Posted by: @kev-m

I'm not sure exactly what the algorithm is but my mediocre ASHP seems to literally test the water.  When it starts up it starts low then ramps up over a few minutes.  I assume it compares actual flow against target, works out how much power to give, gives it, sees what happens to the flow and adjusts.

Yes, that seems to be what's happening. In this graph, blue is the power output the controller has worked out, red is what the pump actually uses and green is the heat produced:

Here's the heating system temperatures for the same period, so you can see how flow, return and the controller's calculation are linked:

Hi @cathoderay

Let me see if I can answer your questions.

The heat pump does not actually know that the heat demand is 6kW, or whatever it may be at the time. If the heat pump is operating in weather compensation mode, it calculates the required LWT based on the outdoor air temperature, and the setting of the weather compensation curve. So the compressor runs at the required loading to heat the LW to this temperature. In your case the LW flows through the external heat exchanger, where some of the heat energy is transferred to the water in the secondary circuit. The RW is therefore cooler by a number of degrees.

During the configuration of the controller the required delta T for the heat pump can be set, normally in the region of 5C or 6C.

The quantity of heat energy being produced by the heat pump is given by the temperature difference between RW and LW and the flow rate. To heat 1000L of water from 20C to 50C requires approximately 35kW of energy, so to heat 1L of water by 1C requires approximately 1.17W.

If the water is flowing through the heat pump at a rate of 10L/min, and the temperature is raised by 5C, then the energy required will be:-

1.17W x 10 x 5 = 58.5W/min      Over a period of 1 hour this will equate to 3.5kWh.

So if the heat demand of your system is 6kWh, then to maintain a delta T of 5C, the flow rate would need to be approximately 17L/min. The heat pump controller controls the delta T, and hence heat output, by varying the speed of the internal water pump to achieve the correct flow rate.

On the secondary side of the heat exchanger the same formulas apply. Ignoring any heat losses, if the water temperature on the secondary side is being raised by 10C, and 6kWh of energy is being transferred, then the water flow rate must be approximately 8.5L/min.

If all your radiators were the same size and your system was perfectly balanced, then each radiator would be receiving approximately 667W/h of heat energy. But of course you have different sizes of radiator and the flow will most definitely not be perfectly balance. The total specified output of your radiators at a delta T of 50C is 25100W. The quantity of heat energy absorbed from the water and emitted by each radiator is again a product of the flow rate through it and the inlet and outlet temperatures. The rate at which heat energy is emitted from a radiator is not linear, but follows an exponential curve, the greater the water temperature the larger the heat energy emitted as detailed below for the largest of your radiators, with an indoor temperature of 20C.

Water Temp.   Delta T   Output (W)

     25               5           233

     30              10          574

     35              15          972

     40              20         1413

     45              25         1889

     50              30         2394

     55              35         2925

     70              50         4651

So as can be seen above, with a water temperature of 40C and no restriction to flow, the radiator would be emitting just over 30% of its specified rating, with a water flow rate of just over 2L/min to produce a RWT of 30C.

From your recorded data, the temperature at each radiator would probably be in the order of 36C, if the water flow through each radiator is in proportion to its output capacity.

If my assessment is correct, then closing the TRV's on some of the radiators will force more flow through the remaining ones and increase their heat output.

@mjr 

Apart from the colours and units, these look the same as my graphs. How are you producing them?  I use the MMSP file and google sheets.

Some measurements from 0800 this morning:

Midea controller: ambient 11 LWT 41 RWT 35 capacity 0kW flow 0.83M3/H current 0A

Room temps (design temp): bedroom 19.0 (19), bathroom 21.0 (22), kitchen 18.6 (19)

At 0800, one room is at design temp, the other two are just below. The primary pump is running, but the compressor is off (despite the display showing the symbol saying it is on). The compressor delta t is 6, about where it should be, and the LWT is close to where it should be (diff between -2 and 11 = 13, 55 - 13 = 42). 

A few minutes later, the compressor started: capacity 4-5kW, current 5-6A (so a COP of around 3.4), but flow rate remained effectively the same (0.84M3/H). 40 mins later the flow rate is still 0.83M3/H. The second decimal place changes every now and then, confirming this is a live reading. But it seems that it is always something around 0.8M3/H whenever I have looked at it. If 1 cubic metre is 1000L, and there are 60 mins in 1 hour, then 0.83M3/H = 830L/H = 13.8l/min.

@derek-m - I am going to have to go back to basics to get my head round this. I keep on getting confused about power (kW) and energy (kWh) and what we mean when we use the word heat, and so related phrases like heat transfer. I think most of the time when we use the word heat, we mean energy (kWh) but much of the time we use kW as the unit (for example heat loss calculations are always given in kW), which is a unit of power. Somewhere fuzzily in my mind I recall energy (in kWh) is the ability to do work (in out case, heat our homes by making the room atoms and molecules move faster), and power (in kW) is the rate at which we can do the work. For example, a 2kW convector heater heats a room faster (it has more power) than a 1kW convector heater, but both will use the same amount of energy (kWh) to get to the same temp say from 10 to 20 degrees, the 2kW heater will do it in half the time. For example, the 2kW of power heater takes 30 mins to heat the room from 10 to 20 degrees, and in so doing uses 2kW of power for 0.5 hours = 1kWh of energy while the 1kW of power heater takes 1 hour to heat the room, but still uses the same amount of energy 1kW for 1 hour = 1kWh of energy. The difference is the more powerful 2kW heater does it in half the time.

Going back to wet systems, I can sort of see how flow rate comes into this, by varying the amount of water that gets heated (twice the flow rate means twice as many litres to be heated per minute/hour), but does that need twice the energy (kWh), or twice the power (kW), or is it both? Perhaps it gets twice the energy, and that is achieved by delivering the energy twice as fast, ie at twice the power.

Apologies if I have managed to confuse others, but I have always been one who has to understand things conceptually before I can do anything with them. Blind use of formulas is to me black box stuff, and it doesn't work for me. 

Going back to flow round my system: I have just looked again, and it is still 0.83M3/H, two hours after I last looked. But maybe that is because other parameters - outside ambient and room temps and LWT and RWT are all broadly the same, so why would the flow rate change?

Finally:

Posted by: @derek-m

From your recorded data, the temperature at each radiator would probably be in the order of 36C, if the water flow through each radiator is in proportion to its output capacity.

But they are less than this, they average around 30 degrees for the warmest spot on the radiator, usually taken as top middle rear panel, the bottoms of the panels and often the front panel are cooler, so the actual average temp is even less.

The table of outputs at different delta t values makes sense, eg as a rule of thumb I reckon a rad puts out about 50% of heat at delta t 30 compared to its output at delta t 50, and that's pretty much what your table shows. This rule of thumb could perhaps usefully be incorporated into the letterhead of every heat pump quotation, to alert customers to the fact that going from fossil fuel delta t values to ASHP delta t values means you will need to double the size of your rads (and/or convert some to K3s, to avoid insanely large K2 rads).   

I will experiment with closing some TRVs and report back on what happens. 

@cathoderay your flow rate seems low to me - pretty sure mine is well over 1m3. The heating isn't on at the moment, so I'll look when it is running later. As an aside, have you tried cleaning out the gauze filter?

@cathoderay all fair points, you should go back to freedom / your installer with this data and they will look after you. under sizing is very rare these days, mcs is pretty much designed to over spec everything. If this is what's happened they will make sure its all sorted for you. as someone mentioned, if it cant cope at +3 its massively undersized or not working properly.

@batalto - it would be interesting to compare flow rates, given we have similar units. I haven't cleaned the gauze filter, the one adjacent to the heat pump, but I have cleaned the magnetic filter on the secondary circuit from time to time. It usually has some magnetic sludge and particulate debris, but not, in my eyes, very much, though I don't really have a benchmark to compare.   

@grahamh - thanks, your comment is reassuring. Once my installer is back at work I will start the ball rolling.

@cathoderay give it a clean out - mine was completely full of crap and caused a low flow error on the system. Apparently its quite common in systems without gauze filters and it takes a few weeks to get all the rubbish out. I still periodically clean mine out, but I was amazed at how much was in there. You can look back in my main thread to find pictures. If it is a bit gunked up that will not help you with flow rates. All you need is a big wrench and you'll be good to go.

https://renewableheatinghub.co.uk/forums/renewable-heating-air-source-heap-pumps-ashps/ashp-in-hampshire/paged/6

@batalto - thanks, I will check the filter and report back. Before I check it, presumably you need to turn the heating and DHW off before doing it?

@cathoderay its a quick job, you can just close the valve and take it out - the system wont error out that quickly. You'll probably have it back in before any issues at all - under low flow it just spits out an error you can reset. If it worries you an easy fix is just to turn down the thermostat to turn the heating off, then do it.

@batalto - OK, thanks, now done. I did turn the DHW off and room stat down. A fair amount of crud, but that is after almost two months of near continuous running, so maybe not too bad:

Cleaned and replaced, system back on. It's really rather small, given the pipework it's on. It must itself act as a throttle, even when it's clean.

@cathoderay that should probably help the flow rates somewhat. I had to clean mine out quite often over the course of two weeks before I got everything. Now I only do it when I think about it and there is almost nothing in there. Give it a check a couple of times over the next week or so, you'll find a lot of junk in it.