@cathoderay 

Great the PHE seems to be ok!  Any pipes outside clearly need to be lagged, but indoor ones probably not.  Any pipework inside the thermal envelope will be helping dissipate heat into the house, improving matters.

Good point about air bubbles in the PHE!

Often there is a magnetic filter in the system, prior to the PHE.  It's there to pick up bits of metal that rust off the steel radiators, and prevent the fine channels of the PHE getting blocked.  Do you have ?  It's a long term issue mind.  I remember when a plumber wanted to fit ones years ago I first thought it was a voodoo water softener thing, but no, the magnets do pick up lots of black sludge.

As to the rads - there's often many ways to connect up radiators.  We actually have a really old 1-pipe system, but it's weird and unusual so forget it; usually they are now configured in a 2-pipe flow and return artery system, every rad connecting to the main flow and return.  The rads themselves have 4 connection points - top left, bottom left, and again top right bottom right.  I think it's best with a standard 2-pipe system to connect to the two lower connections, where the colder water will otherwise congregate.  

Rads are not a consistent temperature over their surface, it depends on the water flowing throughout.  They may have sludge inside preventing circulation.  Or the circulation pattern may be inappropriate - eg. top - top may just leave a pool of cold at the bottom, with no natural convection reason for circulation.  They may have an airlock inside - there's a little valve at the top to allow manual air bleeding.

You said you tried to balance the rads, have you progressed with that? We had one rad stealing all the heat flow as it was wide open and it was causing all the other rads to be very cold to the touch.

@robl 

thanks, I think I totally misunderstood what a heat exchanger/PHE was.  I think I do now.  We don't have one and it seems to be fine without is.

@robl - lagging - they are all lagged, point taken about once they are inside the house, but they are actually in a low level loft space (ie roof space over a single story bit of the house), which doesn't need (full) heating or contribute to the main indoors heating. 

Yes, there is a magnetic filter just before the PHE, and it does collect sludge and crud (I am planning on checking it again later today).

The rads are on two pipe flow and return pipework, I know because I put most of them in when I first did the CH. I also have the bills for supplies, and so can see how much 28/22/15 pipe got fitted (ground floor all buried in solid floor, so no way of inspecting it now, but the lengths purchased give a good idea). All the connection are on the lower radiator connections, lockshield one side, TRV on the other (not necessarily on flow or return, ie randomly fitted because no way of knowing with the old boiler out which was the flow and which the return).

Sludge shouldn't currently be a problem, the rads are all new, and the system had a powerflush. That said, the magnetic filter does continue to pick up sludge. If it is hideous when I check later, I may post a photo here... The circulating pump is currently on max constant pressure, so flow should be adequate. But, most of the time, most of the rads have their lower half at around the rooms ambient temp, and so that area of the rad is contributing little or no heat. As mentioned, a number of the K3s have noticeably cooler front panels, probably because a K3 is a K2 with an extra panel, so the flow an return connections are between the read and middle panel, rather than in the middle. I have bled them fairly often, occasional air, but mostly water straight away. The circuit also has an automatic air vent at one of the higher points in the pipework.

@batalto - initially there was a lot of thievery going on, but now they are all at least partially balanced. The trouble is they are erratic. Some rads just decide to run cool for a while. At the moment they are all within the mid 20s to mid 30s range give or take (again, they are volatile) but I think I can say no one radiator is stealing the heat from a lot of the others.

@cathoderay 

You could try turning all rad TRV's off except one, confirm that one works as expected, move through them all one by one?

Our plumbing is old, and flexible pipe was used for some of it.  It takes weeks for all of the air bubbles to finally come out:-(  Some new pumps now have a "bleed" setting, which jerks the speed setting up/down, helping encourage air to move along to a bleed point.

I think the constant pressure pump setting is ultimately best for 2-pipe rads with TRVs by the way, as it should automatically save elec power by turning down the flow as rads lock off one by one, so any remaining rads have the same flow they always had. If in doubt just stick the pump on max though.

However, best for the rad TRVs to not lock off; we don't want the heatpump to be working hard at high temp low COP into rads that have locked off and don't want the heat.  

@robl - the all off bar one, one by one testing is a good idea, might highlight a problem rad/section of pipework. And a good point about constant pressure maintaining steady flow in the rads that are open, as things change, though to date I have had all TRVs fully open all the time. I will wait until it is a bit colder (forecast for later this week) and see what happens.