Stiebel Eltron DHC-E 8/10 instant water heater

[ProDave]

I have just installed our  Stiebel Eltron DHC-E 8/10 instant water heater.  I have had it over a month now, but this was prompted by another "run out of hot water" incident this morning *

 

This is an electronically controller instant water heater that modulates it's power to maintain a fixed water output temperature set on it's dial and can heat with a power up to about 10Kw

 

It is connected in line with the output of the unvented hot water cylinder.  The idea being most of the time it will do very little, but in the event of the hot water running out, it will take over and at least produce some hot water so ones shower can continue (although you will probably have to turn down the flow rate to maintain a sensible temperature)

 

Just a few observations.  This has gone to the top of my list of most awkward appliances to make the electrical connection to. Whoever designed the cable entry arrangement needs to be condemned to a lifetime of fitting cables into them.  I was also confused by two L terminals.  The manual says L L (L N) so my best interpretation of that is the left hand L terminal is L and right hand L terminal is in fact N

 

I had deliberately left the UVC cold while I was installing this heater. The tank temperature was probably in the region of 30 degrees and I was able to get a hot enough shower temperature with a decent flow rate, though the heater was flashing it's light to indicate it was running at full power so not achieving it's 50 degree setpoint.

 

In normal use I have the tank set to 48 and the Stiebel Eltron set to 50 so it will do a little bit of uplift.

 

* This is the third (or is it 4th) "ran out of hot water" incident.  We have a 300L UVC.  This morning SWMBO had a "hair wash" shower and I timed her at over 15 minutes in the shower.  Immediately afterwards, daughter had a shower and about 5 minutes in, complained it was getting cold.  SWMBO tells me this is unacceptable.  As part of my "research" I did the bucket and timer test and found that the rainfall shower head is delivering 17 litres per minute. That will empty a 300L tank in just under 18 minutes.

 

I am trying to educate the ladies that a shower tap is not a binary device, it can be anywhere between fully off and fully on and it does not need to be fully on.

 

In the mean time I might look at some form of flow restrictor, but I don't want anything that makes the water flow noisy.

[newhome]

7 minutes ago, ProDave said:

I am trying to educate the ladies that a shower tap is not a binary device, it can be anywhere between fully off and fully on and it does not need to be fully on.

 

 

No chance of that believe me. When fully on feels so much better than half on there is no chance of the flow being turned down . 

 

[ProDave]

8 minutes ago, newhome said:

 

No chance of that believe me. When fully on feels so much better than half on there is no chance of the flow being turned down . 

 

Hence the flow restrictor idea.

[newhome]

Just now, ProDave said:

Hence the flow restrictor idea.

 

You'll be rumbled! 

[Jeremy Harris]

Sorry, @ProDave, I should have warned you about the lack of space to fit a 10mm² cable at the bottom of the unit.  Another 10mm or so would make all the difference.

 

I've not heard any noise at all from the flow restrictors I fitted, including the one that was in the shower pipe for a while.  Our shower runs at around 10 litres/minute and seems fine.

[ProDave]

Just now, JSHarris said:

Sorry, @ProDave, I should have warned you about the lack of space to fit a 10mm² cable at the bottom of the unit.  Another 10mm or so would make all the difference.

I will admit to pulling out the cable clamp and not using it. And that silly grommet thing now has a cut in the back so I could feed the cable in and then fit the grommet back afterwards.

 

I never have that trouble with most electric showers, it just needs a bit more room.

[joe90]

@ProDave very interesting thread as I too have a 300litre tank and ASHP like you, I bought an Eltron like yours and is my plan “b” (cable pulled In ready). I am lucky that er indoors has a short shower as do I (but at other ends of the day usually) and my daughter does not live with us?. However, Christmas is coming and we will have two and a half guests (granddaughter) so I will monitor the situation. My DHW tank has two tank stats, one higher than the other, I have wired both of them with a change over switch so I can either have half a tank of hot water or 3/4 tank of hot water (guests or no guests. ) @Nickfromwales Was worried that as the Eltron is a 15mm fitting it might cause a restriction on water flow!

[Nickfromwales]

30 minutes ago, joe90 said:

@ProDave very interesting thread as I too have a 300litre tank and ASHP like you, I bought an Eltron like yours and is my plan “b” (cable pulled In ready). I am lucky that er indoors has a short shower as do I (but at other ends of the day usually) and my daughter does not live with us?. However, Christmas is coming and we will have two and a half guests (granddaughter) so I will monitor the situation. My DHW tank has two tank stats, one higher than the other, I have wired both of them with a change over switch so I can either have half a tank of hot water or 3/4 tank of hot water (guests or no guests. ) @Nickfromwales Was worried that as the Eltron is a 15mm fitting it might cause a restriction on water flow!

If you look at @ProDave's specific instance its not quite 'normal' as in he has turned his cylinder stat down to the absolute lowest temp he can get away with, which then massively reduces the quantity of DHW you can yield from a 300L tank. 300L is a bloody decent sized tank, even from an ASHP, so running out of water is ( IMO ) self inflicted ( sorry Dave ).

Is the Steibel on a cyl stat so it can only be energised if the cylinder falls below a certain temp? If not, thats going to routinely use grid electricity to fortify DHW production? How do you have it set up?

 

Joe, in times of known high DHW consumption you simply hit a 30 min or 1 hour boost button and pop the immersion on to fortify the DHW production  The immersion will take the tank from the ASHP set point up to way over 70oC and give you oodles of DHW at full flow rate. I genuinely don't think running huge cables to massively powered but still very slow flow rate instant heaters, and having to buy said device, is a good idea. The cylinders already have an immersion fitted, so why not just use that? Deliciously simple AFAIC.  

[Jeremy Harris]

The Stiebel Eltron is clever enough to not turn on (or at least not turn the heating element on) when the incoming water supply is above the set point.  It then modulates from zero power up to it's maximum rated power depending on the temperature differential between the inlet and outlet, trying to raise the outlet temperature to the set point as the inlet temperature reduces below the set point.  This is heavily flow rate dependent, so the maximum temperature uplift the unit will give is dependent on flow rate.

 

Here is the graph I drew up from when I was planning on using the unit to boost preheated water up to about 45 deg (the dotted red line):

 

[PeterW]

I’m with @Nickfromwales on this one. I’ve set one of these up to go to 52c on the ASHP but boost to 65c overnight on E7 using the immersion. That will take about  85 mins with 3kw immersion and cost around 35p at low rate. 

 

Using the 8.8kw Stiebel in-line during peak time would allow me to only use it for 10 minutes for the same cost so that’s a very short shower. I would also have a cold tank and still need to boost every time for hand washing etc. so it will cost more over the day too. 

[ToughButterCup]

11 hours ago, ProDave said:

[...]

I am trying to educate the ladies that

[...]

 

Bet that went well...

[Nickfromwales]

40 minutes ago, JSHarris said:

This is heavily flow rate dependent

One of the reasons I wouldn't fit one. This is a case of prevention is better than cure IMO. Just hit the immersion and job done, thats if you actually need it thats is. A 300L UVC set up correctly should give bags of DHW. 

Lets say Joe Blogs asks for one to be fitted. All up I bet you'd be in for a minimum of £500.00  

£500 worth of electricity would get you around 3,500 kWh of juice.

Put that through an ASHP and use a multiplier of 2.5 ( SCoP ) and you have ~8,700 kWh to spend on "boost". You then dont have to worry about running out of water and telling the family to slow the flow down when you run out of hot water  Turn up the stat Dave ! 

Case dismissed.

 

 

As a note, these instant heaters are routinely fitted ( multiples of ) in places like McDs and KFCs as doomsday devices, so if the UVCs go off for any reason the restaurants ( lol ) can simply switch manual lever valves and divert to the instants in order to tick the boxes required to stay open.

[Jeremy Harris]

In my case the idea was to just be able to boost when needed for the rare occasions when the Sunamp PV ran out of charge and we only had luke warm hot water.  In practice this never happened, so the Stiebel Eltron sat there doing nothing at all, which is why I took it out when I fitted the Sunamp UniQ.

 

As luck would have it, the only time we've run out of water for showers was after I'd fitted the much bigger Sunamp UniQ, for reasons associated with the poorly managed (IMHO) way in which it enables charging (or doesn't, as we found out).

 

 

 

[joe90]

I find it very interesting to put actual £ figures to the various options, my reasoning behind the Eltron was to avoid having cold water if the consumption was unusually high, using the immersion takes time once you have actually run out! but timing it sounds better and there have been various threads on hot tank losses when the temp goes up.(so er indoors might get her warm airing cupboard ?)  It was easy to bang a cable in when the wiring was being done (before heating options settled on) and I found the Eltron cheap on Ebay. I still am considering the option of E7 or E10 when I know more about my consumption.

[ProDave]

The whole point of this particular heater is it only operates when needed. If the input temperature already meets it's set point, it will not heat it. So in the Mac D case they could just do as I have done and have it in the output of the UVC with no need for changeover valves.

 

As to flow rate, the fact the shower, all plumbed in 15mm and now passing through the "restrictions" of the heater, can still deliver 17L per minute says flow rate is not an issue.  If I were plumbing this again, I would have used standard ballofix valves for the showers, not the full bore ones, that would have reduced the shower flow a bit.

 

I don't want to heat the tank water any hotter from the ASHP I am trying to get the best COP and least defrosting possible from that.  Shortly I will be installing solar PV. For much of the year excess from that will keep the tank hot so I would then expect this run out of hot water to only be an occasional winter problem, which the Stiebel Eltron will deal with.  A case of making it a robust system that will cover all eventualities.

 

Like @joe90 I anticipated I might need this (thanks to @JSHarris for originally finding this particular heater and suggesting the idea) and had already installed a 10mm cable and spare rcbo in the consumer unit.

[Nickfromwales]

These were never plumbed in series, always in parallel as an ‘alternative’ source of DHW due to flow restriction. 

17ltr/p/m is ok but for two showers at the same time you’ll be pushing it. 

Plus, at anywhere near 17lpm the Steibel would be giving you huge quantities of cold water.  

@Stones, how many times have you run out ? ?

[joe90]

Thinking about it my ASHP is giving me lots of hot water, no idea if it’s defrosting so don’t know it’s running cost yet. After the turmoil of getting my ASHP to run I have not thought about it much lately, if I remember correctly the command unit can determine the temp of heating water but not DHW temp??? I must get back to trimming the whole heating / DHW debarkle.

[ProDave]

55 minutes ago, Nickfromwales said:

These were never plumbed in series, always in parallel as an ‘alternative’ source of DHW due to flow restriction. 

17ltr/p/m is ok but for two showers at the same time you’ll be pushing it. 

Plus, at anywhere near 17lpm the Steibel would be giving you huge quantities of cold water.  

@Stones, how many times have you run out ? ?

Yes but the Steibel is the "safety net"  If you are the unlucky one that has the last shower and it runs cold, then the Steibel will let you finish your shower and get the shampoo out of your hair, but you might have to turn the flow down a bit to maintain an acceptable temperature.  That is it's purpose, and sounds infinitely better than nothing but cold water to rinse your hair.

 

That is all it is for, nothing more nothing less.

[Jeremy Harris]

Also, the hot water doesn't usually suddenly run out and turn cold as the tank runs out of capacity, it just gets too cool to be usable.  Boosting it when it's at this "too cool" stage still allows it to be used for a short time, depending on the flow rate.  For example, it would allow you to get a comfortable shower of over 40 deg C with the water from the tank only being at 30 deg C, even at 10 litres/minute.

 

Interesting to look at the performance of electric showers, in terms of flow rate.  For mains water coming in at 8 deg C (not untypical, that's about the temperature we used to get from the mains at our old house) then a 10 kW electric shower can only provide a shower at 38 deg C (about the temperature we seem to prefer) at a flow rate of about 4.75 litres/minute.  Lots of people see to manage OK with showers that work at this low flow rate, we did for around 5 or 6 years, until we had the combi boiler installed at the old house.

 

 

[Stones]

2 hours ago, Nickfromwales said:

These were never plumbed in series, always in parallel as an ‘alternative’ source of DHW due to flow restriction. 

17ltr/p/m is ok but for two showers at the same time you’ll be pushing it. 

Plus, at anywhere near 17lpm the Steibel would be giving you huge quantities of cold water.  

@Stones, how many times have you run out ? ?

 

We haven't, ever, in the near two years we have been in, and that's with multiple showers or combinations of bath being drawn then a shower immediately (or concurrently) after.  Our showers run at a flow rate of 17l/min, and nobody takes a quick shower in this house.  Our DHW is heated to 49C / 50C, 300l cylinder, recharge kicking in when the cylinder temperature drops to 40C.  Our ASHP is an 8.5 kW so it may be as simple as we have a bit more recharge capacity that stops us running out.

[Stones]

@ProDave what water temperature do you have set for the ASHP to kick in to recharge the cylinder?  

[ProDave]

16 minutes ago, Stones said:

@ProDave what water temperature do you have set for the ASHP to kick in to recharge the cylinder?  

I have set the hysteresis to 3 degrees, so with the tank heated to 48 it will start re heat at 45.

 

I think another issue may be thermostat pocket location.  Our tank came with 2 thermostat pockets.  The highest one (about half way up the tank) was hopeless, you only got half a tank of hot water.  It is now in the bottom pocket but even that is nearly 1/3 of the way up the tank, but there is no other option.  So I suspect we might not really be getting 300L of hot water in the tank.  I guess a test might be to temporarily borrow the immersion heaters thermostat pocket and try the heat pump thermostat in  that.

 

A lesson here when buying the tank would have been to specify an extra thermostat pocket really low down perhaps.

[joe90]

@ProDave I did not think of this also, our lowest stat pocket is a third of the way up the tank so my 300litre tank will only ever heat about 200litres!!, the two immersions are half way up the tank and slightly above that, so immersions will only heat at most half a tank. Unless the tank does  not stratify well??? Is this correct or not?

Edited December 16, 2018 by joe90

[Jeremy Harris]

I made the mistake of forgetting to specify a thermostat pocket on our buffer tank, but managed to make a small hole in the insulation, making sure it went at a tangent to the side of the tank and exposed a good length of the side of the copper.  I then coated the end of the capillary probe from a simple mechanical thermostat with some thermal compound, along with a DS18B20 sensor, and pushed them both deep into the hole, sealing it up with low expansion foam.  This seems to work fine, and although it may not sense the exact water temperature in the tank, it's probably within a degree or two, and the offset is pretty consistent, so the small temperature error doesn't really make any difference in terms of functionality.

[PeterW]

You’ll find that you’re not getting full temperature as the way the coils work - and the sheer size of the coil - then it needs a slightly higher temperature to get the cutoff.