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Electric showers...


EverHopefull

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I am planning a TF house from kit company and have been thinking more lately about hot water demands. We will have one family bathroom with a bath and an en suite with a shower. My thought process is to have electric showers to avoid the hot water storage situation. Only heating on demand. I would be having a hot water store that would be fed by ASHP and a Pv diverter but feel that the demand situation will be more economical not running daily showers from the hot store. All appliances are now cold fill so the bath is going to be the only thing stored water will be required for. Hot taps in kitchen and bathroom i also wonder if instant under sink units will be more suited as only very small demands. Any thoughts? 

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The running cost will be a lot higher with instant water heaters and electric showers.  Worst case from an ASHP in wet and cold weather might be a COP of about 2.5 heating water, probably an average of around 3, so using stored heat from the ASHP would be roughly 1/3rd the running cost. 

 

Showers are our biggest hot water usage by far, around 6 kWh to 7 kWh per day, for the two of us.  With ASHP heating the water at E7 rates that would cost about 16p to 19p/day.  With non-E7 run electric showers the cost would be around 92p to £1.07/day, so a lot more expensive.

 

We heat our hot water using mainly diverted PV, boosted during winter with an overnight E7 boost.  Since January this year we've used about 610 kWh of off-peak electricity to provide hot water, so at a guess I'd say that we're unlikely to use more than about 900 kWh of paid for electricity to heat water for the whole year.  That's at 8.148p/kWh, so the total cost for hot water for the year would be around £73.33.  If we used electric showers, then the total cost for the year would be well over £300.

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If just comparing stored DHW against an instantaneous system, then it really just comes down to standing thermal losses and installation costs.

I personally like DHW to be stored for when I want it, but if I could run a 20 kW instantaneous boiler, I suspect I would get the same shower experience.

 

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Even a 10KW electric shower, we find the flow rate disappointing.  If you appreciate a good shower it really needs to be a thermostatic mixer from a pressurised hot water system.

 

Heating stored hot water with an ASHP you need to be careful of your water temperature.  We heat ours only to 48 degrees C.  You don't want to go a lot hotter than that with an ASHP.  And because you are storing the hot water at a lower temperature than you would with a boiler, you will use more "hot" with less cold added. So we chose a 300 litre cylinder.

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On 05/10/2019 at 15:59, Adrian Walker said:

[...]

instant water heaters for basins, [...] are the most efficient way in terms of energy, water and time. 

[...]

 

The wealth of advertising claims  and shared ignorance on Tinternet on this issue is starting to annoy me. Everyone seems to be pushing one agenda or another - and precious few (none, in fact that I have found) look at the range of contexts in which   instant water heaters make provable financial and (to a lesser extent) ecological sense.

 

Would you be kind enough to point me to anything - reliably evidenced - on the Internet which supports your assertion, please?

 

I ask because I started out on our build with a  determination to do just as you suggest. And every time I discuss it with mates or plumbers, they all pour metaphorical cold water on the idea.

And when I push them, none has any experience  of fitting instant water heaters. None.

And if I have learned one thing on this build - and learned it the very hard way -  plumbers and many other trades folk don't like  doing things just a little bit differently.

Anything which involves just a bit of thought or seems outside the normal run of things seems to cause a switch to flip from On to Off

(With sincerely insincere apologies to @Onoff ?)

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1 hour ago, AnonymousBosch said:

 

The wealth of advertising claims  and shared ignorance on Tinternet on this issue is starting to annoy me. Everyone seems to be pushing one agenda or another - and precious few (none, in fact that I have found) look at the range of contexts in which   instant water heaters make provable financial and (to a lesser extent) ecological sense.

 

Would you be kind enough to point me to anything - reliably evidenced - on the Internet which supports your assertion, please?

 

I ask because I started out on our build with a  determination to do just as you suggest. And every time I discuss it with mates or plumbers, they all pour metaphorical cold water on the idea.

And when I push them, none has any experience  of fitting instant water heaters. None.

And if I have learned one thing on this build - and learned it the very hard way -  plumbers and many other trades folk don't like  doing things just a little bit differently.

Anything which involves just a bit of thought or seems outside the normal run of things seems to cause a switch to flip from On to Off

(With sincerely insincere apologies to @Onoff ?)

 

Leave it with me and I will do some experiments and will provide that data for instant hot water heaters.

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The only instant hot (warm!) water heater I have is in the detached workshop . I ran hot water pipe in 10mm plastic to avoid wating fir hot water, designed the house around central DHW tank (short runs). DHW tank is well lagged and temp like @ProDave is 48’. All running on an ASHP (4KW) ?

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1 minute ago, Adrian Walker said:

 

Leave it with me and I will do some experiments and will provide that data for instant hot water heaters.

 

Thanks, thats a generous offer.

Before you spend too much time on that task, I wonder whether you could tell me about your involvement with the instant water heater sector please? 

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The experiment needs to include low heat loss thermal storage, as that very definitely swings the balance strongly in favour of not having instant water heaters at the point of use. 

 

We heat our hot water for an approximate annual paid for electricity input (our house is all-electric) of about  900 kWh (it's probably a bit less than this, I'm extrapolating from ~9 months usage to date being 610 kWh).  All of that paid for electricity is at the E7 off-peak rate, currently 8.148p/kWh.  I have meters on both the diverted PV used to heat hot water and the off-peak boost supplies.

 

We use roughly 2100 kWh/year for heating hot water, so if we used instant water heating for all of it then all would be at the peak rate (we don't shower or run the hot taps in the middle of the night) so the cost would be 2100 kWh * 15.729p/kWh = £330.31

 

Because we don't use any instant water heating, but store heat in a ~9 kWh Sunamp heat battery, charged by either excess PV generation or an off-peak overnight boost, our hot water cost is roughly 900 kWh * 8.148p/kWh = £73.33

 

If we didn't have any PV in the roof, and relied on E7 off peak electricity to charge the Sunamp, then our hot water cost would be roughly 2100 kWh * 8.148p/kWh = £171.11

 

There's no way that I could make the sums stack up to show that instant water heating makes any sense at all.  The heat losses from the Sunamp are included in the figures above, and are around 800 to 900 Wh/day, so about 310 kWh.  Not enough to be worth bothering with, given that instant water heating costs more than double the cost of just using E7 off-peak electricity, and more than four times the cost of using a mix of excess PV generation and off-peak E7 boost. 

 

It's a bit of a no-brainer of a decision as to which to go for, IMHO.

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6 minutes ago, JSHarris said:

[...]

We use roughly 2100 kWh/year for heating hot water, so if we used instant water heating for all of it then all would be at the peak rate (we don't shower or run the hot taps in the middle of the night) so the cost would be 2100 kWh * 15.729p/kWh = £330.31

[...]

It's a bit of a no-brainer of a decision as to which to go for, IMHO.

 

I wish it were - for me.

Context is important, and because I dont want to hijack this thread , I'll start another (that references this thread)

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I don’t have a sunamp but a very well insulated DHW tank (like @ProDave) and my other half has commented that the airing cupboard is not very warm! I don’t have any measuring facilities or data!. Also part of my theory is that any heat lost from the DHW tank helps heat the house during the heating season (agreed its a loss during the summer, but a small one).

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27 minutes ago, JSHarris said:

8.148p/kWh

Plus 7/24th of the meter rental and 5% VAT.

 

I think electric showers give a saving as they beat less water. Why they are horrible.

I probably have the lowest capital cost system there is, just a vented 200lt E7 cylinder, F&E tank, pump and a combined mixer tap.

Probably less than £500.

6 minutes ago, joe90 said:

I don’t have any measuring facilities

Yes you do.

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4 minutes ago, joe90 said:

I don’t have a sunamp but a very well insulated DHW tank (like @ProDave) and my other half has commented that the airing cupboard is not very warm! I don’t have any measuring facilities or data!. Also part of my theory is that any heat lost from the DHW tank helps heat the house during the heating season (agreed its a loss during the summer, but a small one).

 

TBH, even the higher losses from a UVC aren't going to change the overall balance.  A UVC might lose around twice as much heat as a Sunamp, but that's still nowhere near enough to swing the balance in favour of using instant water heaters.  The only reason for considering instant water heaters is really convenience, in that they may provide hot water at taps a little bit quicker.  In practice I doubt this is that noticeable, as if small bore (10mm or so) pipes are used to feed basin taps, with a radial plumbing configuration from a distribution manifold, then the few seconds delay before hot water flows is probably much the same for either instant heating or tank-fed hot water.

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1 minute ago, SteamyTea said:

Plus 7/24th of the meter rental and 5% VAT.

 

I think electric showers give a saving as they beat less water. Why they are horrible.

I probably have the lowest capital cost system there is, just a vented 200lt E7 cylinder, F&E tank, pump and a combined mixer tap.

Probably less than £500.

 

 

True, but also any fair comparison has to be on a like-for-like basis.  The fact that an electric shower will struggle to deliver more than about 4 or 5 litres/minute is just another reason not to have one unless you really have to, IMHO.  I have our shower throttled down to 9 litres/minute, with a flow restrictor, and that's perfectly OK, but not luxurious.  I'd not really want to have a shower that's much less than this, TBH, as I can still remember what our old electric shower was like from a few years ago.

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My last experience of a "limited" shower was when in the static caravan. That was heated with a 10KW instant propane heater and produced a poor shower flow.  You had to compromise and have it a bit too cold just to get barely enough water.

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