I wonder if anyone could shed some light on this scenario that we are having trouble with:

We are the installers, we have installed an Altherma sized by Daikin onto UFH supplied and designed by Polypipe and installed by the client and his plumber.

Property is 700 sqm, SAP came out at 21 watts a sqm though, due to levels of insulation specced, hence Daikin calculated heat loss at 14kw from coefficient and supplied a 16kw Monobloc with 285 (300) cylinder

Since then it transpires that the SAP was wrong, client forgot to include conservatory which now takes heat loss to over 18kw.

However the unit does not achieve set point flow temp on space heating (45 degrees), even if we isolate off all but 7kw of UFH. This was tested again last week when ambients were circa 0 degrees. It does not have an issue on hot water side.

We have installed a low loss header which holds 15 litres and a 4 litre make up vessel, header is circa 1.5m long. Heat pump f & r offset to UFH f & r and so on. We have a pump on the UFH side of the header feeding to 7 manifolds, all with their own circulators.

Delta T across f & r on heat pump side is 5 degrees and same on f & r to UFH. UFH was designed with a 43 degree flow temp and a 7 degree delta t. When we installed the header Daikin Tech advised to retard the Daikin pump to achieve the delta T, due to it's high flow characteristics and the low static losses to and from the header.

On Monday with a Daikin service engineer on site, we could not get the space heating flow temp above about 38 degrees (with 7kw of load) and the BUH would come in to achieve this, then drop out, followed by the compressor and then the flow temp would decay by 10 degrees before the compressor kicked back in to start the process over.

Some rooms are at set point but it's costing a fortune to run, circa £30 a day. We recognise that the sizing is an issue due to the incorrect SAP, but the client who is switched on, is blaming the unit because it does not reach set point and recruits the BUH too frequently.

We are not using the weather compensation and interestingly there are 2 sensors o/s, 1 on the unit and 1 linked to a Smart Home system that drives the space heating controls. The Altherma sensor is constantly 3 degrees lower than the smart home sensor, possibly due to the air wash.

Daikin could not see anything wrong with our install, Polypipe have been on site to help with balancing, but when I have measured delta t across the manifolds there is only 1 which has a delta t of 5, the others have delta t's of 1-2 degrees. Losses are minimal circa 2 degrees at furthest mf.

Daikin technical are now saying the Altherma pump should be on full chat, provided we can feed enough out of the header to avoid recirc and cycling due to high return temp.

Their view is that the flow is too low to clear the sensors of the excess heat that occurs when the BUH kicks in, causing a shut down and refrig recovery/balance. However from their manual 16kw/5/4.2=0.76l/s or 45.9lpm which is nominal.

We have the EP set at o degrees, and again the manual suggests that the BUH should be recruited when the ambient is less than set point, which it wasn't when we were testing on Monday.

Unit has not gone igloo over winter and all other manual requirements have been met.

I am just not convinced by the flow rate issue yet and wonder if there is a software issue.

Does anyone have any views?