the plates. That pathway must run very freely, you should have a magnaclene on the boiler return pipe, that would protect the sec heat exch from having probs. also a lime scale reducer on the cold water into boiler protects sec heat exch from probs.

 

[AdamWatson]

The water seemed to flow freely through the plate, through both paths in both directions.

Magna clean filter is on the list to save for and buy.

Agile,

do you know what the temps should be across the plate?

 

[Agile]

At the specified DHW flow rate the flow and return temps should be ABOUT 70 C and 55 C on a non condensing boiler.

Its the differential which should not be too high.

But the exact temps to be expected vary between different models and somewhat between different boilers of the same model.

Tony

 

[AdamWatson]

Thanks, I'll try and borrow an IR thermometer and check it. I assume if the main heat exchanger was blocked the boiler would lock out?

 

[AdamWatson]

I've got a thermometer, the water in to the exchanger was 77 and 55 out when the the flame turned off. Does that suggest anything to you?
I was thinking of getting some central heating cleaner and I've got some heavy duty descaler to take the exchanger off again and soaking it for a few hours in them, do you think its worth it?

Thanks
Adam

 

[easyas123Viessmann]

Remove the plate heat exchanger. Fill the primary side with FX2. Wait 30 mins and flush. Repeat. Place back in the boiler and test. Do this all the time and always works. If you want to prevent it repeating fit a Spiro trap to the return to the boiler. Magnacleans have a high failure rate. I have made a rig for full descaling of boilers it can only be used on ally free boilers. When I have used this rig and fitted a Spirotrap 8 have never had a repeat of the problem.

 

[Agile]

An IR thermometer is pretty useless for boiler diagnostic measurements because the emissivity varies so much.

A contact thermometer is required.

Even if your measurements were correct, a differential of 27 C is way too high and indicates a poor thermal conduction between each side.

It needs a chemical to remove lime scale or oxide deposits!

Tony

 

[AdamWatson]

Thanks for the replies. I know the readings are unlikely to be accurate but I made sure I put some tape on the points I was reading as material makes a big difference and kept an equal distance for both so I think the difference should be close to accurate even if the absolute values aren't. I'll get some FX2 and give the exchanger a good soak and save some pennies to get a filter.

Many thanks for the help

Adam

 

[easyas123Viessmann]

Did it work?

 

[AdamWatson]

I haven't had a chance to do it yet, full time job and two small children mean its not easy to find a couple of hours where we can be without hot water. I'm off tomorrow so hopefully I'll try it then

Adam