draining combi boiler heating system

[Tim Smith]

Hi

I need to replace one radiator and also replace the valves on another. Ive never done drained the system before so I thought I would ask the experts.

My main confusion is whether there is valve that I can open on the Vaillant VCW GB240H combi boiler (at least that is one of the three model numbers on the instructions).

I have put a picture of the pipework at

have workred out the following. There is a braided pipe that goes from 1 to 2 with a valve at each end. If I open both these ( as I have done in the past) then water is added to the system. The pipe at 2 is hot so is therefore the heating circuit whereas pipe at 1 is the cold water supply. I therefore assume that 3 is where the cold water supply supplies the hot water system.

The valve at 4 has some kind of locking mechanism on hte right hand side and has a connection to the overflow pipe. Is there something here I can use to drain the system ? Do I turn of the mains to hte boiler first ?

To re-fill the system, do I simply start refilling the system ? Should the boiler be on so I can feel the hot water in the radiators ? I guess I need to open the bleed valves ? Shall I do this on one radiator at a time ?

I tried searching for a step by step guide but failed miserably. Can anyone help ?

Thanks

Tim

[Ed Sirett]

See FAQ on SealedCH for a bit of background.

Identification:

1) The filling loop entry valve. 2) The connection of the filling loop to the primary return from the radiators. 3) A back nut on the mains cold water inlet connection to the boiler. 4) The pressure relief valve.

The slick way to drain your radiators is. Turn boiler to HW only. Turn the valve which is about 3cm to the 'south east' of (3) so that's its slot is horizontal. Do the same with the similar valve about 20cm to the left on the primary flow pipe.

The boiler can still be used for HW. You can work on the radiators as needed. You can remove the water through a drain c*ck somewhere (ideally it is situated outside) it will look like screwfix part #11421. Expect the washer of that drain c*ck to be totally knackered.

Since you are replacing a radiator and that will likely mean replacing both valves, totally draining the radiators is advised. However with care you could be able to work on one valve at a time without draining down the unaffected radiators.

If the boiler pressure gauge falls when you begin draining then turn the boiler off.

Before refilling turn the boiler off and open the isolating valves back to slots vertical. When refilling care should be taken to expel as much air as possible (the boiler has an auto air vent but it could easily be defective or its dust cap closed). I recommend turning the gas isolator off (it's just above the metal bracket joining all the pipes and its on the middle pipe of the three small pipes) it operates with 1/4 turn, this will allow the pump to get the air out of the heat exchanger safely before the first firing. I'm being a bit cautious here but I don't know your level of experience.

HTH

[Tim Smith]

Hi Ed

Thanks for the reply. I did actaully do the work I needed today before I saw your reply. Everything seems to be OK but here's what I did.

1. Shut down valves of radiator to be temporarily removed and removed radiator after draining through bleed valve.
2. Turned boiler to permanently off on timeswitch.
3. The system doesnt have a drain c*ck, so I drained the heating system through the valve of the radiator that I had removed - only got a bucket or two.
4. Got shouted at by wife as hot water didnt work !
5. Replaced valves on radiator.
6. Pressurised using filler loop upto 1 bar (where the red mark is).
7. Cross fingers, turned on boiler and monitored for leaks and also pressure drops.
8. Pressure rose but only within normal limits and still appears to be fine.

So, now fitting this in with what you said.

1. I did try to shut off the two valves you mention that arent numbers but at least 1 of them didnt want to turn at all - so I left them.
2. I didnt turn off the gas valve (again because I hadnt seen your post).

I still need to go round bleeding the radiators and (I guess) filling the water up a bit again.

I assume that everything I did, despite it not being exactly as per your instrucitons, is OK and that Ive not left anything in a dangerous state. My level of experience is none when it comes to boilers but I like to learn and have a modicum of intelligence.

Question though, given that in a combi the hotwater and heating are separate, why didnt the hot water work when I drained the system of water. I surmise the following - both systems hot water and heating pass over the same flame. The boiler realises that there is no water in the heating becuase I drained the boiler as well (since I didnt shut off the two valves) and refuses to fire up. Is this correct ? Therefore if I had shut off the two valves, the boiler would still have had water in it and fired up....

Is this all correct ?

Thanks for the help

Tim

[Tim Smith]

Oh yes...and should I be adding some inhibitor or something now.

Does this have to be done at a particular location in the circuit ?

Tim

[Ed Sirett]

It would probably have moved with enough oomph and the right tool, it hasn't moved probably since the boiler was installed and that was probably in excess of 10 years ago.

I was being very cautious. I think you were safer than I made out because the boiler did not get drained and so little air got in it. In the worst case where there is too much air the pump ceases to work. However in this model that would be sufficient to stop the boiler working.

Yep, but you might as well add inhibitor first.

In nearly all models the gas heats the primary water and that heats the domestic HW in a secondary heat exchanger. No primary water means no heating or HW.

You can add inhibitor any place thats convenient. I usually add it at the non-bleed end of a convenient rad and let the displaced air out through the bleed point. You need to have the rad at least partially drained before adding. See the SealedCH FAQ

HTH