Vokera Compact - hot then cold.. again!

[vokeraques]

Hi there,

I have the Vokera Compact 28 combi boiler. It began to give the 'hot then cold then hot..' behaviour.. The boiler got a service, and the heat exchanger, and tubes were cleaned out.. a lot of magnetite debris were removed. This greatly improved things..

The other day, when the boiler was on to heat the radiators, I took a shower, and the shower exhibited the 'hot then cold..' behaviour a few times.

I then took a shower the following day with the heating system turned off, so the boiler was just providing hot water output to the shower.. The 'hot then cold..' behaviour did not occur.

I am wondering what the reason for this could be? While running both radiator heating and hot water heating, does it put too much strain on the system, therefore resulting in periods of colder water when it can't cope with output? Or is there any other reason for this?

Thanks again for any advice offered..

 

[John D v2.0]

Was the secondary side of the dhw meat exchanger also cleaned out? The issue is probably not enough heat getting into the water from the primary side which could be caused by crud on either side or the equation.

 

[vokeraques]

Hi John,

There are two metal flexible tubes coming out of the heat exchanger and both of these were removed and cleaned out. The casings which these two tubes are attached to were also cleaned out.. What part is the secondary side?

Thanks again..

 

[John D v2.0]

So actually your boiler has a combined heat exchanger. So the other two pipes should have been cleaned out, the ones with magnetite would only be for the radiators
Take a look at this from nearly 11 years ago and we if it helps
https://www.diynot.com/diy/threads/vokera-compact-28-hot-then-cold-then-hot-water.82883/

 

[ianmcd]

just read your post and you are being given some wrong information, your boiler has what is called a bi-Thermal heat exchanger, sometimes referred to as a twin pass heat exchanger, when you call for CH the pump will run to pass the heat to the radiators, when you call for HW the pump will stop and the heat will be transferred to the water that comes out of your tap, you can never get both at the same time you get one or the other, what you have is a problem called step modulation and not something you can sort yourself , there are a few things that cause this,cleaning out the braided hoses was the correct action as these block constantly, you need a decent breakdown engineer to work out why your boiler is not modulating but is step modulating

 

[vokeraques]

Hi there,

Thanks for your replies. This indeed sounds like the issue. I'm wondering if maybe some more gunk has got stuck again in the hoses. I will look into the step modulation issue.. Sounds complicated

Thanks very much!

 

[John D v2.0]

Step modulating not sure what that is. Maybe the step refers to the boiler shutting off the burner completely on a high temperature sensor. Either way clean out the dhw side of the heat exchanger and see if it helps