most efficient HW setup ?

[Iceverge]

This is an unusual requirement indeed

 

You're limited by how much electricity you can suck from the wires. This can be multiplied say X3 by the ASHP and also buffered by extra storage but still you arrive at almost certainly unpractical figures for both. 

 

A third way that came to mind.

 

Run the shower water through a heat exchanger preheating the UVC. They can run at 90% + efficiency but even if you were to achieve 50% overall you'd be 25kW better off re your total power requirement. 

 

 

 

[Nickfromwales]

8 minutes ago, Dave Jones said:

already have the Powerwall, will be taking it from our existing place. will be fitting approx 9kw of solar as its relatively cheap and quick payback.

 

yes 3 phase (all new builds in western powers area are required to be 3 phase now).

That will be sized, most probably, to the electrical requirements of the house, with most of the excess feeding DHW or topping up the battery. Can you fit more PV? 

[Dave Jones]

this is the waste water heat recovery unit im planning on using. basically plumbing all the shower wastes to it.

 

https://recoupwwhrs.co.uk/documents/Recoup_Pipe_HEX_Be-Technical_Specification.pdf

 

 

[Iceverge]

In fact scrap that, have a dedicated UVC for the showers for the "cold feed" that is full of water preheated to ~ 30deg by the drain water. 

 

Your normal UVC @50deg+ will last for ages when blended with almost warm enough water then. 

[Nickfromwales]

3 minutes ago, Iceverge said:

This is an unusual requirement indeed

You've described @Dave Jones to a tee

3 minutes ago, Iceverge said:

You're limited by how much electricity you can suck from the wires. This can be multiplied say X3 by the ASHP and also buffered by extra storage but still you arrive at almost certainly unpractical figures for both. 

Which was one of the main reasons why I have mentioned gas tbh. 22.5 kW of microgeneration ruffles the feathers, then add EV charging, and THEN mention a whopper of an ASHP and they may well shit the bed.

 

Prob be a good idea to choose a PV system where you can volunteer to accept zero export. Wipes out toying with Octopus etc though.

 

6 minutes ago, Iceverge said:

Run the shower water through a heat exchanger preheating the UVC. They can run at 90% + efficiency but even if you were to achieve 50% overall you'd be 25kW better off re your total power requirement. 

Cracking idea, and perhaps could be further simplified by just doing that from a huge UVC stored at 40^oC?

[SteamyTea]

14 minutes ago, Nickfromwales said:

20 minutes ago, Dave Jones said:

must be a calculator somewhere that tells how long to heat X volume of water to X temp in kwh .....

If you say @SteamyTea three times, he will appear

Easy.

 

(4.18 [kJ/kg.K] x kg [mass of water] x ∆T [temperature to increase by] x 0.0002778) / kW [input of thermal source]

 

This is the theoretical minimum time. There are heat losses to consider that rise as the temperature increases. These follow Newton's Law of Cooling.

 

Easier to just add a bit of time, say 15%.

[Nickfromwales]

Just now, SteamyTea said:

Easy.

 

(4.18 [kJ/kg.K] x kg [mass of water] x ∆T [temperature to increase by] x 0.0002778) / kW [input of thermal source]

 

This is the theoretical minimum time. There are heat losses to consider that rise as the temperature increases. These follow Newton's Law of Cooling.

 

Easier to just add a bit of time, say 15%.

Exactly what I came up with. You literally JUST beat me to posting that.

[SteamyTea]

11 minutes ago, Nickfromwales said:

Cracking idea, and perhaps could be further simplified by just doing that from a huge UVC stored at 40^oC?

Why not store at the temperature the UFH needs. Then blend that in with a smaller volume of much hotter water when needed.

Sometimes you need piping hot water, like greasy dishes.

 

I checked my water usage the other day, 170litres/day.

Way too high again, but I do like a hot bath to start the day, gets rid of most of the aches and pains. Maybe I just need to take more paracetamol, would be cheaper.

 

[Dave Jones]

so some more fag packet math:

 

Assume a single 300L tank.

 

Based on incoming mains temperature 8C.

 

Step1:

Target for the ASHP to increase the cylinder temp to 45C an increase of 37C. Would use 12.9 KWH of electric. At 6kw of power per hour from the ASHP would take 2.14 hours.

 

Step2:

Target for the immersion to increase cylinder temp to 65C and increase of 20C. Would use 6.9kwh of electric. At 6kw immersion would take 1.16 hours.

 

Not factored in the heat recovery but would make step 1 much faster.

 

So not an insurmountable task ?

 

 

thats if my math adds up.....

 

 

Edited March 8, 2023 by Dave Jones

[SteamyTea]

5 minutes ago, Nickfromwales said:

Exactly what I came up with. You literally JUST beat me to posting that.

Then you can explain how to incorporate the heat losses better, and how to convert them to energy from the existing known, and unknown,  parameters.

[JohnMo]

35 minutes ago, Nickfromwales said:

Diminishing hot water? When the combi registers DHW flow the pre-heat circuit shuts down and the volume of heated water starts to diminish rapidly.

Not sure I am following. Mine has no option but to be pre heated via a 160l combi thermal store the preheat circuit.is always on. As that's the only way the water flows.  In winter even a few of degrees reheat gets the same performance as you would in summer.

 

KWh for kg of water 

https://www.omnicalculator.com/physics/water-heating

[Nickfromwales]

37 minutes ago, SteamyTea said:

Then you can explain how to incorporate the heat losses better, and how to convert them to energy from the existing known, and unknown,  parameters.

I would, but I'm washing my hair....for a few hours. It's on my to-do list though, and I'll not forget! 

[Nickfromwales]

25 minutes ago, JohnMo said:

Not sure I am following. Mine has no option but to be pre heated via a 160l combi thermal store the preheat circuit.is always on. As that's the only way the water flows.  In winter even a few of degrees reheat gets the same performance as you would in summer.

 

KWh for kg of water 

https://www.omnicalculator.com/physics/water-heating

Ah, so not a Combi boiler, just a combi-nation of components. Gotcha  Now I recall your setup, sorry. Been a looooooong day.

[SteamyTea]

2 minutes ago, Nickfromwales said:

would, but I'm washing my hair

Cut my Sampson locks of yesterday.

All part of my energy saving regime.

 

[Nickfromwales]

Too much off the ears.

[ProDave]

Nature did that for me.

[JohnMo]

35 minutes ago, Nickfromwales said:

not a Combi boiler

Yes a Combi boiler and a combi-nation of components 

[markocosic]

The requirement is daft.

 

If you really must then buy a commercial cylinder with a decent sized coil for the heat pump to dump heat through:

https://www.vaillant.co.uk/specifiers/products/unistor-500-and-800-litre-cylinders-38080.html

 

And plumb it in exactly the same way that you would a usual pre plumb for a heat pump:

https://midsummerwholesale.co.uk/pdfs/unistor-datasheet.pdf

 

 

Your issue will be the mismatch with space heating. The heat pump that you'll need for space heating will be small (their turndown is limited; so a hulking great big one for silly DHW production will struggle to turn down low enough for mild season space heating)

 

The "5 kW" Vaillant unit will do 7 kW at -2C external and 35C flow temp (with a COP of 3 at that) and turns down to about 2.5 kW in milder conditions. (with a COP over 5 at that) It comes with decent heating controls too. That'll suit the space heat side of things.

 

 

 

For the hot water you can get away with something bigger / dumber / optimised for higher temperatures. Samsung's "HT" scroll compressor units would do the trick. Crap heating controls on those but you're literally only using them as an on/off immersion heater so...meh?

 

https://midsummerwholesale.co.uk/buy/samsung-heat-pumps/samsung-stealth-12-heat-pump

 

 

Fit hulking great PV array and just time the hot water heating for when the sun is shiniest and the air is warmest. You'll be sucking your PV dry with the amount of water you use / invariably importing something so screw trying to be too clever about diversion. Battery handles passing clouds?

Edited March 8, 2023 by markocosic

[Dave Jones]

thanks for that, ive got some reading to do.

 

Would I really need such a big lump though ?

 

so some more fag packet math:

 

Assume a single 300L tank.

 

Based on incoming mains temperature 8C.

 

Step1:

Target for the ASHP to increase the cylinder temp to 45C an increase of 37C. Would use 12.9 KWH of electric. At 6kw of power per hour from the ASHP would take 2.14 hours.

 

Step2:

Target for the immersion to increase cylinder temp to 65C and increase of 20C. Would use 6.9kwh of electric. At 6kw immersion would take 1.16 hours.

 

Not factored in the heat recovery but would make step 1 much faster.

[IanR]

51 minutes ago, Dave Jones said:

 

Assume a single 300L tank.

...

cylinder temp to 65C

 

How does that meet your x4 simultaneous 20l/min showers for 20 mins + apparently hot feed to washing machines and dishwashers?

 

If 300l @ 65°C with a 3 hour reheat time meets your requirement, then just have a 500l UVC heated to 50°C by a 12kW ASHP and have a 60 min reheat time. Pretty standard, if a little on the larger side, but able to store plenty of excess PV when the sun is shinning.

[markocosic]

47 minutes ago, Dave Jones said:

Would I really need such a big lump though ?

 

You're the one who wanted 900 litres of stored hot water. 😉 

 

 

A more reasonable option might indeed be something like:

 

- 7 kW Arotherm (still turns down low enough to do your space heating; but offers a usefully higher output to the DHW reheat)

https://www.vaillant.co.uk/for-installers/products/arotherm-plus-7kw-146561.html

https://www.theheatpumpwarehouse.co.uk/shop/heat-pumps/air-source-heat-pumps/vaillant-arotherm-plus-7kw/

 

- An off the shelf 500 litre cylinder from Telfords (which at 710 x 1850 isn't stupidly big; and at 3.3 m2 has a coil just about big enough for the 7kW Arotherm)

https://www.cylinders2go.co.uk/shop/renewable-energy/500-litre-telford-tempest-heat-pump-cylinder-twin-immersion/

 

- A direct electric backup heater for that odd occasion when it's actually cold outside (and your heat pump is busy doing nothing but space heat) or you otherwise need to refill that 500 litre cylinder in doubletime

https://www.cylinders2go.co.uk/shop/stainless-steel-unvented-cylinders/indirect-unvented-stainless-steel-cylinders/telford-tempest-500-litre-stainless-steel-indirect-unvented-cylinder-fitted-with-2-x-6kw-titanium-immersions/

 

(I'd ask Telfords to do you a heat pump type cylinder with the big coil and 2x 6 kW immersions - expect plenty of change from £2k)

 

 

Overall cost not too silly. It's a fairly standard heat pump (for an existing old stock building) with a slightly larger but not in the least bit complex cylinder. Idiot proof. 🙂

[markocosic]

I'd also store at 50C unless you know there's a requirement for more.

 

(in which case you can probably go up to 60/65 with these propane based heat pumps before you give up and pop the immersion on)

 

Incoming mains cold water will be your limiting factor / costly item I suspect.

[jack]

3 hours ago, Dave Jones said:

this is the waste water heat recovery unit im planning on using. basically plumbing all the shower wastes to it.

 

https://recoupwwhrs.co.uk/documents/Recoup_Pipe_HEX_Be-Technical_Specification.pdf

 

I don't believe any domestic waste water heat recovery unit is rated to cope with four showers. The one you linked recommends a maximum of 24 L per minute (total) for system types B and C, and only 16 L per minute (less than one of your showers) for system type A.

 

8 hours ago, Dave Jones said:

Coming from limitless hot water I'll need to keep adding tanks until it doesn't run out from our usage pattern.

 

If you want to "save the planet" as you say, then why not use this opportunity to change your family's usage patterns rather than just replicating the current pattern in the new place?

 

Assuming you have kids who are using at least a couple of these simultaneous showers, then the flow rate is ultimately up to you. A few weeks ago I installed flow restrictors on our kids' showers to limit them to 9 L per minute (down from about 12 or 13 L max per minute beforehand) and the showers work perfectly fine. Not sure the kids have even noticed - I didn't tell them and they haven't mentioned it.

 

6 hours ago, Dave Jones said:

agree ref shower its a preference as there is no shortage of water in the UK. 

 

Depends very much where you are in the UK. There have been hose pipe bans in the south east more than once in the last decade due to water shortages.

[Dan F]

Just now, jack said:

I don't believe any domestic waste water heat recovery unit is rated to cope with four showers. The one you linked recommends a maximum of 24 L per minute (total) for system types B and C, and only 16 L per minute (less than one of your showers) for system type A.

Some can do 2 showers, but (especially if the shower flow rate is high) one per shower is the way to go.

 

That said, looks like Showersave have a commercial option which might be worth looking at: https://www.showersave.com/commercial-new/

[jack]

7 minutes ago, Dan F said:

Some can do 2 showers, but (especially if the shower flow rate is high) one per shower is the way to go.

 

Two showers at once would be extremely unusual in our house, so having two connected to one unit (from memory we have the recoup unit linked above) isn't an issue, especially given the flow rate of each shower is less than 10 L per minute.