Install of a largish MyEnergi Libbi battery

[sharpener]

Situation: a 200sq m stone barn conversion with UFH downstairs and rads upstairs. I am planning to convert from an old oil boiler to a 12 kW Vaillant Arotherm + HP (with appropriate upgrades to the rads).

I also hope to put in a thermal store which I want to heat to 55C at off-peak rates while the HW cyl is heating, this should act as a big heat sink and prevent the HP short cycling due to the small coil. Hence avoiding the need to change the cylinder which due to where it is sited will involve a lot of structural work. I do not care about the consequent long recovery time.

So if I connect the TS in parallel with the existing coil will that be sufficient or is there a better way? A secondary pump would discharge the TS to the rad circuit when I want to heat the bedroom(s) first and last thing in the day.

[Countrypaul]

It would be normal from what I have read to use a motorised valve so that either the HWT is heated or the TS, The MV can be controlled from some HPs so that different flow temperatures are used for the HWT and the TS/CH. If they are in parallel wil you have thermostats on both the HWT and TS?

[sharpener]

I think because of the small coil in the original tank it will need the additional volume of the TS to be in circuit all the time to avoid short cycling. I would fit the sensor to the latter (which will be the larger) and rely on the circulation to equalise the temps. The original tank has a stat which I can re-purpose as an overtemp limit. I may also recirculate the domestic HW side with a bronze pump to mix the bulk contents and improve the heat transfer.

This will all happen on E7 (and again p.m. if there is a PV surplus). It will be a 12kW HP so will easily charge in that time, the main issue is its minimum output is 5kW.

This is not the conventional wisdom which is to have any TS, buffers or volumisers on the heating side of the 3-port valve. But to get the max storage I want the TS to be at DHW flow temp not UFH.

The potential installers don't like the sound of it but I am trying to find any flaws before having another go at them.

[Countrypaul]

We have a 450L TS with a 12kW Ecodan ASHP in a 225m2 house with UFH downstairs and in bathrooms upstairs. I have the room stats set slightly higher from 00:30 to 07:30 (E7 period) and drop back to normal (around 20C) during the day and hen drop to about 18C late evening so that the HP does not come on to have very little effect at peak rate. This works well with very little running of the ASHP during peak times unless it is very cold outside for long periods (0c for more than just one day).

We have PV and an Eddi, I have now almost setup the Eddi & ASHP so that the ASHP should come on when there is sufficient excess PV - not needed over summer as the Eddi often heats the TS fully and we start exporting even though 2 kids are at home and have 3 large (gaming) PCs in use!

[sharpener]

That's almost exactly the kind of setup.

How is yr TS plumbed in and what controls the charge/discharge? Or does it just act as a big general buffer?

[Countrypaul]

The TS has 3 immersion heaters - this was as originally before we moved in I needed to ensure the house pipework etc. did not freeze and we had no ASHP. I therefore got some figures for over winter based on just immersion heater usage to keep the place above 15C (insurance requirement). We also had some windows missing and boarded over though large gaps around!

The immersions are one near the top that will only provide DHW, one near the bottom and one near the middle just below the UFH take off - the bottom and middle were used above. The Eddi has the top one as primary and lower one as secondary.

The UFH has take off just above the mid point and return near the bottom. The ASHP has take off near the bottom and return below the mid immersion (and therefore below the UFH take off).

The DHW has 2 coil in the TS one at the lower half and one at the upper half with 3 takeoff points, bottom (also the input), middle and top with TMVs at the middle and top, the idea being that it can mix warm (or hot) waterwith cold at the mid point but if that is insufficient it can mix how water from the top. The idea being that this preserves the hot water at the top of the TS and limits having a full TS of luke warm water.

There is also attachments for a WBS with input at the middle and take off at the bottom (no WBS implemented yet though), and solar connections for solar thermal (again nothing implmented).

The UFH for the ground floor is through a standard wiring centre that turns the pump on if there is any demand, UFH at 150mm centres about 1200M covering 150m2. The UFH upstairs is simply on a timer in winter (usually 05:00 to 07:30).

There is an MVHR ssytem with outlets in each bedroom plus 4 rooms downstairs and extraction from 3 room upstairs and 3 downstairs (Kitchen, utility & WC).

[sharpener]

Thanks for that! A big universal buffer then. And I thought my concept was complicated.

I take it this is something of your own devising, is it controlled natively by the Ecodan controls and the mixing valves, or by your own raspi or Homely or Modbus or something? How did you persuade yr Mitsi installer to go along with it or is it all DIY?

I plan that the TS will feed just the upstairs rad circuit, 300litres will store enough about energy to heat the master bedroom up for an hour before we get up and again in the evening. Even so I cannot find an installer who knows anything about adding a TS.

No immersions (or coils) in my TS, they would need further application to DNO, only the existing one in the DHW tank (which is fed from a diverter of my own design). As it is already there I would claim if asked that it is not part of the HP system which I need their permission for. Another reason why I do not want to have to replace the tank.

Can you see any reason why what I am proposing would not work?

TIA

[Swwils]

Flawed plan just size the HP for 6C and then have secondary heating for coldest points.

No buffer no store.

[Countrypaul]

The Ecodan allows use of an external thermostat (zero volt contacts) to call for heat. I use a W1209 on the TS near the bottom but well above the ASHP draw off point. The downstairs UFH is simply controlled by the wiring centre which supports 12 zones, but allows upto 4 zones o be controlled by one thermostat (mine is Heatmiser). Due to an oversight when wiring the place (forgot to put cables for wired thermostats in place) we ended up with a wiring centre for wired thermostats so have used 5 Salus wireless ones to control the zones along with one wired and one on if anything else is. The kitchen has 4 zones, lounge 3, study, utility, hall, playroom and WC(no thermostat) 1 each.

It is not my design, but came from various souces I looked at and finally a supplier that seemed to think the same way (TMS Thermal Store iirc). They suggested the multiple TMV approach for the DHW and it all seemed straightforward as far as they were concerned. For installation of the TS, the plumber who was helping me during our renovation was quite open to installing a TS even though he never done one before. He is Polish but been in this country a long time, and it much more than just a plumber, almost a jack of all trades, but won't touch brick laying. The TS is copper fron Newark Cylinders - again they were familiar with the concept and had no problems with the tank fitting locations we specified. The TS is under a sloping roof and its size limited by both the location height and getting the TS through doorways (and upstairs) though the two of us managed to carry it without any real problem.

For the Ecodan, the supplier rep seemed very familiar with Thermal Stores (as you have found many are terrified) and other than a couple of problems with parts the installation seemed to go easily enough (1 day). I did find a couple of problems, the Ecodan was set to HW only mode and running at 55C continually. Covid interferred, so I talked to both the installers and Mitsubishi and changed from using the themocouples they supplied direct to the Ecodan, to setting it for external thermostat. The electricity meter signal for the Ecodan did not work at first, so it did not produce meaningful figures, this turned out to be a simple case of reversing the wires at one end.

I dont know how your UFH is setup, but if you have separate zones is there not a chance of short cycling if the ASHP supplies the TS, UFH and DHW, what happens when the TS is upto temp, the DHW is and there is only one zone wanting heat? Similarly if the thermostat is only on the TS, would you not be in the position of the UFH not heating as the ASHP has been turned off because the TS is upto temp?

[sharpener]

Flawed plan just size the HP for 6C and then have secondary heating for coldest points.

No buffer no store.

Flawed plan, MCS requires a system rated to 99% probability OAT, in my case -0.2C (Plymouth).

No MCS compliance no BUS grant.

I dont know how your UFH is setup, but if you have separate zones is there not a chance of short cycling if the ASHP supplies the TS, UFH and DHW, what happens when the TS is upto temp, the DHW is and there is only one zone wanting heat? Similarly if the thermostat is only on the TS, would you not be in the position of the UFH not heating as the ASHP has been turned off because the TS is upto temp?

There are four legacy UFH zones but I expect it to operate during the day on weather comp with all zones open.

Vaillant provides the usual DHW priority, with a programmable max duration. When the TS and DHW are up to 55C the control will revert to the UFH at <45C flow, this should happen by 0700 when E7 goes off. Meanwhile the bedroom rads will get 55C flow from the TS.

Afternoons there is the best chance of surplus PV, if not the outside air will be at its warmest and little requirement for space heating, the TS will recharge ready to heat the bedroom again before we go to bed while the UFH continues to keep the downstairs warm or maybe on setback.

Will that be OK or is there an underlying problem<g>?

PS I sympathise with yr Polish plumber, I don't like the wet trades, am happiest with carpentry, plumbing and electrics in equal measure. My bricklaying looks OK but is too slow to be useful on anything bigger than a bike shed, plastering a total no-no.