Willis Heater Integration with a Thermal Store

[JamesDRG]

Hello fellow members, this is my first post so please be gentle  

 

We are working on the heating design for a remodel/extension to a property built in 1990.

 

Our primary source of heat will be an ASHP with Solar to (maybe) follow in the future. We also have a log burner with back-boiler.

 

All heat sources will "charge" a 500L thermal store which we have situated on the first floor. Log burner will use a standard gravity fed system to the TS (out of scope of this thread).

 

The CH side of things we are fairly comfortable with (wet UFH on ground floor, floors dug out with 200mm celtox insulation, upstairs rads, all fed from TS).

 

Where I would value your opinion is on the DHW.  

 

I have been reading with interest @oranjeboomand @Jeremy Harrisyour successes with the humble Willis Heater (I plan to use a similar setup as an interim/backup for the UFH until the ASHP arrives).

 

For the DHW I was thinking of integrating a Willis Heater with a direct vented HW cylinder, using the attached "Willis" diagram which many of us are familiar with. The advantage of this is it would require little change to the DHW arrangement we have now (we have a header tank in the loft, gas boiler).

 

In the summer months we would use the Willis Heater to provide DHW "on demand". For example, we put on for 10 minutes before a shower, 1 hour before a bath. That is fine for our needs.

 

In winter, I want to make use of the TS for DHW. With that in mind, I have proposed the second schematic where we essentially have a similar thermosyphon arrangement with the Willis Heater, repeated (in parallel I think?) using the indirect DHW coil in the TS. I hope that if we say light the log burner of an evening (boiler output 5kw), once the TS starts to charge this heat will be transferred to the vented cylinder and provide an extra 150L of hot water. 

 

I would like to hear people's view on this approach. Some things I have top of mind:
 

1. I will need to think about we make the Willis a little "smarter" in not actually activating should there be sufficient water in the DHW cylinder. The thermostat in the Willis could be sufficient? 
 

2. For on-demand heating in the summer months, perhaps there is some optimisation we could do to feed water to the Willis heater again via the coil in the TS so we can utilise this captured from the ASHP. Or would the thermosyphon effect be OK here? The TS would reach about 30C so this should be transferred to the HW cylinder. 

 

My assumption (which is only that ) is it is more efficient to run the ASHP at a very low output temp (say 30C) and provide the additional "top of" of DHW temp using the on-demand heating using the Willis. 

 I look forward to hearing what people think

Edited December 11, 2022 by JamesDRG

[PeterW]

New TS with 2 or 3 immersions - top one is your “boost” then the rest heat lower as required. The Willis won’t work as you have it - you’ll get pull through via the tank. 

[JamesDRG]

Thanks for the input Peter.

 

In your proposal would you do away with the second cylinder and take the DHW from the DHW coil in the TS?

[JamesDRG]

On 11/12/2022 at 22:03, PeterW said:

The Willis won’t work as you have it - you’ll get pull through via the tank. 

I could add a valve as shown in the third picture that will close whilst the Willis is on?

[JamesDRG]

[joth]

On 11/12/2022 at 20:00, JamesDRG said:

All heat sources will "charge" a 500L thermal store which we have situated on the first floor.

ASHP charging a TS to run UFH sounds inefficient , what temperature would you be running the ASHP flow at for this?

Ideally the ASHP should run directly at the desired UFH temperature, say about 30°. If you're not using the TS for DHW then the amount of use you'll get from it seems quite limited 

[PeterW]

A 3kW Willis is nowhere near powerful enough to provide instant hot water - you need upward of 6-7kW, 10kW for a decent flow at 45°C

 

I would be putting a TS in with multiple immersions, or a UVC with a thermal separator from the boiler stove and build it out properly

 

 

[JamesDRG]

2 minutes ago, joth said:

ASHP charging a TS to run UFH sounds inefficient , what temperature would you be running the ASHP flow at for this?

Ideally the ASHP should run directly at the desired UFH temperature, say about 30°

Thanks @joth

We were going to run the the ASHP low enough so it can serve the radiators upstairs - which we have oversized. We might get away with 40C if we can get enough insulation in the house?

 

 

4 minutes ago, joth said:

If you're not using the TS for DHW then the amount of use you'll get from it seems quite limited 

I think this is a fair point, the main reason in the short-term would be for the stove, but if we were to add solar in the future then I think this would "solve" the DHW problem (at least in my head!).

[JohnMo]

At the moment you have the two cylinders in parallel, could you do them in series.

 

Also you are setting it up for rubbish flow, as the system has no head available - couple of metres only.

 

A thermal store will give you mains pressure, UVC the same.

 

For the thermal store you need a huge coil 6m2+ or tank in tank to be efficient for DHW at heat pump temps.  You also need a pressurised thermal store/buffer for a heat pump to allow direct charging.

 

You can get split thermal buffers, stores, with two operating temps designed for heat pumps and stoves.  Off the shelf solution, but tend to be a big capacity

 

Some ideas here also

 

https://originaltwist.com/2019/11/26/heating-system-for-heat-pump-2020-revision/

[JamesDRG]

4 minutes ago, PeterW said:

A 3kW Willis is nowhere near powerful enough to provide instant hot water

Defo agree with this point. But in our design we are not looking for instant hot water - we would run the Willis just ahead of when we need it.  So for a shower, should take 10-15 minutes of heating. We can have this linked to Alexa via Shelly relays so it's not too much of a pain to just give a bit of notice before a shower  (or time it of you shower more or less at the same time). 

[JamesDRG]

3 minutes ago, JohnMo said:

At the moment you have the two cylinders in parallel, could you do them in series.

Thanks @JohnMo Hmmm interesting idea. I'll sketch that out in a bit and see what that could look like.

 

4 minutes ago, JohnMo said:

Also you are setting it up for rubbish flow, as the system has no head available - couple of metres only.

 

A thermal store will give you mains pressure, UVC the same.

Yes, agree with the above. We have a pump already fitted for the showers and this is more than fine. Filling a bath takes a bit longer but again we don't mind this really. 

What I am not sure with a DHW from a TS is how much of a temp drop you would get from the TS vs DHW out? Ie what is the minimum TS temp you could get away with to still have a hot shower.

[JamesDRG]

9 minutes ago, JohnMo said:

You also need a pressurised thermal store/buffer for a heat pump to allow direct charging.

Good call on this - yes a buffer tank in the plan for the ASHP. 

 

Hmmmmm feels like I am breaking some KISS principles!

[JohnMo]

Some reading material 

679346837_AccumulatorsBuffers.pdf

[PeterW]

1 hour ago, JamesDRG said:

So for a shower, should take 10-15 minutes of heating. We can have this linked to Alexa via Shelly relays so it's not too much of a pain to just give a bit of notice before a shower  (or time it of you shower more or less at the same time). 

That would give you 3-4 mins run time of shower. Why not just put the immersion in the tank on ..? And why not heat the whole tank off peak on E7 ..?

[JamesDRG]

1 hour ago, PeterW said:

Why not just put the immersion in the tank on ..?

The Willis heater heats the very top of the tank with hot water and is more efficient than an immersion near the top, which is invariably heating a larger volume of water and promotes more blending of water.

 

1 hour ago, PeterW said:

And why not heat the whole tank off peak on E7 ..?

As I understand it, you cannot have E7 meters installed anymore.

[SteamyTea]

26 minutes ago, JamesDRG said:

I understand it, you cannot have E7 meters installed anymore

They just do the switching though a smart meter.

My neighbours have smart metering and E7.

I still have an old radio switch, but some people still have mechanical timers I am sure.

Mechanical timers would be great as you can wind them on a few hours.