Are you measuring with a thermometer or your hand? I think humans aren't very accurate in the 23-33°C range.

Posted by: @mjr

Are you measuring with a thermometer or your hand? I think humans aren't very accurate in the 23-33°C range.

Speak for yourself.

I have been calibrated and I am accurate to +/- 10C approximately. 😋 

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? 

My flow temp is 34 deg now and my kitchen rad is definitely cooler on the bottom (with my uncalibrated but able to register differences quite well, hand).  Rads with cold bottoms are normally poor flow - sludge or flow rate.  I don't think I have either of these issues.  Like you, when they heat up the effect is less/non-existent. Maybe it's something to do with the convection effect on these big double finned radiators?

@mjr, good point. The thermal imaging camera is coming out tomorrow.

Posted by: @editor

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? 

My flow temp is 34 deg now and my kitchen rad is definitely cooler on the bottom (with my uncalibrated but able to register differences quite well, hand).  Rads with cold bottoms are normally poor flow - sludge or flow rate.  I don't think I have either of these issues.  Like you, when they heat up the effect is less/non-existent. Maybe it's something to do with the convection effect on these big double finned radiators?

I have much the same. Currently running on standard Midea weather comp curve. My Mk1 human thermometer (back of the hand) reports rads as never being warm, sometimes they are definitely cool all over, sometimes less cool at the top. But the room temp has been fine, at design temps. I think this is simply how low temp systems work, it's a sort of trickle charge rather than boost charge. Once the building fabric is up to temp, the heating system just runs a trickle charge to keep it topped up. It has been very mild so far this autumn here in the south, that may be another factor.

Most of the time I use my Mk 1 HT, more than adequate, if I want actual numbers I use a point and click IR thermometer, bearing in mind a white gloss rad's heat emissivity may not be what the IR thermometer expects. But as relative rather than absolute readings they are probably fine.  

The top to bottom rad temp differential and flow rates came up last spring, when I also tried a constant LWT and set the circulating pump to max flow. Maybe the rads were a little warmer, but the most noticeable thing was my electricity consumption was much higher doing that. Not sure whether that was because of a low COP due to constant high LWT, or the circulating pump being full on all the time, or a combination of both.

The other problem I found was there seems to be no straightforward way of knowing what the actual water flow rates in the rad circuit are. Despite manuals running to 100s of pages, the pump controls are extremely basic, in effect low medium or high (ie not very much, a bit more, quite a lot), based on flow, pressure, or something called proportional all set by pushing a button. What the actual flow rate in my system is is anyone's guess, apart from anything else it will depend on resistance in the system, meaning there is no way of knowing whether my top to bottom rad temp difference is due to inadequate flow, or something else, but that won't be sludge, the whole system was power flushed after the installation was completed. 

If anyone can throw any light on how to measure actual water flow rate in secondary pipework, I think that might be very useful.    

Edit: FWIW, I've just taken the following readings with the IR thermometer while the room temp is 18 degrees, which is 1 degree below design: wall, matt white emulsion paint, 1 foot to the side of the rad is 16 degrees, top of rad, white gloss paint, is 25 degrees, bottom of rad is 17 degrees. The wall is sucking heat out of the room, the rad is trickling heat into the room.  

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? 

Hi Mars,

I have been giving this issue some thought and have come to the conclusion that the water within your radiators must be obeying the laws of physics. 😎

There are possibly several explanations for what is happening within your system.

1) Your heat pump and water pump could be switching on and off, which if the off period is long enough you will experience stratification within the radiators. As the water in contact with the metal of the radiator cools, it will become more dense and migrate to the lower section of the radiator, whilst the warmer water collects in the upper area.

2) If you have zone valves for UFH and radiator circuits, it could be stopping the flow through the radiators and hence having the same affect as (1).

3) TRV's on your radiators could be throttling in and hence reducing the flow rate through the radiators, thereby not supplying sufficient energy to heat the whole radiator with the help of (1) above.

4) Standard panel radiators are constructed with an upper and lower header, with the water flow tubes produced during construction, connecting the two headers. Normally the flow and return pipes connect to the bottom section of the radiator, one at each end. One of the pipes will connect to the lower header directly, and the other pipe will connect to the upper header via one or more of the water flow tubes at that end. If the flow pipe is connected at the end feeding the upper header, then the warm water pumped into the radiator will go into the upper section first. Since the flow of water going into the radiator is via a 15mm or less diameter pipe, when it reaches the much larger volume of the radiator the flow rate drops quite considerably. The warm water flowing into the radiator will therefore fill the upper header and then start pushing the colder water down the radiator tubes and out via the return pipe. Obviously, as the warm water pushes down the radiator it is cooling along the way, and when your system is running in weather compensation mode, the water entering the  radiator is probably already cooler. The combined effect could be that by the time that the warm water is reaching the lower section of the radiator, it has cooled to the point where the radiator is cool to the touch.

Check to see if your heat pump and water pump are being switched on and off.

Check the operation of any zone valves if you have them.

Monitor the temperature gradient on one or more of your radiators using the thermal camera, noting the operation of the heat pump and water pump.

Post the results for posterity. 😎  

Some interesting things to note as temperatures have dropped. Despite outside temperatures dropping to 9-10C, the heat pump's weather compensation continues to run at around 35C. TRVs are set to temperatures that they can't hit so they're not shutting down or closing and the pumps are running all the time as far as I can tell and there's a constant demand for heat.

We are now, however, beginning to see the value of K3 rads over double panel radiators. We have double panel rads in the bedroom and a Stelrad K3 in the TV room. The difference is notable.

The rads all feel more even to the touch at the moment and the Flir One thermal imaging camera (it's on sale at the moment on Amazon if anyone's interested) has confirmed this. I didn't get a chance to use the camera when they felt notably warmer at the top and cooler at the bottom, but I'll capture that when it happens again and post it here.

So here's the master bedroom double panel rad – top (24C) and bottom (23C). About 1C difference. I would have expected them to be a tad warmer if I'm honest.

Now for the TV room – Stelrad K3 top (30.5C) and bottom (28.1C). 2C difference.

Given that the TV room, due to our piping and north positioning, has always been the coldest room in the house, it's remarkable the difference in heat between the K2 and K3. 

What really sums this up is the notable heat coming "out" the top of the K3. Streaks of lovely, gentle warmth. As a consequence the TV room feels warm and cosy all the time. 

In terms of UFH – another interesting visual. Last winter when we scanned the floor with the thermal imaging camera we could clearly see the UFH pipework. Today, it's just a sea of consistent heat at 23-ish Celsius. Again, I would have thought it would be warmer, pushing 28-30C based on flow temperatures, but rooms are adequately warm,

@editor - useful images. Using my rather more basic IR thermometer I continue to have a roughly 5 degree difference between the top and bottom of most of the rads. But that's not the only anomaly. Some recent readings (all in degrees C):

Ambient: 9     Room stat setting: 19.5     Room temp showing on room stat: 18    Heat pump running in Midea weather comp mode (55@-2/37@15)

Room stat calling for heat throughout, rad circulating pump on throughout. All K3 except two K2 rads. DHW hot so no DHW call during period of obs.

LWT/RWT (from Midea controller): varies between 36/35 and 50/46. Observing it over a 20 min period, at the start of the period the compressor was off and at around the 36/35 point it started again and then rose over the 20 mins to 50/46 when it cut out again, and the LWT/RWT temperatures started to fall. It appears to be cycling: off until it cools to around 35, then on until it reaches 50, then off again until it cools to mid 30s, then on again. This does not seem consistent with weather comp mode at all. At 9 degrees ambient it should be running at a steady 43-44 degrees all the time (until the room stat says stop). I do note this point is about half way between the current cycling range. 

The PHE (plate heat exchanger) primary in/secondary out temps are just about OK, within 5 degrees of the heat pump LWT most of the time, and most of the time the secondary out is within a few degrees of the primary in. Conclusion: the PHE is 'losing' heat, but not very much. 

The heat pump cycling could explain the next observations: all the rads are either cool or luke warm, mostly ranging from 24 - 30 at the top, and all about 4 -5 degrees less at the bottom. Just as they start to warm up, the heat pump turns itself off. These temps are always below, sometimes a long way below, the primary circuit RWT temp at the heat pump.

The room temp is currently within a degree of design temp. Over the last few days it has struggled, being mostly at 17 degrees, 2 degrees below design. This may have been partly because of the gales (more draughts in a leaky building, work continues on reducing them). I fear that low ambients will also see the system failing to reach design temp, there was some evidence this happened in the short cold spell at the start of last April.

In summary: how much is the heat pump cycling compromising the room heating (DHW is fine)? Why isn't it doing what it says on the tin, setting the LWT by the ambient temp?

Is this also, or even just, an inadequate secondary flow problem? Seems unlikely, given the circulation pump is already on max proportional (whatever that means - but that's part of the problem, no way of knowing what the flow rate is, and so whether it is adequate or not.

Any thoughts much appreciated! 

PS Should have said, all the TRVs are fully open, lockshields only closed enough to achieve some sort of balancing.

@cathoderay 

Isn't it cycling because the power required to keep the LWT at 43 deg is less that the minimum your ASHP can output?  Mine does that at 9 deg or so. It cycles 2-3 times per hour and when it does, the flow temp varies quite a lot. When it's running continuously the flow temp is very constant. 

Most ASHPs can modulate down to 30-40% of their nominal output.  For a 14kW that's still quite a lot of power. 

@kev-m - that's interesting, and makes sense, and fits with what I observed about the power reading in the Midea controller, around 5kW when it was on, even as it approached the higher LWT. It will be interesting to see whether it smooths out in colder weather.

Any thoughts on the temperature differentials both between LWTs and rads generally, and within rads? It seems to me a bit like the heat is there, but not getting fully transported into the rooms. It goes back to the heat pump, visible in the high RWT, which consistently considerably higher than even the hottest rad.  

Posted by: @cathoderay

@kev-m - that's interesting, and makes sense, and fits with what I observed about the power reading in the Midea controller, around 5kW when it was on, even as it approached the higher LWT. It will be interesting to see whether it smooths out in colder weather.

Any thoughts on the temperature differentials both between LWTs and rads generally, and within rads? It seems to me a bit like the heat is there, but not getting fully transported into the rooms. It goes back to the heat pump, visible in the high RWT, which consistently considerably higher than even the hottest rad.  

Hi Cathoderay,

Correct me if I am wrong, but did we not discuss the problems associated with your heat exchanger last Winter?

I suspect that Kev is correct that your heat pump is stopping because it is producing more energy than is being utilised. But if you are not getting sufficient heat energy to your radiators, then it could possibly indicate that the heat exchanger is not adequate for the job.

What manufacture and model is your heat exchanger?