Combi fitted to old oil fired system. Uneven heating

Hi Guys and Girls.

Last March, right in the depth of 'The Beast From The East', my Stanley oil fired range decided to S**T its self all over the kitchen floor and the repair bill was in the £1000's and getting someone to do it within a month was impossible, and i needed it doing within the week due to a pregnant wife and freezing cold house.

Anyway, i didn't like the range as cooking was a pain in the arse with poor heat control / slow to get up to temp, the hot water ran out when the emersion emptied, oil was rather expensive, and the pan was always to eventually install a combi anyway.

So that all considered, I got a plumber to fit a combi, we had a nice fancy smeg range fitted for cooking, and we freed up cupboard/ loft space when the cylinder and tanks were removed.

As it was quite a rush job, the plumber just connected the flow/return of the c/h to the old range pipes. Relatively straight forward and it seemed the obvious thing to do. Job done! We did have a few rad's blow as the new pressurised system found the weak points created by sediment build up from the old rusty pump powered system but other than that it was great. Heating on at the flick of a switch (actually a fancy phone app) and unlimited hot water!

Happy wife, happy life????........

Now we're a year on and the winter weather is upon us, the system seems to have uneven heat distribution to the radiators. Upstairs is very hot, downstairs is just luke warm to just about take the edge off the cold. And it’s the downstairs where a couple of radiators seem to take all day to get warm which is no good as we only have the heating on a few hour AM and PM.

During the installation, the plumber said he was going to power flush the system to clean out all the rubbish but that never happened due to time. And the old pipes he joined onto from the range go straight upstairs and then the tee off down stairs at a different place.

Could it be a sludged up system that’s stopping radiators getting hot?
Is it that the feed goes upstairs why that downstairs doesn’t get very warm?

I remember at the time of installation that i was sceptical about a power flush of the system due to the blown radiators as i thought it could find more weaknesses, but with the combi running at the best part of 1.5 bar i think that’s just my paranoia.

The wife keeps on nagging at me so thought I’d ask your good selves before calling a plumber as endless bleeding (no more air to bleed) im none the wiser.

Crikey, talk about half a job. I'd be very surprised if the Boiler Manufacturer entertains any warranty claim if the boiler has been fitted to a dirty system for starters, better hope you dont have a problem.


To be honest, pressurising an old system can always be a risk, as you've found out, but provided it's settled now then you may be ok. As for circulation, if the system hasn't been adequately cleaned out, then sludge could well be an issue, but as the new boiler seems to be getting some heat out and round the system, it cant be completely gunged up. Has the system ever been balanced since the new boiler was fitted?

Was a magnetic filter fitted? Otherwise as above really.

Thank for the fast reply's.

Part of the boiler's 10 waranty was to have the same brand filter fitted (ideal). This is a big bras filter on the return (i think) side of system fitted about a foot away for the combi. The plumber included a 10 year free service plan in the installation which give me great comfort.

I am unsure if said filter is magnetic, but after looking in the trusty screfix catalogue I'd say it was.

I am unsure what a balance of the system comprises of but i don't think it was done.

I have drained the system 3-4 times due to removing old radiators and fitting the new ones, as well as changing small amounts of pipework and adding an additional radiator upstairs, and the water coming out was black when drained but I would not say alarmingly black or sludgey. Although I suppose the worst of the sludge could still be trapped in radiators/low lying pipe work.

What does a flush comprise of? I have 2 drain valves on a radiator in the garage, one at either side of the rad. Could i simply connect a hose pipe to either side and flush it out from one side and out the other?

Thanks, Dominic

my guess is that your changing of rads has meant that you didn't re balance the system or it wasn't balanced to start with.
As a quick guide which should get you close is to close alll upstairs lockshild valves and open 1 turn, and repeat the same for down stairs but open 1 turn but you may need 1.5 turns open for downstairs.

Brilliant I’ll give that a go! Thanks you!

Turn off all the radiators that are heating up and only leave the ones that arent heating open, if they now start heating it is a balancing problem and not a sludge problem

Afternoon gents. Now have even heat distribution. I did as above by closing all valves and opening upstairs 1 turn and downstairs 1.5 a turns.

Thanks very much for your help!
Dominic