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.  

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.

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.

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.

Just posting as there has been mention of glycol. 

The flow in the secondary rad circuit was reversed (circulating pump rotated 180 degrees), and the rads started to warm up, but erratically. A powerflush was done (should really have been part of the original installation), without much effect. Over time most rads seemed to warm up a bit (luke warm at best), and on a second visit the plumber flushed the ground floor rad circuit using mains pressure (and in the process somehow managed to trip the pressure relief valve (the circuit is unvented) so at least we now had a pressure test, and no leaks evident (the pipework is unchanged, still the original 28/22/15mm flow and return pipes). Now most of the rads warm up some of the time. I have tried to balance them, but difficult to do when they behave erratically. All TRVs are fully open, and the rad circulating pump is set to max constant pressure.

Ad hoc readings just now with 4 degrees ambient outside and room stat at 21.0 and system in (presumably) steady state: 

All rooms at or a degree or two below design temp (but we are not at -2 ambient...)

All rads luke warm, range 22 to 36 (these are outliers, most are in the upper 20s)

Midea controller reports LWT 47/RWT 42 (this is about right, both for current weather comp settings and differential between LWT/RWT)

IR temp on black insulating tape on PHE pipes: primary flow (in) 40 return (out) 31, secondary flow (out) 42 return (in) 31

Ditto, about 10 mins later: Midea controller 50/44, primary pipework 47/43, secondary pipework 47/41

These figures are actually some of the best from the PHE, the secondary flow out temp is close to/matches the primary flow in (40=>42 !?! and 47=>47). They also show how volatile the readings are, both from the independent measured Midea controller and IR thermometer, ie 'steady state' is not very steady.

But: the rads are nowhere near the heat pump LWT, rads say an average of 28-29, LWT 47-50 degrees, around 20 degrees difference. At the moment some of the loss appears to be happening between the heat pump and the PHE (pipes are all lagged throughout), the PHE isn't doing too badly (which is unusual) and the oddity is the difference between the average of the secondary flow and return (~35, then ~ 44) and the rads (~28). Conventional theory has it that the rads should be ~ half way between the flow and return, ie 35 to 44 degrees, but they are far cooler, ~28 degrees. The rad temps are all top middle warmest panel readings, many of the three panel K£s (not a typo...) have noticeably cooler front panels, and all rads are only warm at the top.

I don't know what to say. Given the above readings, the PHE can at least on occasions function as it should, degrees C in the primary in get transferred to degrees C in the secondary out (but it is not always so), but there is something odd about the transfer of heat from the circulating water to the rads: average circulating water temp 35 - 44 degrees, rads around 28 degrees. It shouldn't be possible...

@robl - just seen your reply which you posted while I was writing the above. Yes, I do get we are using a sort of proxy/shorthand, degrees C instead of watts (power) and kWh (energy), but that is perhaps because it is more intuitive, and also it is what we can measure without fancy equipment. I also know you can't 'disappear' energy, it has to go somewhere (unless and until e = mc2 kicks in). "To be clear- we ideally want the radiators to be within 5C of the heatpump output" - exactly, and that is what we haven't got, but the reasons are not immediately apparent.

PS I appreciate this is a partial highjack of @howiewsl's thread, but it is the same general topic, under-performing systems. FWIW, my current weekly energy use for the heat pump on its own meter is ~250kWh, or £62.50 a week at 25p/unit, or just under £9/day. Ouch. And the weather over the last week has been mild.           

There are so many things that might need  adjustment/upgrading/adding/removing to get the optimum setup. It can sound overwhelming (to me anyway). 

My overriding general initial thought was that the EPC and separate heat loss numbers for a Victorian house with 80% of the external  solid brick walls sounds low. Of course it may be fine. EPC data is of variable quality though as is the MCS  heat loss numbers. You can look online for EPC reports for properties so you could look at neighbouring houses etc as a comparison.

https://www.gov.uk/find-energy-certificate

We found that useful. 

You say you have good insulation in the suspended floors and roof. Is this something you fitted yourself or was that the previous occupant? For example our EPC said our roof insulation was good, i would say it was mixed....

As well as looking at EPCs for similar houses, chatting to neighbours might also be useful to understand their energy use and setups and what they have done to the fabric of their home. We found it useful.

We have 2 power showers in our house, obviously not the greenest of appliances...  the temperature of the water in the tank makes a dramatic difference in our case to the length of showers you can have, especially if you like hot showers. Just wondering what sort of showers you have?

It would be useful to know how many kWh the ashp has used per month since it was installed. Even proxy monthly figures from the smart meter if nothing else is available historically. I am not  sure the £5 figure is helping other posters get an idea of the scale of the issue which may help. 

Doing your own comprehensive as possible room by room heat loss calculation feels like a good idea. By using the information from your installers, neighbours, other EPCs, the knowledge you have gained about your house since you moved in etc. Of course it won't be perfect but it might give you some pointers to inform other things to investigate and change that have been highlighted by others.

Getting the right setup to heat old homes and figuring out what to improve regarding the building fabric can be challenging in my experience. It is possible with patience and some compromises and investment irrespective of your heat source (we have stuck to our mains gas boiler for now so are yet to go through yet another learning curve).