Bear with me, I’m trying to get something straight in my head. So had my Vaillant 5kW ASHP installed and its running very well, I’m really happy with it ( Emoncms - app view ).
Setup is ASHP plumbed straight into the UHF manifold (10 port) and wet screed floor (w/ insulation)
The design temp ASHP installer went for was 35C flow @ -5C ODT, with a heat loss of ~4.5kW, WC curve of 0.3 and Room mod set to extended as the front of the house can get a lot of solar gain. So far, as expected flow temps have been lower (like 28C flow at 5C outside). But one thing I’ve noticed is that the living room is not heating up at the same rate as the other rooms, which all seem to be near enough in sync and all shoot for 21C
The UFH was installed by my builder and plumber back in 2017 and was being heated via biomass first (45C flow temp with blending valve) then last year via a pair of 3kW Willis heaters straight into the manifold (30C ish flow). All open loop. Both kept the living room coming up to temp with the other rooms. Flow rates have remained largely unchanged throughout (bit of tweaking here and there for some rooms that I didn’t want as hot as others but nothing major). I’ve modelled it in LoopCAD and it agreed with the flow rates the previous UFH designer/installer had put down for each loop @ 35C flow.
So what I don’t understand is what I should do to account for the fact the living room is now slower compared to the others. The overall heat curve seems fine, the house is not getting cold overall, ASHP is meeting the demand from what I’ve seen (from 4C up to turning off at 18C outside temp). I just want the living room to heat up a bit quicker compared to the others. So far no much an issue due to solar gain catching the room up in the afternoon.
Should I adjust the other rooms flow down? The problematic living room loop is already ‘max’ on the its flow meter. I think the ASHP just to happens to produce the 14lpm that all circuits need in total, so there is now ‘slack’ to give this one loop more flow? The pump is set to Auto on the ASHP and I read you can adjust it to get more flow (on DWH its on 100% rather than Auto and giving 20lpm, and that’s another question!).
Just trying to understand how it all works. Hopefully that makes sense and I’m not making a complete hash of it!!!
Answered my own question, I went for a play with the settings. Turns out Auto was on 100% which corresponds to the 855lph / 14.5lpm I was seeing. Will have to just adjust the flow rates down on the rooms that are warming too quick and balance it that way, its not far off.
The system will then fix at this flow rate and modulate the DT when less heat is required.
Likely be between DT2 and DT3 at minimum modulation (2kW-3kW ish).
860lph x 4.2 x DT2 = 2kW
860lph x 4.2 x DT3 = 3kW
etc
I think you cannot directly modulate dT. The only thing under control is the flow temperature, which is dictated by the weather compensation curve. The return temperature depends entirely on the emitters. If flow temperature is increased, the radiators/UFH give off more heat as the heating power of a radiator depends on its temperature difference to the room temperature, which in turn leads to a larger dT, as more energy gets extracted from the water. On an even lower level, all that can be controlled is the compressor speed, so I think Vaillant’s control logic is looking at flow temperature, comparing that to the target flow temperature and using that information to modulate the compressor. If flow temperature exceeds the target even at the lowest compressor speed, the energy integral mechanism kicks in to adapt heating power via cycling.
But as both the flow rate and the SHC are fixed from within the mass flow calculation, DT is the only thing that can modulate to change the heat output.
Is what I was implying with the examples.
When low outputs are needed you’ll see narrow DT, when higher outputs are needed you’ll see a wider DT. Because the other elements are fixed.
I just found it odd that on DWH it can hit ~20lpm/1200lph. I guess the pressure drop is a lot less on the DHW loop so it can run higher flow. I thought I read somewhere that Auto was 65% and 860lph with 100% being 1200lph, but I’m probably confusing what I’ve read
Control of the dT for me is by virtue the UFH slab has masses (no pun intended!) of heat store potential and can have a decent dT when the slab is cool. Because the biomass used to only like 2+hr runs, I’m used to loading it up for a few hours once a day and leaving it to release the heat all day long like a big storage heater. I know that its not the most efficient way to do it with an ASHP but with having IOG off-peak, that will be my aim, at least to put proportionally more heat in during IOG off-peak (have HA control it and artificially up the setpoint to drive up the heat curve) and top up as required during peak (from battery and then grid if/when that runs out). ASHP doesn’t quite have the power the biomass had (15kW) so can’t expect it to put a full days heat into the slab
It’s related to pressure drop. You have a “maximum head” or similarly named setting on the indoor unit which goes up to 900 mbar. This setting will also influence how much flow you’re getting in heating mode.
