Changing the pump bypass setting on a Vaillant Boiler

[hatcheth]

Hi

Got a problem with a newly installed radiator.

The radiator is at the end of 14 metres of 15mm Plasic Pipe (its 22m plastic for the first 8m). We put a new radiator in by t'ing off an existing radiator pipe buried in the wall.

The radiator is quite large (5kw), the existing one is about 2kw.

Everything was working fine with the new radiator, so the chases were plastered over yesterday. I went to turn up the heating to get the plaster to dry and the new radiator isn't working.

Firstly I thought it just needed to be balanced, so I reduced the flow on the nearest radiators. That didn't work

Then I switched all the TRV's in the house, so that only this radiator was on full on its TRV. That didn't work but I left them all off for the steps below.

Then I took off the TRV and checked the pin and it looked fine, I swapped the TRV for another on that hadn't been near the plasterers and that didn't work either.

Then I added some water to the system to give it more pressure (about 1.8 cold in the end) and that didn't help either.

I put a boiler fixs in the pipes to the radiator to help with the installation to stop the need for constantly redraining the system which are now burried in the wall (I appreciate that this isn't ideal, but I am an amateur plumber so give me a break please!!). I thought the plasterers might have thought that it was funny to turn them off, to allow the plaster to dry evenly so I wanted to check that wasnt the case:

1. I closed the lockshield and the TRV and opened the drain/vent valve at the top of the radiator, this then produced a nice steady stream of cold water. Then I opened the lockshield and the flow of water increased, I took this to mean that there wasn't a blockage in the return pipe. ( I know its the return as it took longer to get hot when the rad was working)

2. I closed the TRV and the lockshield again, opened the drain/vent valve at the top of the radiator. Again a steady stream of water, which increased in flow when I opened the TRV. I took this to mean that the flow valve wasn't blocked.

3. Whilst I had the TRV opened and the lockshield closed, I left the drain to run for a few minutes and eventually the flow pipe got slightly hot. Hoping that I had cleared a blockage I opened the lockshield and instantly the inlet pipe got cold, like the return was at a higher pressure and the cold water flowed back into the inlet pipe.

Strange thing is that during the whole of the above (when all the TRV's were off and every single radiator pipe in the house was cold), the inlet and outlet pipes at the boiler were both boiling hot, which made me think that there might be a path of easier resistance for the water somewhere under the floor boards.

I am 100% sure that the TRV valve is the flow as I clearly remember it geting hot when it worked.

So now I am wondering if the pump is up to supply this radiator at the end of a long pipe. Its a Vaillant 937, had a quick look at the manual and it talks about two interesting things:
1. You can change the pump operation mode D.019 to run all the time:

Setting for operating mode of 2-stage pump:
0 = Burner operation = stage 2, Flow/overrun = stage 1;
1 = Heating + Overrun = stage 1, Hot water = stage 2,
2 = Like 1, but rotational speed during heating mode
depending on heating partial load,
3 =Always stage 2
4 = Rotational speed during heating mode depending on
heating partial load, otherwise always stage 1

Looks like 3 is the setting to have here?

2. Changing the bypass valve setting:
The boilers have a bypass valve.
The pressure is adjustable in the range between 17.0 MPa (170 mbar) and 35.0 MPa (350 mbar). Approx. 25.0 MPa
(250 mbar) is pre-set (mid-position). The pressure changes by approx. 1.0 MPa (10 mbar) each time the adjusting screw
is rotated. By turning right, the pressure increases and turning left decreases it.
Right-hand stop (turned all the way down): 35.0 (350) If the radiators do not get properly hot in the default setting.
Note: In this case, you must set the pump to the maximum stage


I know that opening the cover isn't strictly legit, but I am willing to give it a go to see if it helps. Any thoughts on what I have done and I am intending to do?

Many thanks for taking the time to read this!

 

[Agile]

I will not recommend that you open the boiler or touch the internal bypass or the software pump settings.

I don't know why the once hot rad is now cold when it was hot before.

When you did the equivalent of the bucket test you should have measured the flow rate from each side. With 1.8 bar I would expect that to be at least 10 li/min.

It may be that some piece of moveable dirt has migrated into one of the paths to the rad.

If the flow rate is seriously lower then I would be forcing mains water into that side whilst draining the other to the outdoors or a large bucket and see if that improves or alters the flow rate.

But you should have done more research as the most a 15 mm pipe is meant to supply is about 3.5 kW and you are taking 7.0 kW so don't expect a very good heat output!

It will also need very careful balancing with reduced lockshield settings on all the other rads. Although the correct solution would be to supply both of these rads from 22mm.

Tony

 

[MJGas]

I will not recommend that you open the boiler or touch the internal bypass or the software pump settings.

I don't know why the once hot rad is now cold when it was hot before.

When you did the equivalent of the bucket test you should have measured the flow rate from each side. With 1.8 bar I would expect that to be at least 10 li/min.

It may be that some piece of moveable dirt has migrated into one of the paths to the rad.

If the flow rate is seriously lower then I would be forcing mains water into that side whilst draining the other to the outdoors or a large bucket and see if that improves or alters the flow rate.

But you should have done more research as the most a 15 mm pipe is meant to supply is about 3.5 kW and you are taking 7.0 kW so don't expect a very good heat output!

It will also need very careful balancing with reduced lockshield settings on all the other rads. Although the correct solution would be to supply both of these rads from 22mm.

Tony

And what Delta and Velocity are you talking about there Tony? As its a 937?

 

[kittedup]

A 5kw rad? What size is it?

 

[hatcheth]

Got my units wrong! It's 6500 btu, so about 2kw. The other rad is about 1 kW.

If we leave the pump alone, is there most likely to be flow at a remote radiator with a high or low pressure in the system?

 

[TCCHeating]

is there most likely to be flow at a remote radiator with a high or low pressure in the system?

Makes no difference at all. A system pressurised to 1 bar and a system pressurised to 1.5bar will both flow at the same rate. The pump dictates the flow rate.

You say that the radiator used to work. So you either have a blockage or an air lock.

 

[hatcheth]

Hi .

Thanks for your responses so far..

I have made some limited progress in the cold radiator issue. I flushed the system out and put mains pressure through both pipes. Doing this allowed the radiator to get luke warm, with the inlet getting very hot but the outlet is still cold (and fully open on the lockshield).

I have then started switched other radiators on and both pipes go cold. All the other radiators are balanced with a good drop in temperature on the flow and return pipes.

I am thinking that it might be time to switch to a plan B. Would switching to a 3000 BTU radiator increase the chances of getting a flow through the radiator and allow it to get warm?

Regards,

 

[TCCHeating]

I am thinking that it might be time to switch to a plan B. Would switching to a 3000 BTU radiator increase the chances of getting a flow through the radiator and allow it to get warm?

Regards,

No. You need to find the cause of the problem. If you have closed all other rads and the problem radiator still isn't getting hot. Then installing a smaller radiator will be an expensive waste of time.

 

[hatcheth]

It does get lukewarm when all the other radiators are off... but then goes stone cold when they start to be switched back on. Its a fairly large house with 15 radiators of different sizes..

The puzzling thing is that with only this one radiator on, the flow and return pipes are both very hot at the boiler. This radiator is the only one which is on and it has a cold return pipe. I am struggling to understand how the return pipe at the boiler is so hot.

I could go back to getting the pump speed increased, but I am guessing that will require a special valliant engineer with the right software tools?

 

[Agile]

The boiler has enough pump power to create sufficient flow in systems with adequate pipework.

I did say that you should have measured the flow rate out of each valve supplying this rad when you did the bucket test.

But nothing seen!

Tony

 

[TCCHeating]

Are you sure that the rad is piped up correctly. I have seen it plenty of times when the flow and return from the rad are both teed into the ch flow pipe. Normally by clueless builders.

 

[Bunnyman]

As tcc said, u won't solve this problem by messing with boiler pump or bypass, this pump will serve many many more rads than u have... There is a blockage in the rad or pipe work , stop dancing round the problem,

 

[hatcheth]

Thanks for the honest answers.

This rad is in the extension, I can see two 15mm pipes coming out of the ceiling into the house and into the extension. So without taking the whole house apart I can't be sure, but it looks like its piped up correctly.

So to be clear, its normal for the flow and return pipes to be hot on the boiler when only one radiator is on ? The return is cold at the radiator.

Cheers

 

[hatcheth]

This is the problem radiator: http://www.ebay.co.uk/itm/310306635036?_trksid=p2055119.m1438.l2649&ssPageName=STRK:MEBIDX:IT

 

[Agile]

I am surprised when you have bought a fancy designer rad that you don't seem to have had a competent plumber to connect it to the system who you could get to solve your problem.

It may not be relevant but many of those rads must have the flow inlet on one particular side and this is hidden in the instructions and not marked on the rad itself. As if most installers read instructions!

Tony