Posted by: @lakey

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I also only really care about this one room as the rest I can oversize rads in, this one I was hoping to limit the conservatism due to wall space constraints! 

I certainly had wall space constraints, which I mostly dealt with by using K3 rads in place of previous K2 rads. The individual room heat losses were taken from the original spreadsheet based heat loss calculation, which almost certainly over-estimated the total and therefore most room heat losses, meaning I ended up with possibly larger rads than I needed (there is a lot more to this than meets the eye...). In other words, if you can get a K3 or two with sufficient heat output to match a spreadsheet based heat loss for the room that will fit on the available wall space, then you will almost certainly be OK. Not exactly very scientific, just a pragmatic solution.   

Well, my current room is heated by 2 off 1800*440 designer rads. I estimate their output to be 1500 W each at dt50, my heat loss is calculated to be between 2.6 and 3 kW for this room. 

My replacements would ideally fit in two 600 wide spaces, but finding things that output 1500 W at dt20 with a max width of 600 is seemingly impossible or expensive (900 each for a 5 column radiator1800*600)

So, I'm really looking to trim that down before I resort to clearing a bookcase off the wall. 

Could a fan coil be suitable.  I had a similar problem which was solved with a fan coil.

@lakey - very roughly, when going from boiler to heat pump flow temperatures, you need to double the effective area of the radiator to get the same output. This sort of increase in size clearly isn't trivial. Tall rather than wide K3s are one option, but hard to come by and not cheap. Another possibility is fan coils. These very roughly need to be half the size of conventional rads, and so you might see something here - a conventional rad needs to be twice as large, but a fan coil can be half the size, meaning you get back to your original size.

I have a very similar problem in my living room. A large enough K3 wouldn't fit where the old K2 rad would fit, and I ended up putting the K3 on an adjacent wall, where it could fit, but it is visually horrible. However, looking at fan coils, it seems I can get one that is roughly the same size as the old K2, and so it will fit, and it has sufficient output at heat pump flow temperatures. I haven't fitted yet because for some unknown reason that rad gets very little heat supplied to it, possibly pipe work obstruction, and even a fan coil can't magic heat out of non-existent heat!

I see while I was writing this @jamespa has come up with the same suggestion of a fan coil. I thought he might, it was he who alerted me to their usefulness in these situations!  

@cathoderay @jamespa  thank.you for the suggestions on a fan coil, they're no cheaper than a large aluminium Eskimo radiator or 5 column cast iron vertical radiator from what I can tell? The myson 19-14 gives ~2kw heat at dt20 with max fan speed and is £900ish. I guess I'd only need one rather than two...

Do they have the option for running without the fan? I think I only really need the fan output for the depths of winter, I'm very unlikely to need it's max output as run my room at 18.5 currently and it's comfortable. 

Posted by: @lakey

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they're no cheaper than a large aluminium Eskimo radiator or 5 column cast iron vertical radiator from what I can tell

@jamespa is really the expert because he has actually fitted and used them, but I found a Mitsubishi I-life Slim 170 Fan Assisted Radiator fan coil with just over 2kW output (which is what I needed) at heat pump flow temps that measures 937 x 131 x 579mm (W x D x H) for £500. OK the output isn't quite what you need (assuming the spreadsheet assessment is correct) but you may be able to find others.

They will run without a fan but output is then less than a radiator of same size.

I thought you were struggling to fit a passive radiator in hence why I suggest fancoil.  If you can fit a passive radiator of the requires size in do so, unless you need cooling.

@jamespa I think I need between 2.4 kW and 3kW over 2 600mm spaces for design conditions, so 21 internal and -3 OAT. I currently run at 18 in winter, and typically the OAT is above 0. 

So I think if my maths is right (which it may not be!), as my indoor to outdoor delta becomes 15 K rather than 24 K, the power range becomes ~1500-1875 W for moderate winter usage. Which is no problem to find radiators 900 W at 600 wide for most flow temps. But then when it's -3 outside I need another 1500W... Hence why wondering if it would work fine without the fan!

I'm sorry but I don't understand your maths.  If you need 2.4-3kW output then you will need a radiator rated for about 6kW at 50C DT. That's a very big radiator, likely too big.  Perhaps you could clarify the numbers.

I'm not sure exactly what question you are asking about the fan, but if your concern is that it won't 'turn down' don't worry.  You will be operating your ashp on weather compensation so the flow temperature will be lower at higher oats, which will automatically reduce the output of all emitters fan or not.

The basic message here is, if it works at the design lowest oat, it will work at all other oats.  Put another way, pick a radiator that has sufficient output at the design oat and higher oats will take care of themselves.

@jamespa my room heat loss is approx 2.4-3 kW (house 7-8 kW) depending on who's done/what tool they've used. The room is a complicated extension with a split floor insulated concrete floor and suspended old timber floor, then large quantities of glazing. 

I understand that heat pumps modulate so that probably negates my maths, but I also intend to run the room at the same temperature I do now (18 degrees) as we run it in a steady state with 55 degree flow temperature.

I was just hoping that the fan wouldn't be running constantly, I assume it's relatively noisy, requires regular cleaning and requires some energy input making it less efficient but a trade off for the price of £800 rads or working out how to do a UFH zone under the suspended floor or taking up a section of laminate. One installer quoted £8,000 for the downstairs to be wholly UFH... 

Currently cost, the initial outlay, is the biggest road block to getting an ASHP for me. 

Posted by: @lakey

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I understand that heat pumps modulate so that probably negates my maths, but I also intend to run the room at the same temperature I do now (18 degrees) as we run it in a steady state with 55 degree flow temperature.

Are you saying your design flow temperature (at the lowest oat) for the ashp system is 55C?   

Fans on fancoils do run constantly, but with a decent one will modulate down to slower speed if less output is needed.  Mine is quieter than my fridge and well below the volume where it cause a nuisance in a living room; I wouldn't want it in a bedroom.

@jamespa I had one company design at 55 because I was struggling for radiator space... I've had others pull out because I couldn't heat the room properly without additional inputs. 

I'd like to design to a 35 or 40 flow temp. 55 is what my combi currently runs at but I can't hit 21 in this room, only 19.5.