It is an 18kW model. I have not calculated the system requirement. It replaced a very old boiler in 2005 and appeared to be functioning OK to start with, even though it seemed to need HW on for best effect.
The Viessmann ov will fire at around 2/3 bars (20-40% output) until the return warms and confirms circulation.
If the boiler is over sized it will never fire at maximum.
Yes, I know that will eventually be the case. I am just trying to get through the winter and don't think this boiler is worth spending more money on at the moment.
Eventually, they get hot all over - although not very hot - but it takes such a long time. It is a bit of a chicken-and-egg situation because, if the boiler is not constantly firing (and I know it can, with HW on), there is not enough heat to distribute to the rads. But if the system is partially obstructed, the boiler may not fire constantly anyway.
What is puzzling me is the precise on/off temperature of the boiler - 41C/60C for CH. It is as if it is internally set. So I have suspicions about the boiler.
i would be looking at the layout and size of the pipework and perhaps consider replacing the return section from the cylinder to the boiler as this is the part that gets choked. the compact is a decent boiler and should last 15-20 years.
a new boiler is going to go the same way as this one unless the system is cleaned out and restrictions removed.
The WB1A is a good boiler but the software plays silly games if there are system flow problems. You will get the restricted temperature rise on/off if the boiler does not clear the heat quick enough. The fact that it runs the HW loop OK shows you there is little wrong with it.
The WB1A is a good boiler but the software plays silly games if there are system flow problems. You will get the restricted temperature rise on/off if the boiler does not clear the heat quick enough. The fact that it runs the HW loop OK shows you there is little wrong with it.
What puzzles me is the precise on/off temperatures of 41C/60C. Now the maximum temperature setting on the dial should be 80C while the 'efficient' setting on the dial should be 60C. Is it just coincidence that it is shutting off at 60C each and every time, or could something on the PCB have failed?
Also, I have run just one upstairs radiator - the second nearest and newest one - and the same problem remains. As running all radiators should increase the flow and reduce the problem, maybe that indicates some blockage very local to the pump, F&E, mid-position valve, return pipe to boiler etc.
The cylinder return is a 15mm T into the 22mm return downstream of much of the CH pipework, and this obviously is flowing easily. The CH pipework has many joints, partly because it has adapters back and forth between 22mm and 0.75".
I spoke to Viessmann technical support today. The boiler works at 20C differential, which could explain the on/off temperatures. It is quite normal for the LCD to show only 2-3 out of 5 bars. He said I had a flow problem, probably the return, and the boiler is OK. There is less flow when CH and HW are both on than HW alone (I did not understand this). The heat exchanger is unlikely to be blocked because the pipes are large diameter. I was not entirely convinced by what he says but have to accept it for the time being.
Prior to calling, I had a look inside to check part numbers. The flow switch looks as if it has leaked, but is quite dry at the moment. Could this be a problem?
No. Why are you not entirely convinced?
As already said several times on here, and by the manufacturer, you have a restriction. Repipe the return from cylinder to boiler. Use some decent flushing product like fx2 or ds40 on a powerflush unit on the rest of the system. Until you do something like this, you will not solve the problem.
I don't understand how, by opening more pipework (adding CH to HW), the flow reduces. What I think may be the case is that software on the PCB puts a more stringent requirement on the flow when CH demand is received.
As already said several times on here, and by the manufacturer, you have a restriction. Repipe the return from cylinder to boiler.Use some decent flushing product like fx2 or ds40 on a powerflush unit on the rest of the system.Until you do something like this, you will not solve the problem.
Repiping will be a hassle, so I want be to absolutely sure it is necessary before doing it. I appreciate all the experience of people on this forum. However, as an aircraft stress engineer, my approach is always to combine experience and explanation before coming up with solutions.
If you have a Y plan 22mm mid position valve then the Viessmann tech could be right; but I have to agree with your view. And your data backs this up.
The Viessmann chap is cleverer than me if he can tell the return is the location of the partial blockage.
On an old ex gravity HW system there is usually an anti gravity valve fitted on the central heating circuit. Powerflushing and (and to a lesser degree, flushes) tend to fill this up with crud. If your Viessmann replaced an old floor stander or wall hung cast iron with unpumped HW I'd be looking to see if the valve is still in place, and if it is, removing it.
If you have a mixture of imp and metric pipe that is a good clue of the age and type, btw.
On an old ex gravity HW system there is usually an anti gravity valve fitted on the central heating circuit. Powerflushing and (and to a lesser degree, flushes) tend to fill this up with crud. If your Viessmann replaced an old floor stander or wall hung cast iron with unpumped HW I'd be looking to see if the valve is still in place, and if it is, removing it.
Thanks Simon. Your posts always open up some useful routes to investigate. Below is the part of the old system - two pumps and, I think, an NRV of some sort on the right. This has all been removed now. The boiler was a wall-hung cast iron one. It always performed better than the Viessmann, but it was condemned and had to be replaced (hung first, condemned later!). View media item 10670I will look around the system for valves and the like. My reluctance to start playing with the pipes is because several of them are hidden behind artexed plasterboard over dry-lined stone walls.
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