Combi Boiler System Cleaning/Flushing.

[Dougie212]

Hi there...

Following on from a previous thread... I have just replaced 2 radiators and have had a number of people tell me I should flush my central heating system, and add various things.. I really haven't looked into flushing the system before, but when we get our boiler serviced its never done, and as such, it may benefit from a clean through.

My boiler is an old vaillant boiler which is awesome..

Ive got a drain plug at the lowest front most radiator so have an easy drain off point. Was just wondering if someone could guide me as to what I need to be putting into the system and is there stuff I add and drain out to remove the gunk that may have built up.

Thanks in advance

Dougie

 

[gigz]

Diy i would. Drain system then leave drain off open and open the filling loop just enough to get pressure to one bar. Turn heating on and close each rad off in turn to clear each rad. Drain again add a system cleaner (i use sentinal x400/x800 but there's a few on the market). Turn heating back on and if using x800 give it a few hours, x400 can be left for a week as its not as strong. Repeat drain and flush each rad again and add x100 (1L for 10 rads approx. Vaillants dont seem to like x800 so personally i would use the x400. Try and make sure you get it all out and make sure all the air is out when you've finished. You can hire a powerflush machine but unless its done properly its a waste (in my opinion).

 

[Dougie212]

thanks so much for your reply and comprehensive explanation!!
could I just ask, when I go around turning each rad off in turn.. do I leave them off and then drain again before adding my x400?

 

[gigz]

No, if you have say seven rads, turn six off, let the mains water push through the open rad, then open another and close the flushed one, and so on. I watch the colour of the water coming out of the hose till it runs clear. Only reason i put heating on is the pump ades the mixing of the muck/fresh water. Sometimes the water is clear from the outset as the muck has deposited itself the the rads/pipes etc. When you add the cleaner open all the rads to get it right throughout the system.

 

[Dougie212]

That makes perfect sense... thank you very much for your help!!

 

[alan999]

gigz I'm trying to get my head around your flushing idea. Why doesn't the water run straight out the drain off without going around the radiator?

 

[DP]

It is not just Vaillant does not like X800, the issue is failing to flush the system properly after using X800. Good chemical, have used it on various boilers including 242. If X800 if not flush out from system thoroughly, it will kill the pump

 

[dilalio]

gigz I'm trying to get my head around your flushing idea. Why doesn't the water run straight out the drain off without going around the radiator?

As long as it's a correctly piped, 2-pipe system, flow will go through the rads to get to the return. Drain cock obviously has to be on the return for this to work and any bypasses closed down!

 

[alan999]

Yes that makes sense as the heating is on and there will not be any back flow. However Isnt there a problem that some of the gunge from the radiator will be pumped past the drain off for one or two circuits at least and be forced through the heat exchanger of the boiler.

 

[Dougie212]

Hi there.. I have just seen some of the responses to my post... Im a little nervous about doing turning the heating on with the drain cock open.... My idea was to drain the system, and turn the flow on without the heating turned on... then re-fill adding the x400, leave for a week, drain, turn the flow on with the drain cock on again (heating off)... the refill adding x100.... do I need to have the heating on to flush the system through..?? I'm sort of hoping not..

 

[DP]

Dougie you need to flush the system properly. Single drain down will not do that. You never get all the water out of the system so will have remnant water with chemicals left in the system when you drain down. On refilling this water is diluted. A conservative estimate is five or more drain downs and refills with boiler run the mix system water. On drain downs collect water in clear container to check water colour and clarity

When using x800, single drain down leads to dead pumps. Have seen this often

Reason for adding cleanser to the installation is to suspend the sediment. It should be removed from the system as thoroughly as possible.

 

[Dougie212]

Understood DP, thank you..!! I just need to confirm... I should refill... run the boiler and empty. I'm guessing I shouldn't be running the boiler as it's refilling.. I know it sounds basic but I'm just very conscious of the pump running with insufficient water running through the system...

 

[DP]

Fill, vent, fill till all rads full. Then run boiler heat system water slightly to aid mixing with what is in the system. Switch off and drain. Repeat as per makers instructions

 

[Dougie212]

Brilliant. Thanks DP.