I had a new Combi fitted in August to replace our near 20yo Vokera Combi.

We had a Baxi 33 Platinum installed.

Once it got cold enough to need the heating back on we noticed 3 of the rads were not getting hot any more.

The feed to all 3 gets hot but the rads only ever get slightly warm.
I have tried to balance the rads and made no difference.
replaced the lock shield and TRV on the 3 and made no difference.
If i turn all the other rads off except these 3 they still wont get hot.

They appear to be the last 3 in the system.
Boiler
kitchen
living room small
upstairs bedroom 3
bathroom
bedroom 2
bedroom 1
living room large (cold rad)
Office (cold rad)
Snug (cold rad)
then back to boiler i think.

How long should the boiler run for when heating the rads?
when watching it when it fires up the temp will rise from 50/60 up to the set temp and rise slightly higher maybe 90ish and cut out again. All in about 10-15 seconds.

Both the feed and return to the boiler are hot. It seems like the heat is getting back too fast to the boiler before it can heat the last 3 rads.

open to suggestions.

Are you 100 its fully bled?
Can you adjust the pump speed?

its probably needing re balanced, see the link

the water will take the easiest route back to the boiler (which its probably doing)

https://www.homebuilding.co.uk/how-to-balance-radiators/

Simple check. Turn ALL the other rads off in the house except one of the ones that doesn't get hot. See if it then gets hot. Then add the other lukewarmers.

If they do work, all teh other rads may only need to be "on" a tiny bit

Tried and it makes no difference. Thay still do not get hot and only slightly warm.
Had all the hot ones shut off. And tried the cold ones one at a time and non get hot.

Sounds like a bypass somewhere...

Was any pipework changed with the boiler? Or a powerflush?

Could be a blockage in a pipe that feeds those 3 rads. In fact that is looking a likely candidate if nothing was changed. But if just one of those rads is on, all the water from the boiler should be passing through it. Otherwise the boiler return would be tepid too.

Some serious investigation needed. Could be a heated towel rail or similar as bypass, and possibly a blockage too?

It was a combo to combo swap. Nothing else was touched. A blockage is what I am thinking too but no idea how to get rid.

No power flush was done.

What do you mean by a towel tad being a bypass?
We have a towel rad in the bathroom but it was here before the boiler swap.

Its unlikely that they are all in one big loop, they are more likely to be in several loops from and to a couple of manifolds below the floor somewhere, if those three radiators are on the same loop it could be a blockage in the manifold caused by debris that got in the system when the boiler was changed. can you get under the floor easily to trace the pipework?

You should have a rad that is not thermostatically controlled just in case they all shut down together this is normally in the bathroom/towel rail.

I’m no expert but we have a couple of rads that play up if the thermostatic valve is closed off, removing the knob by unscrewing the collar and giving the pin that sticks up a wriggle or light tap with a small hammer seems to work . Might also be worth bleeding afterwards

When we had a new combi boiler fitted by BG we had to have the existing system powerflushed before they would guarantee it. There was an awful lot of gunge in the system.

If you can borrow a thermal imaging camera it a fab tool for tracing where the pipes go, where they are hot and cold without lifting carpet or floor boards.

A few weeks ago, I had to repair a central heating leak, that until I tightened the shut off valve, was just a drip
Said valve broke up, and water everywhere, thankfully in the garage!

Leak, and new valves fitted sorted, and system filled up, and left on for a couple of hours, then pumped/flushed out, and have to say the water was quite clean,

However, the main rad in the lounge, despite getting rid of any air, and is the furthest from the boiler, with a run of about 20 metres, there and back, would not get hot,

So I did as said above, shut every rad down, exept the offending one down, pump on max, and it did get hot after bleeding (again!) I then reflushed the system, and the water was pretty dirty

We now have the pump on medium, and all the heating seems to work fine

In the diary for the summer, will be another major flush, and refill with an inhibitor

I do hope the OP can sort it out, as its pretty depressing when the heating doesn't work

steve

do you know what size pipework you have if you have micro bore pipes at radiators then you may have a manifold fitted somewhere which may have a blockage in it if you have standard 22mm flow and return and 15mm t offs to rads i have come across this problem once before on swapping the boiler the new boiler may be piped up with the flow and return opposite to wot they were in the case i had a problem there was a non return valve fitted to a circuit that fed two rads so i took the valve off of the system which cured the fault and had a happy customer

[Edited on 26/1/19 by chris]

I think i might have found the issue, after trying to use pressure to unblock the pipe work i ended up removing a lot of the boxing around the pipes and went over with a magnet and it sticks tot he pipes where it spits off tot he rads that wont get hot.

I have put some Sentinal X800 in the system to see it it can clear some of the crap out but if it fails i'm just going to rip out the old pipes and replace.

i would be looking to get some compensation from your installer as the system was obviously not power flushed before commissioning the boiler this is a requirement of the benchmark and can affect the guarantee of you boiler

the exit valve on the radiators are such a design that they block very easily with any debris, to check if these are the problem, open the bleed valve fully, if this blasts out hot water then the exit valve is blocked. I had this for quite a while which was very annoying as I had to keep draining the system till it was clear.