Weak Shower following a combi installation

Postby panx » Mon Oct 13, 2008 9:52 am


I have just replaced my existing boiler with a Combi (Valiant 837) but my shower has lost 70% of pressure. No pump.

Is this a normal behaviour? or something is wrong.

I have some rads hot a one end (in) but the other side cold and this is downstairs. So I closed the upstairs rad to help the flow but the boiler started making noise. I guess this is the air trapper.

Can this be released from the boiler? or Rads only.

When I flush the toiler upstairs now I get a lot of vibrations in the pipes.... its like a train is passing through. I can stop this by opening the tap in the bath room.

What is going on!!!

Please help
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Joined: Mon Oct 13, 2008 9:13 am


Simply Build It

Postby plumbbob » Mon Oct 13, 2008 9:01 pm

The Vaillant 837 is a superb boiler so clearly this should not be the problem. I fitted a new bathroom with a Mira Excel pumped shower about 12 months ago. Customer was well pleased. Just recently ripped the pump out and fitted a Vaillant combi, The customer is adamant the shower now, without the pump, is far better than before, which frankly surprised me but there you go.

My guess is your problem is you have a poor mains water supply from the street so consequently poor shower performance. Get this checked out.

The boiler should automatically vent any trapped air as it passes through the boiler. Keep an eye on the pressure gauge because as the air escapes the pressure will drop slightly.

Has the toilet now been connected to the high pressure side of the system without a change of ball valve? If it was originally low pressure, the internal workings will have been converted and will now not work correctly.
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Postby panx » Fri Oct 24, 2008 4:27 pm

Thanks plumbbob

You were right! the water pressure was low because the stop cock was not fully opened. This has fixed the noise and the shower. Much happier now BUT!

My rads are not getting very hot and we have removed all the rads and flushed them with water until the slug was out. We also put in cleaner in the system but this has not resolved the issue.

Next, we use the power pump on the rads (flush) but this still has not fix the problem.

We are not sure how to remove this air lock in the system.

Please help.
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Joined: Mon Oct 13, 2008 9:13 am

Postby plumbbob » Fri Oct 24, 2008 8:17 pm

If normal bleeding does fails to remove trapped air the only remaining solution is to break into the system and remove it manually.

Drain one of the coldest radiators (remove it if it's difficult to gain access to the pipes). Place a short length of hosepipe ( 12 inches) onto one of the valves and place the other end in a large bucket. Open up the radiator valve for 3 - 5 seconds, the rush of water should expel any trapped air. If escaping air is heard, vent until it is clear. Repeat procedure on opposite valve.

Do not let the boiler pressure drop to zero as air may be drawn in.

Once refilled, make sure system is properly balanced.
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Postby panx » Mon Oct 27, 2008 11:42 am


I following your instruction as following:

1) blead the rad and removed
2) connect pipe on one end and extracted water. (good flow)
3) connect pipe to other end and extracted water. (good flow)

All the rads upstairs were closed.

I re-connected the rad but no joy!

Is it possible that my new boiler is not able to coupe? The boiler is down stairs and the pipes go up to 2 yards (upstairs).

Upstair I have the following rads:
5 small - single
1 mid - single
1 normal - single

5 doubles - approx 6x11 (aveage)
3 small

I have heard that combi is not good for big house but I am not sure if this is the case now a days.

My shower has excelant flow and my water tap downstairs.

Can I add another pump upstair where my airing cupboard was?
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Postby panx » Mon Oct 27, 2008 4:36 pm

Over the weekend I followed your suggestion in the following way:

1) Close all upstairs rads
2) Bleed the rad
3) connect pipe to one end and extract water (good flow)
4) connect pipe to the other connection and extract water (good flow)
5) reconnect the rad

This had no effect on the rad. :(

More details on the set-up:
The combi is installed in the util room which is downstair and the pipes go up to first floor (1yd).

Upstairs I have the following rads:
5 small rads
1 mid
1 big 5x9
(all single)

5 double rads 5x12 (approx)
4 small

I have been told that combi is not suitable for 5 bedroom house... Is my boiler not strong enough?

Do I need a new pump in the airing cupboard in the first floor where the old water tank was?

My shower has good flow of water so I am not sure is problem exist.
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Joined: Mon Oct 13, 2008 9:13 am

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