Great feedback, Mick, I’ll happily adopt your strategy as a start this winter! In the process I expect to discover undersized rads as I experiment with lower curves. I purposely didn’t upsize them with the heatpump install as I’m a bit sceptical of ‘design’ heatloss which despite care and attention is often overstated.
Ideally I’m looking for all radiators’ valves fully open (inc. lockshields) and achieving 20C room temps with night setback on the lowest curve. This would indicate ‘perfect’ radiator sizing in ideal conditions. Of course it isn’t achievable (!) because e.g. opening the back door to let the dog out, opening bathroom windows, the sun shining on the house, etc. etc. will all conspire to ruin my plan. Can’t hurt trying though, eh?
@Fortune7 - one thing I notice is your flow/return rise quicker than @Zarch. This implies to me there is a blending valve to set the Return temperature. Might this be set too high?
Brian, I can’t comment on @Zarch setup but mine has a 3-way spring-return valve to divert to DHW (normal position is for heating), so the initial jump in temps will be the valve opening. After that the rise in temps will be consistent with the needs of the Sunamp heat battery.
The heat meter is on the return leg to the heatpump. Hopefully this explains?
Something must be heating the Return fluid, else it would not jump so quickly. I’d suggest the return temp could be too high so the HP is not getting an efficient DeltaT.
The return temp. sensor is initially being heated by the residual heat in the Sunamp heat battery, which due to its nature has a higher temp at its core - the PCM material changes phase at 58C. So when the pump starts, a ‘slug’ of water in the coil quickly reaches the heat meter’s ‘return’ temp. sensor, hence the jump on the graph. The flow, by comparison is the true temp of the heatpump’s output temperature, which takes a little longer to rise. Once the flow settles-down, the DT is around 5-7C while it charges the store.
I appreciate this profile is different to a DHW cylinder with water as the store, so looks a little odd when comparing. Well spotted, though!
So the HP must achieve that as a minimum to charge the Sunamp?
If so it seems that the PCM type DHW systems may not be very efficient with a HP unless other factors kick in (like space). Did you already have the Sunamp or was it specified with the HP?
Just interested, that is all. I’ve looked at these systems in the past.
My understanding is heat pumps are most efficient at around 5C DT (i.e. higher flow rate, lower DT) to keep the compressor working in its optimal range for heat transfer.
Good question! My home energy setup has evolved over the last few years; first solar PV, then monitoring which showed large summer pv export to grid. At the time I had a gas combi boiler, so no DHW storage cylinder and no space for one. The Sunamp gave a good alternative as it fits in the lower part of a standard tall kitchen cupboard, where the boiler was, and accepts surplus PV generated up to 3kW. For its small size, it equates to a standard DHW cylinder of about 150l (depending of course on storage temp). Static heat loss is claimed to be 28W/day due to vacuum thermal insulation, so probably better than a cylinder.
As for the ‘charging’ temp, Sunamp specify a flow temp of 70C, which is why the Vaillant unit was chosen. Clearly, this ensures full phase change of the store but at the expense of COP. I’m experimenting with lower flow temps, but it seems ‘above 60C’ works ok. Ultimately, it depends on how much storage is needed. I’m sure it would be ok to heat the store to (say) 45C, i.e. the PCM material would remain solid, but warm, but the capacity would be low. The store’s benefit is to heat the material to above the phase-change point, (can I say ‘superheat’?), to achieve full capacity.
So in short, yes, it was about space, or the lack of it! I don’t have the COP ‘bragging-rights’ of DHW cylinder owners, but reckon my space/efficiency compromise is defendable.
hello to everyone. this is my first post, and I really would appreciate your help
I am about to order my ASHP (Vaillant arotherm plus), and I need to decide between the 55/6 or the 75/6.
I will use it ONLY for heating, no DHW, no buffers, only 1 heating circuit only, only rads, all C33’s.
Last winter, even if it was quite mild (min -6C), we never went above 40C as a flow temp (condensing gas boiler).
Design temp for my area: -12 C, although we did not go below -8 since a few years.
Now my fear: if I choose the 55/6, below -8C, it does not cover my heat loss (missing by ~1kW).
I could use my gas boiler, as a backup (which I keep, anyway, for DHW only), for such cold weather,
Overall, I consider that is better to have a very low modulation from the 55/6 for the normal outside temp, instead of oversize (75/6).
P.S. is there any backup heater sollution from Vaillant (to be installed together with the 55/6) ?
I really looking forward for your advice. thanks, adrian
Funnily enough I was looking at something similar for a friend yesterday.
The 5KW model has a lowest modulation of around 2.4kW at 7C outside (which is when mine sort starts to cycle)
This is 30-35C flow diagram outputs
The 7kW at the same 30-35 flow temps will go down to around 3.2kW at 7C outside
At 40-45C, the MAX output of the 5kW is 4.9kW at -12C
And the 7kW at 40-45C at -12C is 6.7kW
As you don’t have and DHW to worry about and it sounds like you’ve got back up heating. If you’re very close to your requirements on the 5kW, maybe that is the best choice for you?
Vaillant do a backup heater, I think 3-9kW available
I don’t have one but have been trying to find out how they are wired up in case I need one (I’m also erring on the low side not the high side in terms of heat pump capacity). None of the easily accessible Vaillant schematics show how to connect it so its controled by the heat pump.
I was also wondering whether my 3kW immersion heater could be used as a backup. The answer must be yes with the right plumbing and control strategy!
@modeller Interesting. SEER for me is 5.2. As an aside, can you just “turn on” cooling and get it to run or do you have to use “cooling for several days” with “cooling on” to get it to run in cooling mode?
