You are correct that the radiator output (at 50C and room at 18C) is 890w. This already takes into account the lower flow temperatures of a heat pump. At gas boiler temperatures (70C flow, 20C room) it will output 1768w.
Your survey says you need 482w, so your radiator (at heat pump flow temp) can provide almost double. The heat loss survey would have to be way out for the radiator to be undersized. From everything I know, the radiator is more than big enough, the problem lies elsewhere.
I found this table to be really helpful when sizing a radiator for much lower Leaving Water Temps.
Most radiators are quored with a Delta T for gas boiler temps usually 50°C.
Water temp of 70C from the boiler minus the target room temperature of 20C.
This table helps you to see the output the various rad types have at lower temps.
For example a Leaving Water Temperature for underfloor heating or other low temperature system might be 30C.
Hybriding in one room as I do the Delta T of the rad would then be just 10 for a room temp of 20C.
You might have a rad delta T of 20 or so I guess?
Once you know the room heat loss, apply the actual area of your rads to the output in W/m2.
As pointed out above somewhere, its the mean rad temperature minus the required room temperature that determines the rad output, a T50 rated rad will emit 100% output at a mean rad temp of 70C, (70-20) = 50, assuming a required 20C room temp, but you cant have a mean rad temperature of 70C and a flowtemp of 70C, T50 rads are generally specified at 75C/65C/20C, mean rad temp, (75+65)/2, 70C. If you required this rad to emit 890W (1768W @ T50) or 50.34% then at the same flowrate will result in, if you use a 50C flowrate, a output of 46.35%, 820W, to achieve 890W would require flow/return/dT of 52C/47C/5C, to give that required 50.34% output, not a huge difference but if the return temp was only say 35C due to reduced flow, then the rad will only emit 35.4%, 626W at 50C/35C, dT 15C, or only 76% of that at a dT of 5C, so important to check the return temp as well.
I have started on the balancing the kitchen radiator. To start with it was only about a quarter turn away from fully open. I have opened it up fully now.
Before
Thank you for that. For now I don’t have any equipment to measure the flow/return temperatures. I have started on balancing the radiators by trial and error.
This is very useful. Thank you
Good to know. Yes I suspect balancing is the suspect. I also noticed on checking the lockshields on other radiators. The one in the entrance hallway close to the thermostat has the flow nearly fully open. I suspect this might cause the thermostat to register higher temperatures faster and cut the flow to the house. After checking how the kitchen radiators now behave, I will close down the hallway a bit more.
Is this right? One of the engineers who visited insisted that following the path of the flow to balance them is the way to do it as the heat is lost progressively.
I expect this is practically the same as fully opening all radators, then starting with balancing down the radiator with lowest dt between flow/return.
Great, see how that changes things. is there a valve of any sort at the other end? If so open this too.
You can use your hands in the first instance. Its not super accurate but for your kitchen to not be warm we are looking for a really big difference. If your flow temperature is about 40C then feel the pipes at either end of the kitchen radiator. If they feel the same, or similar temperature then water is flowing through the radiator fairly quickly which is what we want. If there is a marked difference or one end is virtually cold then we have a problem. Also check that your radiator feels similarly warm all over, and that the back panel is also warm.
Its also worth comparing with a radiator in another room, living room for example, to see if the pipes feel a similar temperature. We don’t care about minor differences of a degree or two we are trying to find out of for example living room pipes are hot vs kitchen pipes are only warm.
Hopefully it is balancing as its fairly easily resolved with some trial and error. I would be tempted to turn the thermostat up really high (like 25-30C) so it is always calling for heat. This removes it as a factor, if the kitchen gets to your desired temperature with the thermostat really high then you know the radiator is capable of it and you need to focus on the thermostat/hallway problem.
Your radiator schedule shows the hallway radiator as being large compared to the heat loss so it could well be getting “too hot” and affecting the thermostat. I’d take the hallway heat loss with a pinch of salt as they are notoriously hard to estimate due to being funny shapes, have doors, letterboxes, heat rising upstairs, etc.
This is right in the way radiators are normally plumbed. Heat isn’t lost progressively. Heat is piped to each of the radiators and then the heat is lost in the radiator, with only a small loss in the pipework. Usually the radiators are all fed individually with a hot pipe (the flow) and then they all feed back into the return. The flow pipe starts as a single pipe from the heatpump and will then be split into upstairs/downstairs then split to individual radiators. The key thing to think about though is that whatever happens the water flows through the flow pipes (red), then through a single radiator, and then through the return (blue) pipes.
This means that all radiators should be fed by water at the same (or similar) temperature. Yes some pipe runs to some radiators will be a bit longer than others but in a terraced house it won’t be that different and (in theory at least) they should be insulated.
There is another way to plumb radiators and that is with a single pipe that goes around the house and each radiator takes some water and then puts it back into the same pipe. This is relatively unusual, frankly unlikely, in a modern house. However your installers description does sound a bit like it. These are generally not suitable for heat pumps, and if it is the case then your heat loss survey is wrong because it doesn’t take account of the radiators at the end of the circuit getting much cooler water. In this case if you think about it the water is going through multiple radiators, losing heat in each one, meaning the next one starts with cooler water.
I think its very unlikely that you have the plumbing shown in the second picture, but I mention it as possibility because of what your installer has said and the fact it is one (admittedly unlikely) explanation for your problems.
I understand it is good practice when you have finished balancing to leave all lock shields at least a little distance away from being on an end-stop.
This means that the next person who comes along has less chance of snapping the valve shaft (when they think the valve is stuck when in fact it is just parked on an end-stop).
Hi There, You may want to try the following experiment. There is a big gotcha built into the transition from gas boiler to heat pump. You have not said if you are using a programming schedule like you would with a gas boiler. Are you running 24/7 or turning the system on and off at set times using your thermostat programmer.?. I run my system (Vailliant) almost entirely controlled by the weather compensation curve and the way the radiators are balanced. My old system had TRV on all the radiators and these are now set fully open all the time as they affect the balance if they shut down and not in a good way. Today the temperature is at 20c throughout the house and it will stay that way no matter what the temperature is outside (currently 9c) and was the same when it was -9c in January. Heat pumps run best when used to just replace the heat as it is being lost, constantly topping up throughout the full 24 hours. If you are trying to run to a schedule it may be that the room has insufficient time to warm up before your schedule shuts it down again.Might be worth a try.
Good tip. Noted 
That is worrying if that’s the case in ours. I took some pictures of the hallway radiators and it indeed looks like there is a single pipe that takes in and lets out the flow?
I can see the pipe from either end goes towards the middle of the radiator (presumably into the wall?) but I can’t see from the photos that they are connected.
Are you able to answer this bit?
I can see now that your pipes are plastic so less good for feeling. Instead use the silver bits between the radiator and the pipe.
I have compared it with the living room radiator and now I can see the kitchen radiator warming up similarly (post lock shield adjustments). I do not know though what the entry exit temperature is. So it is all just subjective for the moment.
Great observation. Yes, I am aware of how much scheduling has to differ from boiler setups to heat pump setups. That was the only thing (besides the curve) that I was modifying before trying to rectify the problem. To achieve desired temperatures in a decent time. However this is an extremely finicky process. Thanks for sharing your setup. I will start another thread to discuss this in detail if you are interested.
I think you may still have a little bit of gas boiler contamination in your thought process. You may want to read another thread on here that deals with the physics behind setback temperatures and how this affects scheduling. Another BIG input is thermal mass and this directly affects the setback scenario. Gas boilers tend to be used in a way that encourages you to heat just the air in the room to the desired temperature. If you are heating 24/7 then the whole structure of the house ends up at the set temperature which is why a heatpump tends to feel “warmer” than a gas boiler at a particular set temperature. You are not sitting on a sofa with a temperature of 15c in a room at 21c so it feels warmer. The downside is that it takes a long while for the house to cool and a long time to warm up. If I was to start my system up from an ambient temp of 10c it would take about 2 days to get to 20c and if the system stopped completely it would be about a day before I realised it was not working. For me setback does not work very well as a 1c drop takes about 7 hours and given that heating at night is at cheap rate I want to be driving the heat pump then not turning it off. Note that my EPC is a high B and my walls are 9 inch solid brick with internal insulation and some of the internal walls are 9 inch as well so thermal mass is much higher than a modern new build with cheap chipoard walls. A 30minute response time ?. definitely in gas boiler territory. 3 hours maybe.
Just fyi and to save you time, @nhp may have been referring to House Thermal Inertia and Roomstat Setback (some cautionary notes) (and the subsequent far more helpful responses to it…)
