I have used my own calculated ventilation figure from my house software model which is 1.5kW without MVHR to current defined standards. Therefore your total heat load should theoretically be 4kW with ventilation loss. Unfortunately, you do not have calculated individual room heat losses at design temperature so we will have to assume that the installed loops are correctly sized. You can enter the flow temperature (in blue) to see heat outputs. For 4kW it is 30C (32.5 flow, 27.5C return). Also, DT. You do not need WC. At 15C OAT, your loss will be 1kW. Hitachi quote a minimum output of 2.1kW, so a bit of cycling will be inevitable.

So, what’s to be done? What I am going to say runs against the normal grain so you will have to decide for yourself. With a UFH system and HP with DT control, open loop working is a load of bollocks. There is no control and your bedrooms overheat as you have found out. You need to go back to the original design with thermostats for control of each room (as you have them, together with a buffer tank). The design is not that bad. Set the HP to DT control 5C so that there is variable flow. Set output flow temp to 32C as first trial and set up loop flows (see below). I am not familiar with the screed used so it may be higher on emission, requiring flow temp to turn down a bit more. The secondary pump is correctly set to constant pressure. What this will do is maintain the set flow in each loop as actuators turn off i.e. reduce the overall flow more or less in line with the HP pump so that the buffer tank doesn’t go skew whiff. You will hit a limit when HP gets down to 2.1kW but the excess flow from the HP will bypass through the buffer e.g. when 1kW at 15C OAT. You could keep some loops open to mitigate this if you wish e.g. one of the loops in the lounge and the hall loop. Not critical.

To set up loop flows, I recommend putting cheap Caleffi plug-on thermometers on each return pipe. Set 5C on these rather than on the manifold flow meters as flows are very low. You could of course try 3C on the HP to increase flows but 5C is best IMHO. The HP will modulate from your peak 4kW output down to 2.1kW so you may suffer some inefficiency below this level where COP is anyhow high. The smaller Hitachi heat pumps only go down to 1.8kW so there is not much difference. No big deal. However you will have fine control everywhere and you will have perfect comfort and save money. You can see open loop systems on heatpumpmonitor.org where room temperatures in a season vary from 17C to 24C with no mention of bedroom temps. Not for me.

That’s the best I can do. The rest is up to you.

Thank you this is very helpful.  There is a lot to take in.  I have the heating on today and have been doing videos of the Live View.

The funny thing I attempted the flow calculations for the floor this morning.  Not as good/detailed as yours.

I also have a MVHR.

One question, what pump setting should I run on a Grundfos UPM3 (K) Auto?  Currently 3 max, but I think 2 might be OK.

I will drop you a private email

I have used my own calculated ventilation figure from my house software model which is 1.5kW without MVHR to current defined standards. Therefore your total heat load should theoretically be 4kW with ventilation loss. Unfortunately, you do not have calculated individual room heat losses at design temperature so we will have to assume that the installed loops are correctly sized. You can enter the flow temperature (in blue) to see heat outputs. For 4kW it is 30C (32.5 flow, 27.5C return). Also, DT. You do not need WC. At 15C OAT, your loss will be 1kW. Hitachi quote a minimum output of 2.1kW, so a bit of cycling will be inevitable.

So, what’s to be done? What I am going to say runs against the normal grain so you will have to decide for yourself. With a UFH system and HP with DT control, open loop working is a load of bollocks. There is no control and your bedrooms overheat as you have found out. You need to go back to the original design with thermostats for control of each room (as you have them, together with a buffer tank). The design is not that bad. Set the HP to DT control 5C so that there is variable flow. Set output flow temp to 32C as first trial and set up loop flows (see below). I am not familiar with the screed used so it may be higher on emission, requiring flow temp to turn down a bit more. The secondary pump is correctly set to constant pressure. What this will do is maintain the set flow in each loop as actuators turn off i.e. reduce the overall flow more or less in line with the HP pump so that the buffer tank doesn’t go skew whiff. You will hit a limit when HP gets down to 2.1kW but the excess flow from the HP will bypass through the buffer e.g. when 1kW at 15C OAT. You could keep some loops open to mitigate this if you wish e.g. one of the loops in the lounge and the hall loop. Not critical.

To set up loop flows, I recommend putting cheap Caleffi plug-on thermometers on each return pipe. Set 5C on these rather than on the manifold flow meters as flows are very low. You could of course try 3C on the HP to increase flows but 5C is best IMHO. The HP will modulate from your peak 4kW output down to 2.1kW so you may suffer some inefficiency below this level where COP is anyhow high. The smaller Hitachi heat pumps only go down to 1.8kW so there is not much difference. No big deal. However you will have fine control everywhere and you will have perfect comfort and save money. You can see open loop systems on heatpumpmonitor.org where room temperatures in a season vary from 17C to 24C with no mention of bedroom temps. Not for me.

That’s the best I can do. The rest is up to you.

File received thank you.

I have tried the DT mode and it does seem to be better with this setting.  The Gundfros UPM3 is current in constant pressure setting 3 (max).

When the video was taken it had just done a Thermal Off, so was starting up.  Typically it 4 or 5C between the Twin and Twout.

Your situation is interesting because of the parallel with mine. To try and get some handle on this, could you send the following information if you have it?

i) What is your UFH pipe spacing and is it laid in spiral form or up/down?

ii) What is the screed? 7 cm concrete or anhydride?

iii) What is the tog value of the floor covering? Do you have tiles, engineered hardwood or pile carpet?

iv) What is the model number of the secondary pump? Is it Grundfos? Is it switched on by the UFH control box?

v) Do you have thermostats in all rooms?

I do not think that open loop working is appropriate for you. There is no control and you will overheat. However, let's not get ahead of ourselves.

UFH is mainly 200mm.  
Screed - 17mm Ambi Tak.  
Mainly engineered wood and carpet and tiles.
Grundfos UPM3 (K) in constant pressure mode.  Switched on by the UFH control box
Thermostats in all rooms and off, except one in the family room.
I have attached a copy of the floor design

Any help on how I private email you?

Your situation is interesting because of the parallel with mine. To try and get some handle on this, could you send the following information if you have it?

i) What is your UFH pipe spacing and is it laid in spiral form or up/down?

ii) What is the screed? 7 cm concrete or anhydride?

iii) What is the tog value of the floor covering? Do you have tiles, engineered hardwood or pile carpet?

iv) What is the model number of the secondary pump? Is it Grundfos? Is it switched on by the UFH control box?

v) Do you have thermostats in all rooms?

I do not think that open loop working is appropriate for you. There is no control and you will overheat. However, let's not get ahead of ourselves.