My initial reactions are this.

I don't know where you are in the country but 14kW is quite high for 240sq m suggesting some insulation might help.

You might want to run it open loop for the cold side and have trvs on the warm side to adjust for solar gain when necessary, but have the warm side lockshields adjusted so when there isn't solar gain both sides remain about right (basically running open loop throughout on WC on dull days, on bright days open loop on WC cold side, modulated by the trvs warm side

If you post a plan it might be easier to comment.

Solar gain would result in an increase in return temperature, and the heat pump will throttle back or turn off, reducing heat input - provided you're running open loop without a buffer.

It's self regulating.

Posted by: @chrislay

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There are a lot of posts on this forum suggesting heat pumps should be run with a single open loop. I struggle with this concept in our house due to temperature variations. It's about 240 m2 but L shaped and the main rooms face SSW. It has quite large windows. Four bedrooms, two living rooms, main one facing south, the other facing N and E. Kitchen/Dining facing S and W but shielded from most of the S sun due to a loggia which is great in the summer! Also an office. The hall is double height but does not get much sun (limited windows and loggia) and is effectively between the living rooms and kitchen/dining.

On a sunny winter day we get lots of solar gain. Last week when it was minus 1 outside and lots of sun the main living room stayed over 21 degrees for about four hours with no heat. However on a dull day, same outside temperature, it will struggle to get to 19 degrees with heat. Also other parts of the house don't benefit from the sun and are much colder. On the same sunny day without heat they would drop to say 14 degrees or below. Due to the layout of the house it is far removed from an open plan arrangement and some rooms, such as the office (cold side) are really quite separate.

It's impossible to set a flow temperature that will work in all rooms with no zoning or local TRV control. Also, it is running with weather compensation but of course not sun compensation. I know systems like Homely will look at sun levels but that only has one temperature sensor and if that was in the main living room it would turn the flow temperature down so low we'd freeze in other rooms. 

I have an opportunity at the moment to re do our heat pump set up completely - it's a three year long story that I'm not going to delve into - and I really want to get it right this time.

The heat loss is about 14kW and one option I'm considering is basically two systems - one for the sunnier bits and one for the cold area. This would be reasonably easy to achieve, but still not convinced about the sun factor unless I go for something like Homely. Would the living room be representative of temperatures in the other living areas - I'm doubtful. Also the sun makes virtually no difference on the cold side and I'd struggle to think where I could put a single temperature monitor that would be representative.

Anyone any thoughts?!

The idea that James proposed of using TRV's in the solar gain rooms should work, and is probably the easiest and simplest solution.

An alternative would be a mixer tank for the solar gain area, which could be used to lower the LWT going to those heat emitters. If you look through the Ecodan manual they suggest such a system. 

Posted by: @hughf

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Solar gain would result in an increase in return temperature, and the heat pump will throttle back or turn off, reducing heat input - provided you're running open loop without a buffer.

It's self regulating.

I think the problem OP has is that some rooms get too hot while others remain cold.

Posted by: @derek-m

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The idea that James proposed of using TRV's in the solar gain rooms should work, and is probably the easiest and simplest solution.

An alternative would be a mixer tank for the solar gain area, which could be used to lower the LWT going to those heat emitters. If you look through the Ecodan manual they suggest such a system. 

My assumption is that the solar gain doesn't happen every day/all of the time so it needs an time-variable adjusting solution, hence my proposal of of TRVS in the warm area only.  Obviously if the warm area is permanently too warm, then a mixer to reduce FT will work,  or reducing the area of the emitters in that area, or possibly turning down the lockshields a lot (but many lockshields have only a very small proportional range, they are basically on or off, so equally this may not work)

There is scope IMHO for heap pump optimised TRV heads featuring proportional control, slow response (or possibly variable response time, a sort of learning mode and an operational mode) and which never quite shut down.  These could, in principle, self-balance for the correct temperature and then operate as limiters, ie just the function we need.  Electronic TRV heads already have all the required hardware, they just need different firmware.

@chrislay if you would like to post a plan we might be able to do better.

Ah, if some rooms are too cold then that needs to be fixed by either increasing emitter size, or improving insulation in that room.

We run on WC but the UFH heating was set up with separate loops to every room. Each has its own thermostat, which provides the upper temperature limit in that room.

But even with proper insulation I think it's sometimes impossible to get all rooms up to a desired temperature. Our entrance lobby/downstairs loo, is a new pitched roof (south facing) addition to the outside of the original 20" thick stone wall of the building. It has its own UFH loop and, as new build, is cavity wall insulated. On cold winter days it can be difficult to get the temperature above 12C to 14C, when beyond the internal door through the original exterior wall, it's a cosy 20C. As a small external addition it has a high surface area to volume ratio compared with the rest of the property (we have internally insulated the external walls) and I think it suffers disproportionate heat loss as a result (plus maybe more through the front door). I gave up trying to heat it properly as it seemed that the ASHP would be running constantly just to reach that level, even when the rest of the house was OK. The thermostat in that area is now set to 14C.

Mike

Posted by: @chrislay

↑

There are a lot of posts on this forum suggesting heat pumps should be run with a single open loop. I struggle with this concept in our house due to temperature variations. It's about 240 m2 but L shaped and the main rooms face SSW. It has quite large windows. Four bedrooms, two living rooms, main one facing south, the other facing N and E. Kitchen/Dining facing S and W but shielded from most of the S sun due to a loggia which is great in the summer! Also an office. The hall is double height but does not get much sun (limited windows and loggia) and is effectively between the living rooms and kitchen/dining.

On a sunny winter day we get lots of solar gain. Last week when it was minus 1 outside and lots of sun the main living room stayed over 21 degrees for about four hours with no heat. However on a dull day, same outside temperature, it will struggle to get to 19 degrees with heat. Also other parts of the house don't benefit from the sun and are much colder. On the same sunny day without heat they would drop to say 14 degrees or below. Due to the layout of the house it is far removed from an open plan arrangement and some rooms, such as the office (cold side) are really quite separate.

It's impossible to set a flow temperature that will work in all rooms with no zoning or local TRV control. Also, it is running with weather compensation but of course not sun compensation. I know systems like Homely will look at sun levels but that only has one temperature sensor and if that was in the main living room it would turn the flow temperature down so low we'd freeze in other rooms. 

I have an opportunity at the moment to re do our heat pump set up completely - it's a three year long story that I'm not going to delve into - and I really want to get it right this time.

The heat loss is about 14kW and one option I'm considering is basically two systems - one for the sunnier bits and one for the cold area. This would be reasonably easy to achieve, but still not convinced about the sun factor unless I go for something like Homely. Would the living room be representative of temperatures in the other living areas - I'm doubtful. Also the sun makes virtually no difference on the cold side and I'd struggle to think where I could put a single temperature monitor that would be representative.

Anyone any thoughts?!

Here are some thoughts for consideration, they may depend upon the location of your heat pump and whether it also gets some solar gain.

You should indeed treat your home heating system as two separate areas as far as control is concerned.

I will deal with the colder area first, the temperature of which I would suggest should be regulated using WC control, which should be adjusted to maintain the desired temperature in the coldest room, which should also have any lockshield valves fully open and of course no TRV's. The temperature in the remaining rooms in the colder area should be set by adjusting the lockshield valves if at all possible, or TRV's as a last resort.

The accuracy of the above of course would be dependent upon the OAT sensor on the heat pump not suffering from solar gain itself. Where is your heat pump located?

There are options for the temperature control in the warmer area, which again are dependent upon actual conditions.

If all the rooms suffer from about the same temperature increase due to solar gain, then I would suggest a zone valve controlled by a suitably located thermostat, which would shut-off the water flow through the heat emitters when the room temperature reaches the desired level. This option of course would require the pipework to be arranged into a warmer zone and a cooler zone.

If room temperatures vary with solar gain or it is not possible to arrange the pipework into two zones, then individual TRV's on the radiators would be my suggestion.

A completely different solution could be some form of ducted air system, where the solar gain heated air is moved from the warmer rooms to the cooler rooms.

Thanks all for the suggestions. I'm currently taking to some installers and waiting to hear what their solution is. I'll come back later when I can hopefully report from a comfortable house with an efficient system!