@cathoderay

Yes, you are probably correct to add fuel burnt.

The idea of using a fixed LWT of 50C was as a temporary measure, to possibly make it easier whilst tracing back along the pipework, not for permanent use. Obviously go back to weather compensation mode after the test.

You could still carry out the test in WC mode if you prefer.

One thing that does concern me is the fact that the secondary water pump was turned through 180 degrees, so is now pushing the water in the opposite direction around the system, though without knowing the exact layout it is difficult to know if this would have any effect.

I would suggest measuring the temperature of the flow and return pipework on each radiator. In theory, the flow pipe temperature should be reasonably close to that of the water leaving the PHE. If it is much lower then this could indicate that there is little or no flow through the radiator. Ensure that the TRV's are not stuck in the closed position and that the lockshield valves are sufficiently open. As a last resort you may have to fully open all the valves and then re-balance your system.

What is the physical size of the PHE and the model of your water pump?

@derek-m - the secondary water pump was rotated 180 to put it back in the correct orientation, the plumber had left it reversed (he thought it might improve things) so there was non-contraflow. The second rotation put it back to how it should be (contraflow). The pump is a Grundfos UPM3 Auto. The PHE is approx 45cm long by 10cm wide by 10cm high excluding the foam jacket, which appears to be about 1cm thick. It has the name Secespol on the jacket. As a picture is worth 1000 words, here is an annotated photo. It is not perhaps quite as neat an install as I would have liked, but I accept space is very tight:

Some of the foam insulation is off the pipework, and you can see both the black insulating tape on the pipes in places for the the IR thermometer, and on the LHS, the glass bulb thermometer for double checking both the thermocouple multimeter thermometer and the IR thermometer. Most of the time they are all within a degree or two of each other, so I am reasonably confident they are accurate enough for our purposes. The Grundfos pump is just visible top left, and the black cylinder mid right is a filter on the rad circuit return to the PHE. It only had a small amount of sludge in it when I last checked it about a month ago.

I did in fact measure all the rad tails, in and out, when I did the 'whizz round' earlier today, that's where the ranges I quoted earlier came from. With two exceptions, they are all 10 degrees or more below the PHE out temperature. This is with the rads nominally balanced (they are by no means perfectly balanced) and the TRV heads off so fully open. The two exceptions are within two degrees of the PHE out temp, one is even a degree higher, but their associated rads are cool, like all the other rads. I think they may be measurement error. The more general observation is that as a rule the rad in tail is usually 10 degrees or more below, in cases over 15 degrees below, the PHE out temp. This is consistent with the bypass theory, only there doesn't appear to be a bypass!

The actual readings in the order I took them (immediately before, the PHE out was around 35 degrees):

Room in/out

Bed 3: 25/21

Bath: 19/17

Bed 2: 22/15

Bed 1: 16/13 (this was the cold rad, it is same as the others now that i have opened the lockshield valve a bit)

Living: 33/18 (but rad no warmer than the others)

Dining: 19/14

Kitchen: 36/16 (36 makes no sense given PHE out, and rad was no warmer than the others)

Loo: 20/15

PS kW/kWh - adding fuel burnt works especially well with oil CH, 1 litre of kerosene/RBO = 10.3kWh/units (which incidentally makes oil at under £1/litre considerably cheaper than electricity so those COPs need to be pretty darn good to make our ASHPs more economical to run)    

Posted by: @editor

I noticed something this autumn while running weather compensation. With flow temperatures of 32-37C, the rads aren't "hot" but they are doing a great job keeping rooms cosy and comfortable. What I've noticed is that the rads are warm on top and noticeably cooler at the bottom. This applies to all rads in the house. They've all been bled, so that's not a contributing factor. When running at a set point of 40C or 45C, the full rad is warm. Has anyone else noticed this? 

What you describe is mentioned in the book Heat Pumps for the Home by John Cantor. Essential reading for a heatpump geek! 

https://www.amazon.co.uk/Heat-Pumps-Home-John-Cantor/dp/1785007793?crid=16YIMIO1SLKBJ&keywords=John+cantor&qid=1667726673&sprefix=john+cantor%2Caps%2C247&sr=8-1&linkCode=ll1&tag=myhomefarm-21&linkId=9630b313d0846bdac73e96d7a35509f3&language=en_GB&ref_=as_li_ss_tl

Page 51 has a thermal image of what you describe. Essentially it seems to be common but not an issue with boilers high flow temps. 

it is a slight issue for heat pumps as it lowers the mean radiator temp and so it's output.. he mentions long short radiators are slightly more efficent for their size as the stratification is less. 

He suggests this will reduce as radiator design improves for lower flow temps in the future.

But he also mentioned going back to the old school radiator plumbing method of   'top inlet, bottom outlet, same end'  but if your rads are in then that won't help.. 

But your observations are a common thing.

Joe

I have been giving some thought to how to test for an invisible bypass and have come up with the following: close all rad lockshield valves except two, the landing and bathroom, these being closest to the airing cupboard where both the circulating pump and PHE are located. Set the pump to 'proportional pressure' minimum and then close the landing and bathroom rad lockshields and observe the temps in the PHE secondary inlet and outlet. Test hypothesis: there is no bypass so the secondary inlet (and in time the outlet) should cool down as there is no flow round the system.

(1) any thoughts on whether this is a viable test? Might there always be some flow eg through the lockshields? From what I recall these do really shut off the flow, enough to be able to remove a rad eg to decorate or replace.

(2) any thoughts on whether this will damage the circulating pump, as if there is no bypass, it will be pumping against in effect a blocked system?

@cathoderay

Rather than closing the lockshield valves, which will mean having to re-balance the system, it would be much easier to close the TRV's.

To be on the safe side I would leave one radiator with the TRV left fully open, and then see if this radiator gets sufficient flow to reach acceptable temperatures. Then open the TRV on a second radiator and close the TRV on the first radiator. Carryout this procedure putting each radiator in service one at a time. This may indicate that you have sufficient flow for one radiator, but not for all of them at the same time.

Did you trace back along the warmer pipework to try to locate the possible bypass?

@cathoderay

Many years of fault finding on highly complex systems has taught me to carry it out in a logical and systematic manner. Obviously fault finding via text messages is far from ideal, and relies heavily on the information being passed back and forth.

Rather than closing all the radiator valves, my next suggestion would have been to fully open all the radiator valves, both lockshield and TRV, and then see if you appear to get a greater flow rate through the PHE, and which radiators are receiving the most heat energy. It should then be possible to close in the lockshield valve, or adjust the TRV, on the warmest radiator, and see if some of the heat energy is diverted from the warmest radiator to the other ones. Then repeat this process until all the radiators are achieving approximately the same temperatures, and are providing sufficient heat energy.

Your system is apparently providing insufficient heat energy in the secondary circuit, whether this is due to a low flow rate through the radiators because of partially closed valves, because of some form of bypass, because of inadequate pump performance, or merely the fact that the PHE is undersized and is supplying insufficient heat energy in the first place. This is what we are trying to identify.

Posted by: @derek-m

Did you trace back along the warmer pipework to try to locate the possible bypass?

Not yet, it is going to be a last resort as it will mean moving furniture and taking up carpets and floorboards.

Posted by: @derek-m

Rather than closing all the radiator valves, my next suggestion would have been to fully open all the radiator valves, both lockshield and TRV, and then see if you appear to get a greater flow rate through the PHE, and which radiators are receiving the most heat energy. It should then be possible to close in the lockshield valve, or adjust the TRV, on the warmest radiator, and see if some of the heat energy is diverted from the warmest radiator to the other ones. Then repeat this process until all the radiators are achieving approximately the same temperatures, and are providing sufficient heat energy.

That makes sense, and is a less drastic form of testing. On the path of least resistance principle this may also throw some light on any invisible bypass may of may not be located eg is all the upstairs rads behave as expected but the downstairs ones don't, this may suggest any pipe anomalies are perhaps more likely in the downstairs circuit. Based on past adjustments during balancing, partially closing the lockshield valve on a warm rad does cause a cooler rad to warm up as expected. Later today (I have some other things to attend to first) I will systematically go through the process and takes relevant notes.

I wonder, did you have any thoughts on the circulating pump and PHE details such as they are in my post with the photo?   

@cathoderay

I sent an e-mail to Hexonic asking how to identify which model of heat exchanger you have installed. Obviously it is the weekend, so don't expect an immediate reply. Have you tried removing the insulation to see if there is a nameplate giving details?

Can you provide full details of your water pump, or a closeup photo of the manufacturer's nameplate so that I can see the details. The different models have different capabilities and control settings.

Posted by: @derek-m

I sent an e-mail to Hexonic asking how to identify which model of heat exchanger you have installed. Obviously it is the weekend, so don't expect an immediate reply. Have you tried removing the insulation to see if there is a nameplate giving details?

That's very good of you (sending the email). I haven't removed the foam insulation because as you can see in the photo, it is tightly wedged in place. It might be possible to lift the top a bit but at the moment it (the top) appears to have been glued to the sides of the insulation, so removal may end up being destructive. I would want to know where I am looking for a nameplate so I can go in locally in the right place. Hopefully Hexonic will give us some clues.

Here's a photo of the pump, taken shortly after installation when it was the wrong way round (upside down) making the flow left to right on both sides of the PHE ie not contraflow. It is currently the right way up, so there is contraflow. Although upside down, you can read the letters/numbers etc.

@cathoderay

Thanks for the photo, I will have a look at the manual.

Have you checked that the two valves on the inlet and the outlet of the pump are fully open?

Posted by: @derek-m

Have you checked that the two valves on the inlet and the outlet of the pump are fully open?

Yes, and they are (I've just double checked). 

@cathoderay

Which way is the flow through the pump? Is it pushing the water into the PHE or pulling it out?

Could you accurately measure the distance between the centres of the inlet and outlet pipes on the PHE, since that may help identify the model.