Boiler flow and return temperature differential

16 Apr 2015
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United Kingdom
I have a Veissman Vitodens 100 boiler which has been in situ for about 10 years. I have a british gas service contract for about 7 years and at the last service they noticed a leak from one of the rubber hoses inside the boiler. I had been losing pressure over say 1 month period so was happy they had located a leak. So they duly replaced the clip, cutting off about 3cm of the hose but since then the boiler simply does not work as before.

The set up is that the system is an S plan, pressurised, fully pumped with a 22mm MagnaClean next to the boiler return flow. We have 9 radiators, 2 towel rails and a hot water tank.

The boiler fires up and heated water starts to flow round the system but it can take up to 30 minutes for the return to get up to a reasonable temperature. At no point does the boiler lock out, although it did once.

British Gas came and bled the pump, checked the magnaclean and despite there being an obvious restriction are stating that a 25 degree drop between flow/return after 30 minutes is acceptable despite it being the warmest day of the year so far.

My own view is that somehow a restriction has resulted after the leaky hose fix or the BG service. They will have turned off the valves under the boiler and the Magnaclean valves. Is there any way these can stick on re-opening ?

BG are now suggesting a £600 powerflush which they say is not covered by Homeserve 200 although looking at the T&C's it seems to suggest that it's not covered if they have ever advised this before, which they haven't.

It's all pretty frustrating as BG just want to sign the repair off but I know the system is not working correctly.

Is there any way I can test the flow rate of the system to prove my theory ?

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British Gas are really struggling to work out what is going on and why there is no flow around the system. The rads upstairs get slightly warm, presumably due to convection, but the hot water needs boosting with the immersion.

BG have now replaced the pump, and proved that they can pressurise the system and drain from beneath the boiler and have verified the stop valves below the boiler are open. The next thing they are going to try is to sonic clean the heat exchange as they say the boiler is starting at a slow burn modulation and switching off before getting to full gas flow.

It's really puzzling. Any one got any other ideas ?
Its quite possible that the hose was containing a bit of dirt which has moved into a restricted place such as a valve inlet.

With a BG contract you are somewhat restricted because you have to rely of them doing what they chose to which may well not identify the problem. Also as an employee he will be in a hurry to move on to the next job.

You did not make it clear if he replaced the hose or just reclipped it?

The original BG engineer cut off about 2cm from the hose and then used a new clip to secure it.

There is no sign of sludge in the manga clean device so I am presuming sludge us not a problem. Perhaps the sonic clean will sort it out. Fingers crossed.
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The sonic clean is, I think, only used on the plate heat exchanger which has no function when heating the rads.

It sounds to me as if he is lost.

Even if the system water looks clean there are often chunks of dirt and many types are non magnetic and so not collected by a magnetic filter.

The Viesmann heat exchanger can be blocked at the connections....there's a tight bend where the stainless steel coil joints. It's possible any debris stuck to the hose wall has been disturbed and found it's way to the hex causing a partial blockage.
OK thanks .. forgot to say this is a non-combi set up, so no plate exchanger in the unit just the circular stainless steel one.

So a blockage in the heat exchanger could give the symptoms I am seeing, ie good pressure but no pumped flow rate. I can see from the schematics that the pump is on the return side of the boiler so a blockage in the exchanger would stop flow on the output side as the pump tries to push water through it. Is that correct ?

Yes...the temperature differential would appear to be high indicating poor flowrate through the boiler....and on this model the hoses are always suspect (the rubber attracts system debris) and if it's dislodged the connections onto the heat exchanger which are restrictive and form a tight turn can block.

Before any powerflushing is carried out the boiler needs thoroughly checking.

In nearly every case of BG claiming a powerflush was required I found the problem was often very simple....the worst case I came across was a stuck float valve in the F&E cistern...lack of system water causing an overheat. They wanted £800 off the customer :rolleyes: They had even changed the pump but failed to check the cistern just a meter above them :rolleyes: You couldn't make it up.
Excellent this sonic clean if the heat exchanger ought to sort it out then. It's happening after the bank holiday. Will let you know if it fixes the problem.

Thanks for the advice.
Sonic clean?

They might have an ultrasonic cleaner for small plate heat exchangers but a cleaner to immerse a primary heat way.

They need to pull off the hoses and connections to the heat exchanger.

Of course the problem could be elsewhere but if the problem really did start with shortening the hose then that would indicate a boiler issue to me.
H'mm allegedly that is what the BG guy is coming to do on the 6th May and as there is no plate heat exchanger in this (non combi) boiler then I might be getting very annoyed with them.

The problem appeared the day after the hose shortening event and has not bee right since
Sonic cleaning a giononni type heat exchanger is not a problem, we had 2 large machines on my old patch.
I too would be checking the rubber hoses for blockages.
To be honest i dont recall a veissmann being done but glowworms have been successfully cleaned, they were always done if a powerflush was done as it is almost certainly gonna have dirt in the h/ex if thre's enough in the system to need a flush.
I've known profiles and suprimas to have been done as well.

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