Instantaneous Water Heater

[Triassic]

I've been looking at using an instantaneous water heater to supply hot water to a kitchen and a bathroom sink. Having read a number of posts and asked various questions, the Seibel  Eltron has been mentioned a number of times, however when I search to buy one I'm presented with a range of others makes, some of which a considerably cheaper that the German model.

 

So my question is, are all instantaneous water heaters the equal?

[joe90]

I plan on using the Eltron as above mainly because it's modulating and been recommended by better minds than my own. I.e. It will only produce heat if the water flowing through it requires heat input, i.e. The hot water tank has run out of hot water.

[Jeremy Harris]

As above, but there are broadly three groups of these things. 

 

Some have a small reservoir of hot water, in an insulated tank, and are intended for use with wash basins, for hand washing.  They aren't true "instantaneous" water heaters, they are really just a miniature UVC with an electric heating element and a pressure release valve to deal with expansion.

 

The second type are the ones like an electric shower, with a fixed, or hand switchable, heating element, that delivers a fixed amount of power to heat the water.  Vary the flow and the temperature varies, so the temperature control on these is by varying the flow - if you want hotter water, cut the flow rate, if you want cooler water, increase the flow rate.

 

The third type is like the Stiebel Eltron DHC-E that we have.  It measures the flow temperature and adjusts the power to the element to maintain the set temperature, if it can.  This means that it will, generally, have two advantages.  The first is that, as long as the flow rate doesn't exceed the maximum, it will deliver water at a constant temperature, as set on the front panel, irrespective of water flow rate.  If the flow rate drops, the heater drops the power to the heating element, electronically, to keep the temperature the same.  The second advantage is that it will only use as much electricity as needed, so can be a fair bit more economical. 

 

In our case, it's a backup in case our thermal batteries in the Sunamp PV don't get enough charge.  The water will still be pre-heated to around 30 to 35 deg C by the ASHP and plate heat exchanger, and all the Stiebel Eltron has to do is lift that up to 42 deg C, it's set temperature, that is the bare minimum we think is acceptable for hot water (it will just run a comfortable shower OK). At 35 deg C input temperature, the Stiebel Eltron only needs just over half the available heating power, at 10 litres/minute flow rate.  If the preheat temperature drops to 29 deg C, then the Stiebel Eltron would be running at full power to maintain 42 deg C at 10 litres per minute.

 

Hope this helps explain the differences.

Edited January 26, 2017 by JSHarris
typos

[Triassic]

How do they cope using incoming cold mains water, i.e. No pre-heat?

[Jeremy Harris]

43 minutes ago, Triassic said:

How do they cope using incoming cold mains water, i.e. No pre-heat?

 

The electronic heat control models,like the Stiebel Eltron DHC-E, will only use as much electricity as needed to deliver the set temperature, so if you turn the flow down on, say, a washbasin tap, then it will turn the power down in order to stop the water getting too hot.

 

The fixed power models will deliver a water temperature that depends on flow rate, so close the tap a bit and the water gets hotter, open the tap and it gets cooler.

 

Both will be similar when first turned on, in terms of heat-up performance.  The difference is really like that between a thermostatic mixer shower, where the mixer tries to maintain a fixed shower temperature, no matter what happens to the supply water pressure or temperature, and an electric shower, where you have to vary the water flow rate in order to get the temperature you want, and may need to do that whenever anything affects the flow rate, like another tap being turned on, dropping the local pipe pressure slightly and reducing the flow at the electric shower, so increasing it's temperature, or the opposite when the pressure suddenly increases and the flow rate increases, making the electric shower run cooler.

 

edited to add:

 

I've just read that back and it doesn't seem that clear!  If you think of a standard instant water heater as being like an ordinary electric shower (they are near-identical internally) and the electronic control instant water heater as being like a thermostatic mixer shower, that's what I was trying to compare above.

 

Also, if you have vulnerable people around then the electronic ones can be a help, as they can be set to never exceed a maximum temperature, to avoid scalding.

Edited January 26, 2017 by JSHarris

[ToughButterCup]

@Triassic, they take cold water only, or preheated water.

[Steptoe]

Most of them nowadays are thermostatic controlled, right down to the piddly little 2 or 3kw over basin ones you get in workshops,

You could get a cheapish (but still perfectly OK) 9.5kw dual outlet for under £200

 

[Nickfromwales]

I think there is a big jump between thermostatic and fully modulating. The former still is quite crude, with the latter near flawless in its delivery characteristics. 

[Steptoe]

20 minutes ago, Nickfromwales said:

I think there is a big jump between thermostatic and fully modulating. The former still is quite crude, with the latter near flawless in its delivery characteristics. 

Yep, there is, but depending on the purpose of it would it warrant the massive price difference for a wash basin,? 

 

EDIT : Most showers are still merely thermostatic, and for myself at least, there is absolutely no discernible pressure change or temp difference in my shower, and its a fairly low end 9.5kw.

 

Edited January 26, 2017 by Steptoe

[Nickfromwales]

Probably best to first define which heater is best suited to what application / why etc.  

Some of the smaller instants are 'over sink' which means they cannot be connected to a tap or other outlet, then there are under sink instants, and then both variations with varied volumes of stored water, commonly referred to as water heaters ( not instants ). 

18 minutes ago, Steptoe said:

Yep, there is, but depending on the purpose of it would it warrant the massive price difference for a wash basin,? 

That would all depend on whether or not you wanted to see it or not I suppose. And, more importantly, if it needed to be connected to a tap or if you'd be happy with the rather crappy swivel spout / arm outlet. 

For basins in particular, anti-scald measures are more of a concern than flow rates imo

[Triassic]

I've built a small one bedroom cabin at the bottom of the garden for us to live in during the main build. As it's small I'm looking at instantaneous water heaters, i.e. Without water storage. I want to provide hot water to a kitchen sink and to a bathroom sink. I'm planning to put the unit in the cupboard under the sink. My thinking was that the sink will be used more than the bathroom sink, so putting it in the cupboard keeps the pipe run short, the bathroom is two metres away. It will be fed directly off the cold mains feed.

Edited January 26, 2017 by Triassic

[joe90]

In my case I have a cheap over sink unit in my workshop to wash my hands ( no hot water in my workshop) but I will be having a fully modulating unit plumbed into the hot feed leaving the DHW cylinder so if we have guests for example and run out of hot water the unit will deliver hot water without having to wait for the cylinder to reheat. Also because it is fully modulating, if the cylinder has not quite got back up to temperature then it's ability to make up the difference means seamless hot water.

 

The  other option I then have is if the ASHP cannot deliver the DHW temperature I want without going into defrost mode which ruins the COP the difference can be made up automatically.

[Steptoe]

37 minutes ago, Nickfromwales said:

Probably best to first define which heater is best suited to what application / why etc.  

Some of the smaller instants are 'over sink' which means they cannot be connected to a tap or other outlet, then there are under sink instants, and then both variations with varied volumes of stored water, commonly referred to as water heaters ( not instants ). 

That would all depend on whether or not you wanted to see it or not I suppose. And, more importantly, if it needed to be connected to a tap or if you'd be happy with the rather crappy swivel spout / arm outlet. 

For basins in particular, anti-scald measures are more of a concern than flow rates imo

 

youre not going to be putting one of those in your house , well, I'd hope not anyway,

no, I was just referring to the instantaneous hidden dual outlet types,

as I said, it really depends on how much you want to use it, and just how high spec you wanted,

I was only pointing out that self regulating temp and flow controlled units can be had fairly cheaply, not as all singing or precise as a modulating unit, but quite effective for a lot of people,

I suppose its like a Quooker, some people see them as overkill, others wouldnt be without one, personally, I love them, but only at the right price, and as my BiL sells high end kitchens to trade,,,,,,,  

 

[Nickfromwales]

Agreed. 

Anorher way to cheat around a cheaper unit is to fit an aftermarket TMV on the outlet to cap the max temp . 

[Nickfromwales]

9 hours ago, joe90 said:

The  other option I then have is if the ASHP cannot deliver the DHW temperature I want without going into defrost mode which ruins the COP the difference can be made up automatically.

I think you mean that you will set the ashp to NOT go into DHW mode purposely? The cylinder will reheat via the cyl stat, regardless of what the instant is / isn't doing. 

I think that logic needs a revisit unless I've mis-understood you?

[Jeremy Harris]

If I've understood Joe right, I think he's looking at something like a hot water tank that's mainly kept at an efficient ASHP flow temp (so typically no more that 40 deg C to prevent defrosting lowering the COP a lot) and then using a modulating instant water heater to provide the boost up to a good hot water temperature at the taps.  Such a tank would need an anti-legionella boost every couple of weeks, to around 60 deg C or so.

 

I can vouch for this working well, as it's our standby DHW system in the event the Sunamp PV doesn't get enough charge, or runs out of charge due to heavy hot water demand.  The ASHP heats our buffer to around 35 to 40 deg C, and this then preheats the incoming cold main to 30 to 35 deg C.  The modulating instant water heater then boosts this up to the 42 deg minimum acceptable temperature for us (I tested this, anything under 42 deg C and the shower's too cool, anything over 42 deg C just wastes energy by heating the water to a higher than needed temperature).

 

One thing to watch is that all these modulating instant water heaters work in the same way; they control the power to the element by pulsing the heating element on and off, with a variable duty cycle at the mains frequency zero crossing point so they don't cause a lot of EMI.  They do this to avoid the electrical noise that would be caused by phase control, and they use a low frequency to get a wide range of control and stay within the regs on flicker frequency and amplitude.  However, if you have dimmable LED lighting, then you almost certainly will see some flicker when the heater is modulating.  We only had one dimmable LED power supply in our house, in the WC, and it's how I know that the heater makes the LEDs flicker! I replaced the LED power supply with a wide voltage range, non-dimmable one and the problem went away.

 

It's hardly surprising that there are small voltage fluctuations with these modulating heaters, ours is switching around 10 kW at a rate of a few Hz, and that's bound to cause a small pulsed voltage drop on the whole installation.  The drop is within the limits allowed, but enough that something as sensitive as an LED, with its very fast response time, to be affected by, if it's not eliminated by the power supply.

Edited January 27, 2017 by JSHarris

[Nickfromwales]

 

Just the wording "if the ashp cannot x y z"  that was a bit misleading   

We'll get there I'm sure

 

[Ferdinand]

Yanking this thread back from the Elysian Fields...

On 26/01/2017 at 15:30, JSHarris said:

Stiebel Eltron DHC-E

 

Is this range still a good option to heat a tap on a washbowl in a cloakroom?

 

The application is for a gym changing room where we currently have hot showers but no hot water tap for hand washing. I have a whisper in my head that it may be a regulatory requirement somewhere.

 

Our supply to this is not preheated, and the most need I can see for expansion would be one further bowl ie 2 taps.

 

If I could use something more like 3kW than the 10kW discussed here to avoid any need for running new electrics it would be an advantage, I think. If I can save £100 or £300 that would be another extra little improvement project I can do more quickly. And I have a list of dozens of small improvement projects I would like to carry out which are gradually being done one by one.

 

Having run a few numbers I think that I probably want 2-3kW per sink, and that I cannot rely on averaging to reduce my demand since they may all be in use at once. So it may be do something inexpensive now and accept that new wiring might be needed should we add another sink. I'll first have a detailed look at the electrics just to check that there is not a heavier supply close by.

 

Ferdinand

 

Edited January 8, 2019 by Ferdinand

[ProDave]

For just a hand wash basin, how about a low volume storage water heater, typically anything from 10L to 20L and with a 3KW element or similar size.

 

This sort of thing just as an example not a recommendation to buy. https://www.ebay.co.uk/itm/10L-2kW-Under-sink-Water-Heater-by-ATC-3-sinks/142934692689?epid=2254439348&hash=item2147925351:g:OxYAAOSwnXpbl8P4

[Ferdinand]

9 hours ago, ProDave said:

For just a hand wash basin, how about a low volume storage water heater, typically anything from 10L to 20L and with a 3KW element or similar size.

 

This sort of thing just as an example not a recommendation to buy. https://www.ebay.co.uk/itm/10L-2kW-Under-sink-Water-Heater-by-ATC-3-sinks/142934692689?epid=2254439348&hash=item2147925351:g:OxYAAOSwnXpbl8P4

 

Thanks for the suggestion for a 3kW option. £150 is also a price point for the non-elecronic version of the Siebel, and smaller versions of the Electronic will be in-between.

 

It's a tricky call. 

 

Current use of the facility is relatively limited to peak periods (8-11, 4-9, occasional at w/e) with a some 6-8 early morning, but our membership could easily double this year - so more potential demand. Currently there are quiet periods, but we are working to increase usage, and filling in the gaps etc.

I need to look at our constraints, I think, and do a short project-memo to lay out the options.

 

Ferdinand

 

[Nickfromwales]

5 hours ago, Ferdinand said:

It's a tricky call

Don’t fit an instant, such as a Steibel as you’ll regret it. 

Incrementally something like this is far more bang for the buck. 

If you see a higher demand in the not too distant future then for a handful of tenners more you can go to his bigger brother. 

If you do a bit of storage you can heat on economy perhaps, rather than peak on demand consumption. 

Case dependant of course.  

[tanneja]

This is an interesting post. We have a Worcester Bosch combi boiler, and rue the 30-60secs it can take for a basin to provide hot water.  We would have 5 basins, all relatively local to each other (but a bit removed from the boiler (part of the issue probably)), where my partner might think I was the best thing since sliced bread if I managed to deliver immediate hot water from any one of them on demand, as hand washing with cold water really grinds their gears.

 

After looking at the options, I can't see which is the most elegant solution.  I am space constrained, part of the reason for having a combi boiler in the first place.  Some time has passed since the last post in this subject, with the devices available today, is there a clear winning tech for my issue?

[Triassic]

What about an instantaneous boiling water tap, the cheapest appear to be around £250 - £350 ?

[tanneja]

On 09/01/2021 at 20:04, Triassic said:

What about an instantaneous boiling water tap, the cheapest appear to be around £250 - £350 ?

Thanks @Triassic, this wouldn't only be for boiling water, but warm water for bathrooms.  Would you think a boiling tap appropriate?  Are you perhaps thinking it being used in a mixer tap format.