@gajolos Ah, unless the occupants of the first floor apartment were cold blooded or didn’t like to complain, it sounds as though the original installation took care of all the heating needs before the addition of the ground floor accommodation. As discussed, the pumped water will take the least line of resistance, hence the ground floor in preference.

The next question then is, does the ground floor overheat? Were this the case, some adjustment to the flow downstairs may encourage more flow to the first floor. You could easily find out if upstairs could still be served when the ground floor area is restricted a little (but not to the degree that the ground floor becomes colder than required) by reducing the flow with the TRV’s (if fitted on ground floor). If no TRV’s but less heat is tolerable, try reducing the flow via the LSV’s by turning slightly clockwise to shut them off a little.

Please bear in mind what I stated before, I am not an expert! If the temperature at present in the ground floor rooms is as you require it to be then leave the LSV’s alone! Some other means of encouraging the water flow to the first floor is required and perhaps someone with plumbing experience might be able to advise further. Regards, Toodles.

@gajolos 

I havent fully followed this but as I understand it you have two apartments, the downstairs one is heating sufficiently, the upstairs one not.  Im presuming that there are no operational thermostats or TRVs (ie they are all set to max), its all open loop and operating 24x7.  

Assuming that my assumptions are correct then @toodles is right, this is about balancing, ie shifting the balance of water flow (and thus heat transfer) between the two.  First make sure that the flow overall is as high as it can be by maxing out any pump adjustment available and ensuring at least one LSV is fully open.  Then reduce the flow downstairs to increase the flow upstairs until they are at the same temperature.  They might both settle at a temperature which is too low in which case you may need to edge the flow temperature up a couple of degrees.

Dont get involved with buffers, it will simply amplify your problems.  If the upstairs pipework is too restrictive for some reason so that a balance cant be achieved then an auxiliary pump for the upstairs might be needed, however this is a bit of a last resort not least because you then have to think carefully about pump control etc. but also because it shouldn't be necessary if both have been similarly plumbed and radiators have been similarly sized, unless the pipework connecting upstairs to downstairs is very long or very contorted.  Are you sure radiators have been sized correctly upstairs and downstairs or is it possible they have been sized with different flow temperatures in mind?

The next question then is, does the ground floor overheat? Were this the case, some adjustment to the flow downstairs may encourage more flow to the first floor. You could easily find out if upstairs could still be served when the ground floor area is restricted a little (but not to the degree that the ground floor becomes colder than required) by reducing the flow with the TRV’s (if fitted on ground floor). If no TRV’s but less heat is tolerable, try reducing the flow via the LSV’s by turning slightly clockwise to shut them off a little.

Please bear in mind what I stated before, I am not an expert! If the temperature at present in the ground floor rooms is as you require it to be then leave the LSV’s alone! Some other means of encouraging the water flow to the first floor is required and perhaps someone with plumbing experience might be able to advise further. Regards, Toodles.

I'm more interested in trying to find out if by heating the ground floor apartment ( 45sq.m), the second floor apartment (130sq.m) "loses" somehow due to the ground floor apartment and if a buffer and a secondary circulator will work better and in a more cost-effective way for the whole function of the heat pump.

I think advice from someone more knowledgable than I is required for this setup. I will say that the use of buffers is to be avoided whenever possible but whether such a system as yours would benefit from using a buffer might help is something a more experienced reader of the forum might be able to advise on.

May I suggest to help anyone who might advise you, that you provide a sketch or schematic with approx. lengths of pipe runs along with their diameters where known please? Also, if you have the details of heat loss calculations, this would help greatly.

Is this a new installation, or has one ‘house’ been added to an existing working system and if so, did the system previously provide the level of heating required? Any relevant details might help to establish what is going on when the system is attempting to heat both ‘houses’. Regards and sorry, I can’t help you. Toodles.

Something else I'd like to ask. Maybe a buffer and two different circulators, one for each one of the two houses, will be better for the circulation of the water to the radiators of the second floor apartment? Or, maybe, a circulator just for the second floor house? I don't know if this is possible, to use a buffer in parallel connection to the inner unit of the pump and have a circulator just for the second floor apartment. Just thoughts, maybe someone who has done it  know better.

I would suggest that you check the TRV’s are not preventing the flow by opening the upstairs valves to Max and see if this helps first. If the heat is still not flowing uphill to those radiators, check the Lockshield valves on the opposite ends of the radiators to the TRV’s. Try opening these a little to increase flow. I have written an article on setting up LSV’s which you will find here: 

https://renewableheatinghub.co.uk/radiator-balancing-lockshield-valve-guide/

You may find that the downstairs circuit needs some tweaking later but carry out any adjustments in small stages.

Regards, Toodles.

That's why I asked if the ground floor apartment because of the pipes that end to the radiators from the pump are shorter ( due to the fact that the heat pump is on the ground floor), affect the leaving water that end to the second floor apartment through the pipes from the pump (that are obviously longer), resulting in loss of heating water in the second floor apartment.

*Not that I am suggesting you ought to have an LLH, in fact most systems work better without one!

Regards, Toodles.

However, the problem is that you need to have the BRC21A52 controller of the Daikin pump in your house as a room thermostat. This code doesn't work along with another room thermostat. This BRC21A52 controller is next to the inner unit of my heat pump, so it can't be used as a room thermostat. The heat pump is connected to two houses, so even if I wanted to put it in the main house, the heat pump won't work properly for the other house.

There is another code 8-00 in this controller that is

" Remote controller temperature control - leaving water control

When using the remote control delivered with the unit, 2 types of temperature control are possible. Default = 1 which means that the remote controller is used as room thermostat, so the remote controller can be placed in the living room to control the room temperature. Set to 0 to use the unit in leaving water temperature control. "

So, to use the BRC21A52 controller of the heat pump as a room thermostat you need to set code 8-00 at "1" and put it in your house. If you set the 8-00 at "0" then the settings in code 9-02 don't matter.

That's all I have find out after searching and many trials with the controller.