Daikin Altherma 3 HHT not getting up to heat

[signorjim]

Hi there - I wonder if anyone can help. New Altherma system takes hours to get up to temp after a DHW cycle or defrost. The refrigerant quickly heats up to 31 degreees c and then ticks up by a degree or so every fifteen minutes meaning that the leaving water temperature doesnt get up to temperature for a long time. Its pushing 17 litres per min, fixed, radiators as heat emitter. Cant figure it out. It feels like there is a setting somewhere making it go v slowly...

We have normal radiators in the house (apart from UFH in the kitchen which has the water temp lowered by a valve)but the Altherma has three possible settings - UFH, Fancoil (restricted to max 55c but you can alter the delta T between 1-10c) and radiators (max 70degree lwt but you cant change the delta T - its stuck at 10). 

The DHW is set on schedule for an hour mid-afternoon and an hour at midnight and mostly works fine but occasionally have to top it up.

Is it possible12kw is not powerful enough? I imagined that a defrost cycle knocks the temp back for 8-10 mins or so when the unit defrosts but then should get to temp again (we also have the additional small reservoir to minimise water temp drop during defrost). If it cant get up to temp in between defrost cycles then the whole thing is a bit useless for three months a year...

 

[PhilT]

51 minutes ago, signorjim said:

Hi there - I wonder if anyone can help. New Altherma system takes hours to get up to temp after a DHW cycle or defrost. The refrigerant quickly heats up to 31 degreees c and then ticks up by a degree or so every fifteen minutes meaning that the leaving water temperature doesnt get up to temperature for a long time. Its pushing 17 litres per min, fixed, radiators as heat emitter.

 

Need a lot more detail please! "up to temperature" - what does that mean - as per the system design? What do you mean by "normal" radiators - are they the old small gas boiler size, or new extra large/low temp radiators? Are you running in "fixed flow temp", "weather compensation" or "auto (target room temp)" mode? The focus of your post is all on flow temp but the fundamental question is - despite your issues, are your room temps getting to where you want them to be? In current seasonal daytime conditions my average flow temp (large-radiator system) is typically around low to mid 30's when my room temp is at or approaching target temp, so possibly things may be working as designed?

[JohnMo]

Fundamentally a heat pump runs as detailed below

 

1. you or it sets a target flow temp, depending on fixed flow or a compensated flow temp.

2. Heat pump starts and monitors return temp and delta T

3. First it maintains delta T, so if you have a target flow temp of 40 and delta T of 5, but the return temp is 25, it will flow 30 degs until the delta T reduces below 5. As the delta T reduces the heat pump add more flow temp until it gets to 40 degs, while maintaining the delta T.  

4. Once at 40 degs the ASHP will modulate down from full power to a lower setting to maintain, 40 deg and the delta T.

 

Two things cause a slow ramp up, the heating system has been off for too long, or you are trying to bring temp up too quick.

 

How exactly do you operate the heating?

Do you have a buffer, if so how big?

How long does the DHW heating cycle take?

How many loops of UFH do you have?