Thermal store

[sharpener]

1. As discussed elsethread here is the proposed diagram for a Thermal Store in conjunction with an Arotherm Plus.

 

I don't know if there is an approved Vaillant way of doing this (as distinct from a Buffer) but I haven't been able to find one. Perhaps there is more in the documentation for installers.

 

The idea is to charge up the thermal store using off-peak electricity in the small hours and free PV in the daytime, and use it to heat the rads in the master bedroom for an hour morning and evening when electricity is much more expensive.

 

Calcs show heating a 210l cyl by 12 deg will store 3kWh which is enough to heat the bedroom, and this could save about 360kWh of electricity per year.

 

Referring to the diagram below:

 

• Normally the two 3-port valves A and B are in the straight-through (black) position and the rad zone is controlled by valve R1 as per the normal arrangement in the Vaillant schematics.
• To charge up the TS the 3rd party programmer sends a demand signal to the VR71 where it is programmed as Zone 3, this activates Valve A and the flow is diverted into the store.
• To discharge the TS through the radiator circuit the programmer actuates Valve B and the pump next to it directly to feed the rads. The HP can be on or off during this.
• In normal operation all the return flows (including the auto bypass) would pass through the lower part of the tank so it would also act as a volumiser.

 

2. Recirculation of DHW on the secondary side

 

You might recall I have done some tests which indicate that the heat transfer is likely to be increased by a factor of 2 by circulating the bulk water in the DHW using a small bronze pump e.g. https://www.pumpsukltd.com/lowara-ecocirc-pro-15-1-65b-bronze-circulator-1-230v.html.

 

This will mean a heat transfer rate of about 6 kW; the Arotherm Plus can turn down to 6kW so it will not need to cycle on and off. This would be much easier to implement than a replacement cylinder or fitting a heat exchanger. Getting the existing cylinder out from above the airing cupboard would require a lot of structural work.

 

We can live with the longer reheat time as the tank will be heated during the afternoon and in the small hours when we are not using the hot water anyway. If we have a lot of visitors there is also the 3kW immersion which we can run off the PV diverter.

 

 

Would appreciate all sensible comments!

[Marvin]

1 hour ago, sharpener said:

Would appreciate all sensible comments!

😄 

 

Not necessarily what you wanted to know.

 

 How does the Arotherm plus deal with freezing?

 

Can the water circulate through the ASHP and a tank of some description if the ASHP decides to circulate?

 

Were you considering frost protection drippers or antifreeze?

 

The primary circuit will all need antifreeze (if that's your choice) including the UFH, rads, thermal store and associated pipework. 

 

 

[JohnMo]

ASHP generally (not seen a modern one that doesn't) a pressurised heating system, so the thermal store will have to be unvented. It will need antifreeze valves or its going to cost a fortune in antifreeze .

 

You seem to spending £2k+ to save a possible £100, but not convinced. You making what should be a simple system super complex. Why not just run the radiators you want in the cheap period, can't see why your bedroom would loose heat that quick, that an hour so earlier or later would make a big difference

 

You can always get the recip saw out and take the existing cylinder out in bits.

[sharpener]

3 hours ago, JohnMo said:

ASHP generally (not seen a modern one that doesn't) a pressurised heating system, so the thermal store will have to be unvented. It will need antifreeze valves or its going to cost a fortune in antifreeze .

 

You seem to spending £2k+ to save a possible £100, but not convinced. You making what should be a simple system super complex. Why not just run the radiators you want in the cheap period, can't see why your bedroom would loose heat that quick, that an hour so earlier or later would make a big difference

 

You can always get the recip saw out and take the existing cylinder out in bits.

 

Yes that is where we started. Does anyone have any thoughts about the proposed circuit, that is what I was hoping for? @Marvin yes my proposal does allow for the water to circulate through the bypass valve and back through the TS under all circumstances so defrosting is catered for.

 

Neither of us sleep well in a heated bedroom so turning the heating on before the end of cheap rate (0700) is not what we want to do, the time-shifting is part of the scheme. Ditto the evening burst, it is always peak rate then and no PV.

 

Installer A says he cannot get the HW tank out and would decommission it in situ. It sits on a shelf in a void above the airing cupboard and the top is 120 mm below the rafters. It cannot go up, down or sideways so we think he is probably right and that the whole landing wall was put up after the tank went in. He proposes to fit a new HW cylinder and a 100l buffer in the utility room which will take up more space than the oil boiler the HP replaces.

 

Which is why I am trying to persuade him to leave the tank as is and fit the secondary pump instead as per the top right corner of my diagram. The marginal cost of the TS would then be the difference between his 100l buffer and a 200l TS, plus 2 x 3-port valves, not £2000 (I already have a spare pump and the 2-channel timer). His proposal is silent on frost protection. I would prefer glycol, since the water will need additives anyway and dumping it in the middle of winter would take some sorting out. 100l extra fluid would mean only 10-20l more glycol.

 

OTOH Installer B thinks he can put in a new tank where the old one is. If he can get the new one in then he can surely get the old one out without sawing it up, complete with its G3 valve group. So in that case we can use it as a pressurised TS at negative cost as he would not then need the 100l buffer he is proposing, and also mean only one large vessel in the utility room so take up less space. His proposal does include antifreeze valves.

 

Installer C also quotes for fitting antifreeze valves and expressly mentions inhibitor. His budgetary quote was 1.5 times the others and includes new pipework throughout the house so we are not going with that.

 

 

 

Edited July 4, 2023 by sharpener

[dpmiller]

we run a TS but with the heat pump on a coil (so no massive amount of glycol needed). I've it configured to have all CH flow passing through the coil as a buffer, and the tank heated farther during the DHW times. The CH can therefore run off diverted heat from the PV as well as from the boiler stove which is plumbed direct to the TS.

 

My observation is that you can slurp heat out of the bottom coil of the tank quick enough that the bottom of the tank goes cold. Are you sure your scheme won't suffer from stratification and not actually hold as much heat as you anticipate?

[sharpener]

2 hours ago, dpmiller said:

we run a TS but with the heat pump on a coil (so no massive amount of glycol needed). I've it configured to have all CH flow passing through the coil as a buffer, and the tank heated farther during the DHW times. The CH can therefore run off diverted heat from the PV as well as from the boiler stove which is plumbed direct to the TS.

 

My observation is that you can slurp heat out of the bottom coil of the tank quick enough that the bottom of the tank goes cold. Are you sure your scheme won't suffer from stratification and not actually hold as much heat as you anticipate?

 

Thanks, good question @dpmiller.

 

No coil proposed. Yes I could do the same as you and have a cyl with an HP size coil. Is your tank then just an inert TS or does the water in the body go somewhere? If not I think I would want a secondary pump to stir the tank for the reason you outline. Though it could all be unpressurised, so eliminating the G3 kit. But I would prefer to buy another 10l of glycol than install yet another pump.

 

Site is on S coast so -10C protection is probably overkill, the HP has its own FP so we are talking about grid outages really and if all else fails I have a 10kWh battery too. Main concern is if we are in the Canaries at the time.

 

It started as a thought experiment to make use of the existing tank if the installers replace it. But as you will have seen we don't now think it can be got out in one piece, however the cost of upsizing their proposed 100l buffer to 200l instead is not great.

 

You will see from the diag that when charging, the flow through the tank is from top to bottom, and when discharging the opposite. But at a min flow for the HP of something like 20l/min a 200l tank will turn over in 10 mins or less anyway, so several times during the charging process. If the HP maintains a constant delta T across itself then effectively the whole body of water will heat slowly without stratification, that is my hope. On discharge much the same in reverse with the rads acting as a heat sink.