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Crofter

Electric shower vs. instant water heater

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Planning my first fix plumbing and wiring and so it's decision time for my DHW.

As a small building used for holiday lets I will not have a boiler or the need for any bulk hot water storage. I also don't want to be paying for standing losses on a house that might go empty for weeks at a time.

 

So the options are:

a) Electric shower + two individual under-sink IHW units, one in the kitchen and one in the bathroom

b) Electric shower + multipoint IHW unit feeding both the basin and sink. I could site this near the CU which would be about mid way between the two outlets.

c) Multipoint IHW feeding the shower, basin, and sink.

 

The last option is very tempting as it would in some ways be the simplest, and possibly the cheapest. Also prevents the possibility of overloading the power supply. But you would want the IHW to produce higher temps, for dish washing, than you'd need for a shower- and I am right in thinking that it's quite inefficient to have to dilute hotter water back down again?

 

If it was my own house I would live with the shower temp water, and top it up with a kettle if I needed to soak some dishes... but it's not my house, and everything needs to be idiot proof :)

 

I know that @JSHarris has had success with his IWH, albeit in a quite different application.

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The main issue with electric IWHs is that they are of limited power, and with cold water coming in they struggle to raise the temperature enough for a decent shower, although that depends on your definition of decent.

 

We used to have a 10 kW electric shower, before we put a gas combi boiler in a few years ago.  It was sort of OK, but the flow rate used to drop to a very low rate in winter, when the incoming cold water was around 4 deg C.  In summer it was just about OK, but even then it wouldn't deliver over around 5 litres/minute, and a lot of people would consider that a bit low for a shower. 

 

When we installed the combi, the biggest difference (apart from the BIG reduction in our energy bill!) was that we could have a hot shower at around 10 litres/minute (I re-plumbed the bathroom at the same time that we changed the boiler and got rid of the electric shower).

 

So, I guess a lot depends on your target market for guests.  Personally, I quite enjoy staying in unusual, out of the way, places and will happily accept a few minor niggles, like less than perfect showers.  My other half is, unfortunately, the opposite! 

 

You're right that it's better to heat water to just the right temperature for the purpose, and only heat it when you need it, unless you can get an E7 or similar tariff, where the off-peak price is so much lower that it makes a lot of sense to have a well-insulated store of hotter water.  UVCs can be pretty low loss, and I believe there's still an option in your neck of the woods to have a very good off-peak tariff (I can't recall what it's called, but Dave mentioned it a while ago).

 

I think your best bet might be to look at the relative cost of running either a few IWIs (accepting that the flow rate will never be more than around 5 or 6 litres/minute) and that of using an off-peak tariff and a decent UVC.  The latter would have a higher capital cost, and take up space, but would definitely give a better "customer experience" and could be cheaper to run, IF you can get a decent tariff.

 

BTW, you can put an immersion on a timer, or just pop in and turn it off, when you have no guests booked in, which gets around the standing loss wastage when not in use problem.

Edited by JSHarris
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Thank for that Jeremy.

In my own house I actually lust after a basic electric shower, because the gravity fed mixer we have is utterly pathetic. I installed it in exactly the same way as the one I put in our previous house, except this one has slightly more head and a cleaner pipe run- but the unit itself is some unbranded junk, and the previous one was a Mira. Even having ripped out the NRVs built into the unit (naughty I know, but it's fed from a header tank so cannot contaminate the mains) it is awful, with a very poor flow. So I have become quite accustomed to miserly showers and forget that some people accept nothing less than a Niagara scale deluge.

 

The tarrif to which you refer is 'Total Heating, Total Control' and gives access to cheap rate electricity 24/7 for space heating and DHW- but I don't think you can get it for new builds any more. It would lend itself to IHW rather than stored systems.

 

Don't UVCs need some sort of annual check? Especially if installed in commercial premises?

 

The priority has to be getting the house up and running ASAP- if I miss this summer season, that sort of blows any other cost considerations out of the water (regardless of how it's heated...)

 

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34 minutes ago, JSHarris said:

BTW, you can put an immersion on a timer, or just pop in and turn it off, when you have no guests booked in, which gets around the standing loss wastage when not in use problem.

You can get a system now that will do it remotely via your phone.

 

Would it be possible to fit a larger IWH supplying water hot enough for a shower and with a large enough flow.  Then, using that same IWH, pipe of to the kitchen and ad another inline water heater to boost it up a bit further.

 

I would seriously consider an E7/10 (or funny Scotch tariff) for DHW.  The standing losses from an ordinary cylinder can easily be reduced by fitting secondary insulation.  Not a hard job if you do it at the time of fitting (not that hard to retro fit either).

 

Just seen your post Crofter.  I fitted a small shower pump from Screwfix (the smallest they do) and I have a good shower.  My system is a vented one and so far has not given any trouble at all.

A Header tank and 200 lt cylinder, a hot pipe off it to the bathroom, tee'd off to the basin and the bath/shower, an inline pump under the bath (does hot and cold), to the bath taps and the shower.

Easy and simple.

Edited by SteamyTea

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Our house had the electric shower because the old boiler was a system one with just a low head from the loft tank.  There was nowhere near enough hot water pressure for a shower, and all the cold feeds came from the mains, so were around 4 bar..................

 

When we first bought it it had an 8 kW electric shower, and when that packed in I had it replaced and rewired with a 10 kW, but to be frank the 10 kW one wasn't noticeably better.  We also found that even the very best electric shower would only last three years, if we were lucky, before they failed, almost certainly because of our hard water (not a problem where you are!). 

 

The key thing is really that there's a finite limit on the power you can draw for an instant water heater.  IIRC, the biggest single phase ones I found were around 12 kW.  With cold incoming mains water, 12 kW isn't going to be able to deliver much flow at shower temperature, say 40 deg C.  Our 9.6 kW IWI can raise the water temperature by about 13 deg C at 10 litres/ minute, so if you had cold water coming in at, say, 6 deg C, then the DHW would be at 19 deg C at 10 litres/minute, which is barely warm.  If the flow rate is halved, to 5 litres/minute, then the thing will increase the temperature by around 26 deg C, so gets a 6 deg C incoming cold supply up to about 32 deg C, which is a bit luke warm and not really hot enough for a shower. 

 

I did some tests with our existing thermostatic mixer shower, that's fed off the combi at mains pressure.  The lowest comfortable temperature for me was around 38 deg C, and that for my other half around 42 deg C.  The combi can chuck around 28 kW into the water, though, so is massively more powerful than an electric shower.

 

I'd be inclined to explore ST's ideas above.  Yes, a DHW UVC does need an inspection, but (and I'm sure Nick will correct me if I'm wrong) I'm pretty sure the annual check is just a visual inspection, with no testing required.  I'm not sure what you have to do with a self-catering holiday let, do the same rules apply that apply to guest houses?  If you want to get around the UVC annual test problem you could look at using a vented thermal store.  The heat losses are higher, but that may not matter too much if you can get a decent off peak tariff and manage the thing reasonably well.  The heat losses in winter aren't really losses, either, as they go towards heating the place.

Edited by JSHarris

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All food for thought...

I guess a single tank would actually be the easiest/cheapest system- so big brownie points for that.

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Out of interest why is the heat loss higher from a vented cylinder to an unvent cylinder?

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As a bit of a cheap no hassle fix for tour shower flow issue,

Have you tried an ECOflow (iirc) showerhead ,? (I think Mira make them)

They obviously can't increase the flow rate, but they sort of bleed air into the stream and make it 'feel' as if there is more water coming through as it is now in droplets rather than a steady stream.

I fitted one on my mothers (very poor flow mixer) shower and it feels so much better. 

IMO, the heads that come with almost every shower should be thrown out and a decent one fitted.

We currently have a Mira Flex electric shower and most in the house have it set at half flow rate to keep the pressure down a bit,  and never above 60% on the temp as its so hot, 

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15 minutes ago, Triassic said:

Out of interest why is the heat loss higher from a vented cylinder to an unvent cylinder?

 

I should have been a bit more specific, as I was thinking of our old vented thermal store, and that's probably not typical, as we had to use a combination one, due to a lack of room to fit a separate header tank, and for some reason I had it fixed in my mind that Crofter had limited height, too.  If not, then the difference between a UVC and a vented cylinder fed from a header would be a lot smaller, just a bit of extra pipe heat loss from the pipe from the header, which shouldn't get too hot as it goes in at the bottom.

 

I'm inclined to think that a standard gravity fed tank with a pump, as ST has suggested, has a lot going for it.  DIY install is fine, no need for special safety inspections on the plumbing, plenty of water flow and pretty easy to reduce the heat losses.  It may well even be cheaper to install that one or more decent IWHs.  Our 9.6 kW Stiebel Eltron was around £230, IIRC, and realistically that would only give a shower that's around 4  to 5 litres/minute, and anyone used to a more standard shower flow rate (the average is around 10 litres/minute, the big ones can go well over 20 litres/minute!) would probably complain a bit.  I lived with a 10 kW electric shower and it's 4 to 5 litres/minute flow rate for around 8 years, and sort of got used to it, but boy was there a difference when we put in a 10 litre/minute mixer shower! 

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My shower delivers at 11 lt/min.

If I had plumbed it in separately from the bath, and fitted a different (larger) shower head, it would probably deliver 20 lt/min, as that is what is pumped out of the bath taps.

 

I don't have a thermostatic mixer, just a cheap mixer tap with a shower fitting.  Something like this:

http://www.screwfix.com/p/swirl-deck-mounted-bath-shower-mixer-tap/41966

It does not give me any trouble at all.

 

The pump is similar to this:

http://www.screwfix.com/p/stuart-turner-showermate-eco-s-positive-head-shower-pump-1-5bar/58337

 

I fitted it over ten years ago and have not done a thing to it.  Was a easy half days work to install, including running the cold feed from the header tank and fitting the RCD and DPS outside the wet area.

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8 hours ago, JSHarris said:

Our house had the electric shower because the old boiler was a system one with just a low head from the loft tank.  There was nowhere near enough hot water pressure for a shower, and all the cold feeds came from the mains, so were around 4 bar..................

 

 

I fitted a venturi shower about five years ago because we had similar problems. It does require soft water to prevent the internals getting furred up but it has worked well up to now.

 

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13 minutes ago, SteamyTea said:

My shower delivers at 11 lt/min.

If I had plumbed it in separately from the bath, and fitted a different (larger) shower head, it would probably deliver 20 lt/min, as that is what is pumped out of the bath taps.

[...]

 

@SteamyTea, is your shower supply cold only?

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No, that is the whole point.

 

I have a very basic vented cylinder for the hot water.  From that a hot pipe goes to the bathroom for the basin and the bath.  The pump goes after the Tee to the basin and just pumps the hot water to the bath mixer tap.

A secondary cold pipe had to be fitted from the header tank to the cold side of the pump (as the cold was originally mains pressure).

 

It seemed odd at first to have a pump running when filling a bath, but I got used to that.

I could have fitted a separate shower mixer, and if it was a new installation I would have, but this was a quick and cheap solution to not having a shower.

 

In a soft water area like mine, there is no need to ever worry about plumbing.  Scaling up just does not happen and I think that is one of the biggest problems with plumbing.

Scale is akin to having a huge variable resistor fitted to your electricity supply, and ever day you turn it down a notch.

If you have hard water, you have to fit a water softener.  No argument.

Edited by SteamyTea

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1 hour ago, PeterStarck said:

 

I fitted a venturi shower about five years ago because we had similar problems. It does require soft water to prevent the internals getting furred up but it has worked well up to now.

 

 

This sounds like a great solution if you don't live in a hard water area, and where Crofter is the water is soft to the point of being below neutral pH, I think.  It's not something I'd heard of, but it makes perfect sense if you have reasonably good cold water pressure, as it saves the bother of fitting a pump, plus there aren't any moving parts.  I made a venturi aerator to draw ozone into our water supply at the feed from the borehole, and was surprised at how effective it was.  Even my home-made venturi matched the differential pressures predicted by Bernoulli around  280 odd years ago - I reckon he was a pretty clever chap for his time!

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@Crofter we run a holiday let. The only complaints we have ever had are the showers not being powerful. We have good water pressure with plenty of flow, but restrict the flow as part of the water conservation rules we followed. Took them all out in the end.

For some reason it is one thing everyone expects( especially European visitors) , so try to get it right

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I find this thread great as I also am planning my DHW strategy. I also think a hot water tank with loads of insulation and E7 is a good idea, good to hear that a vented cylinder if well planned build is not much poorer performance wise than unvented. With comments regarding incoming water temperature I wonder if with a warm roof design ( like I am having) that the cold water tank within the loft would raise the temp of the water being supplied to an electric shower enough to be acceptable, yes I know the water would rob the envelope ( within the loft) of a bit of heat but would it be detrimental?

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Fixing the current house shower is on the to do list... after the new house is done!

 

So, IHW or electric shower may lead to disappointment and bad Tripadvisor reviews, especially in colder weather. Got it.

A normal vented cylinder is doable, and obviously cheaper to buy and install (DIY?). It's a bungalow, but the highest I could put a header tank is about 4m above FFL, or a couple of metres above the shower head. Not convinced that's enough without having to put a pump in?

 

I've got my eye on a UVC on eBay that looks perfect, but of course DIY install then goes out the window- how much might I be looking at for this?

 

Edit to add- great water pressure here, I don't have a figure but it catches people unaware and visitors end up with wet trousers when they wash their hands...

Edited by Crofter

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4 minutes ago, joe90 said:

I find this thread great as I also am planning my DHW strategy. I also think a hot water tank with loads of insulation and E7 is a good idea, good to hear that a vented cylinder if well planned build is not much poorer performance wise than unvented. With comments regarding incoming water temperature I wonder if with a warm roof design ( like I am having) that the cold water tank within the loft would raise the temp of the water being supplied to an electric shower enough to be acceptable, yes I know the water would rob the envelope ( within the loft) of a bit of heat but would it be detrimental?

 

That's a really interesting idea- fit a header tank as a buffer to take the chill off the incoming water prior to electric heating. I'm also warm roof construction so it could be feasible, but I don't know if I've got enough head.

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I'm pretty sure that electric showers are really designed for mains pressure, so probably don't like anything less than about 1 bar or more.  There's no such limitation on a power-modulating IWH, like the one we have though.  The only potential problem is that all the single phase IWHs I've seen only have 15mm pipe work, so they could be a bit restrictive on a gravity system.  I think Nick would be the best to advise on this, as my instinctive feeling is that the pressure/head loss in an IWH when run from a gravity system is likely to be a bit too high.

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13 minutes ago, Crofter said:

Not convinced that's enough without having to put a pump in?

You will. and should, put a pump in for a shower.  Gravity fed showers need a lot of head, and generally have increased flow, which kills your storage capacity.

 

There is a noticeable difference between winter and summer with my shower temperature settings.  My header tank is in the loft, which gets warmer in the summer, means I turn the leaver down a bit.

I am not convinced that fitting the header tank inside the heated envelope would make a huge difference overall.

 

With a rental place you will be running the cylinder at 65°C anyway (legislation for legionella), so just work with that.  It is easy enough to work out the heat losses, and how to mitigate them (just add 200mm of celetex inside the airing cupboard, make a larger cupboard, and don't forget the base and the top).  Make sure the wiring is correct too.

 

I like to keep things simple when designing; a header tank, an vented cylinder, an immersion heater or two, simple controls (on or off) and a shower pump.  Can't get much easier than that for a DIY project.

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Is the loss with a header tank the vent pipe only?, is it possible to lengthen it ( serpentine) but still uphill all the way and insulated?

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Basically yes.  But if you insulate it well, then the losses are reduced.

They are probably small compared to the cylinder losses though.

It is all do to with surface area and temperature differences after all.

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2 minutes ago, SteamyTea said:

Basically yes.  But if you insulate it well, then the losses are reduced.

They are probably small compared to the cylinder losses though.

It is all do to with surface area and temperature differences after all.

ST, it was your tank insulation and reduction of losses compared to the  sunamp unit that convinced me to stick with a well insulated DHW tank in my build. Watch out now for threads from me about all the other nuts and bolts of my build ?

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The main problem is making sure there is enough space around the cylinder to add insulation.  But that is a problem with all insulation.  Just treat it as you would a house, insulate and make airtight.

 

The other thing I have noticed about all the cylinders in all the houses I have had, the immersion heater has always been fitted at 90° to the opening.  So basically, up against a wall.  This makes adjusting the thermostat hard, and in this house it would be impossible to remove and replace the element without removing the cylinder first.

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Any thoughts on using a venturi hot water supply booster rather than a pump, ST?  Crofter has high mains water pressure, soft water and it would seem that a venturi pressure/flow booster could well provide a pump-free solution.  I know that pumps are pretty reliable, but they do have moving parts, and the idea of boosting the gravity hot water with the spare energy in the cold water flow, with no moving parts to ever go wrong, seems attractive to me.

Edited by JSHarris

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