Oversized boiler - efficiency issues?

Not open for further replies.
No shame in checking or trying to obtain further information from a manufacturer for non routine matters.

The OP's keen interest in his boiler performance would make one expect that he would have engaged someone very familiar with Vaillant boilers although he was only described as a Gas certified gentleman!

Because the internal adjustable bypass is a rather Vaillant feature, then I would expect anyone regularly installing and servicing the boiler to be pretty familiar with them and not to need to call the makers.
Sponsored Links
What you expect, and what is actually the real world are two very different things Tony.

For example, when someone makes a bold statement, most people would expect them to be able to back it up. ;) does not appear to be the case in your world. :p
That is quite true!

I expect RGIs doing boiler installations and servicing to be very competent at what they work on.

But manufacturer's tech helpline people say that most of the calls are for very basic questions which the caller should already know.

One exception is apparently Alec who calls them with questions about control aspects often outside their ability to answer.

It was even suggested that Vaillant provided a replacement PCB just for him with altered characteristics!
Sponsored Links
Pipe 22mm and quite long pipes; but boiler now range rated at 18 kW so that should be ok I guess. One more question to the person that put this oversized boiler there...
The limiting factor when considering pipe size is the velocity of the water (metres/sec). If it is too low there is a danger of sludge etc accumulating in horizontal pipes; too high and the water can be heard flowing through the pipes. A velocity of about 1metre/sec is normally considered OK. A 22mm pipe can carry 14.7kW @ 11C boiler differential and 26.8kW @20C boiler differential without exceeding 1 metre/sec. The difference is due the different flow rates (litres/sec) required for 11C and 20C differentials.

The pump and ABV are the same on all versions of the 637, so your graph is the correct one.

Yes, the flow rates given in the tech data are those required to provide a 20C differential at max power. This does not mean you have to maintain this rate all the time, which is a mistake frequently made by many. It would probably be better if manufacturers stated the maximum permitted differential across the heat exchanger.


If you look carefully at the graph you will see that the Y-axis is labelled "lift". This is the head available for overcoming the resistance of the heating system, i.e the index circuit. If the bypass is set too low there will be insufficient head to circulate water through the index rad; if too high there will be excess head, which will cause the bypass to open. The horizontal lines are similar to the constant pressure lines on Grundfos Alpha pumps.

I have shown on the chart the flow rates for both 18kW and 6.5kW at 11°C and 20°C differentials. If the flow rate remains constant the differential will reduce in direct ratio with the change in output; i.e 20°C differential at 18kW will become a 7.22°C differential at 6.5kW.

"Restricting the flow" is the wrong way of looking at the problem; the task is to provide the correct flow through a radiator. So if you have a 22mm pipe carrying water at 775 litre/hr (18kW @ 20°C differential) and this is fed to 18 1kW rads, via 15mm connections, then each rad will require 775/18 = 43 litres/hr (0.72 litre/min, 0.012 litres/sec). Given that a 15mm pipe can carry 12kW (20°C, 1.0 m/sec velocity), a lockshield valve will have to be closed down a lot to restrict the flow rate to what is required through the rad. Water is not pushed through a rad by a pump, it is sucked through because the pressure at the return connection is lower than that at the flow.

The simple answer to your question is to balance the system so you are working in the flat part of the curve and the differential does not exceed 20°C. This must be done with all TRV heads removed, pump set to operate in stage 1 and bypass adjusted so the furthest rad is heated.
he was only described as a Gas certified gentleman!

What does " Gas certified gentleman" actually mean ?

As I understand it a Gas Safe certified person is a person considered to have the knowledge of how to carry out gas work in such a way that the installation is safe. It does not include knowledge on how to determine if an appliance is safe. It does enable the person to notice some potentially hazardous faults in an appliance

Only if there is a tick in the box "Domestic Boilers" on the card can the holder be considered to have the knowledge of how to work safely on a boiler.

That tick does not mean the person is competant in or even capable of working all boilers. The manufacturer's instructions for that make and model of boiler should be available to the person working on the boiler.
If flow over heat exchanger is over 1200 l/h, with a differential of around 5 degrees
flow through radiators is only around 700 l/h, with a differential of around 11 degrees
1200 litres/hr @5°C differential equates to 7kW; 700 l/hr @ 11°C is approx 9kW. So more heat is being used up than is created!

Where did you get the 1200 and 700 from?

What were the flow and return temps at the boiler?
@Agile yes, all the way down, clockwise. Says the same in the manual.

Well in that case it should be well away from any bypassing in normal heating.

I have not encountered any faults on them myself but potentially a weak spring or dirt on the seating come to mind.

Well worth investigating in my mind!

I am even surprised that Vaillant tech did not suggest that.

@D_Hailsham, Yes, forgive me my errors rounding up and down. When boiler is at minimum power from the display so there is some reading errors there, plus it reports in whole degrees rounded up or down. So I have unfortunately not found a free energy source ;)

Boiler measuring (d40 and d41) a temperature diffential of around 5º (4.5 to 5.5 in reality). Assuming boiler is near minimum (only graph on display no exact figures) that would equate to around 1200 litres (Even minimum power is a moving target between 6.4 and 7.1 kW depending on water temperature). But all calculations give me *around* 1200 litres per hour.

However, measuring the flow and return pipe just before the boiler myself (with digital 2 temp sensor probe I bought for this) I get, at the same time as the measurements above, measurements in the range of 10 degrees. That would indicate around 570 litres.

So yes they are ballpark figures. It might be 1149 and 574 litres/hour respectively. Question remains - why? As you showed in the graph in the purple line, that is where I want to be?

At least my radiators are on the purple line, my boiler is working much further to the right on the curve, with differential of around 5 degrees, nearer the red line?

I'm baffled.
I don't see why you are baffled.

Clearly the boiler is reporting a differential of 5 C whereas the actual at connections is 10 C.

My only concern is the too high return temperature which is going to reduce efficiency.

Obviously caused by an over active internal bypass.

Yes, the flow rates given in the tech data are those required to provide a 20C differential at max power. This does not mean you have to maintain this rate all the time, which is a mistake frequently made by many. It would probably be better if manufacturers stated the maximum permitted differential across the heat exchanger.

Ok, good to have that clear. Glad I'm not the only one misunderstanding. And it does seem to match the 20 degrees max differential.

I am baffled as was hoping somebody could say

'if you have such a large difference from what the boiler reports obviously you have a '... fill in problem.

An over active bypass would be one such problem, that would help. At least then I can get a good vaillant engineer to replace it!

I am even surprised that Vaillant tech did not suggest that.
Afraid the service person didn't...

Thanks again now I will definitely get a vaillant service engineer to investigate. Dirt on seating sounds like a possibility, it's only 4 year old boiler but obviously there's every chance the installer just threw it in without cleaning the system...

Wasn't my house back then so can't be sure.

I cannot think of any other cause than the ABV.

But perhaps some others might think of something.
Never trust any installer arrogant enough to think he knows it all. There's no shame in checking details with the manufacturers.
Well said Dan , you've hit the nail on the head as far as Agiles arrogance is concerned.

DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.

Are you a trade or supplier? You can create your listing free at DIYnot Local

Not open for further replies.
Sponsored Links