Hot water system for the garden room

[MikeSharp01]

Just working out the plumbing for the 30 m² garden room (GR). Small sink, washbasin and shower. To keep it simple I think an instantaneous water heater should be OK for the job. Reading here ( @JSHarris ) and elsewhere the Stiebel Eltron DHC-12E looks like a good bet (£202 on Amazon). There are a couple of issues firstly the unit will be 45M, run, from the Electric meter, 16mm buried in ground comes out OK and because the unit derates a bit, assuming I have read the blurb correctly, through voltage drop I will have a few amps to play with on the 63A consumer unit for lighting in the GR when the unit is running. The second complication is that we, note the use of the word we, would like the Stielbel unit in the cupboard under the sink, I cannot see a problem with this as the brochure shows a unit, or very similar, under a work surface and hopefully most of the heat goes into the water and so it won't warm up the cupboard too much. The big question is can the Stielbel unit feed a thermal mixer shower tap, many of the cheaper units say they must not be connected to such devices but I can find no mention for this unit, or should I just set the temperature output of the Stiebel unit to shower temp and run it straight or use a manual mixer?

[Nickfromwales]

I'd go for a cheaper small unvented tank to serve both hot outlets and an 8.5kw electric shower. The steibel doesn't have very good flow rates for showering and the voltage drop will be evident on the lighting, it will dip up and down when the unit kicks in / out. 

Youll be looking at 12-15kw for the instant for one big enough to cope with all 3 outlets, but you can size smaller if you wish, ensuring that the utility sink never gets used whilst someone is showering. 

The steibel unit comes alive when fed with preheated water, but will struggle with a very cold incoming main. 

[ToughButterCup]

@MikeSharp01, we're using a pre-warmed supply for our Stiebel. Stiebel provide data sheets explaining how much they can raise the temperature of incoming water - can't find the datasheet  now but when I do I'll post it here ( or link to it) .

Ian

[Jeremy Harris]

At full chat the Steibel Eltron DHC-E (9.6 kW model) can raise the temperature by just under 14 deg C at 10 l/m.  At 5 l/m it can raise the temperature by double this figure, so a bit under 28 deg C.  This is the uplift above the incoming feed water temperature. I'm using an acceptable minimum preheat temperature of 29 deg C to ensure that we have hot water that's a bit over 42 deg C at 10 l/m; our shower flow rate is a bit under 10 l/m.

[MikeSharp01]

Thanks all - very interesting, looks like I may have to rethink a little. I guess I can ensure that the kitchen sink is not used when the shower is in use so that will help. I think mains water is about 6 degC so that plus 28 only gets us to 34, not enough for a shower. @Nickfromwales if I go for a small unvented cylinder won't I be in need of part G sign off which I had hoped to avoid or is there some sort of low volume limit? In my original thinking I was considering using an electric shower and an inline heater but I figure, @recoveringacademic I have not found the Stiebel curves either but based on what @JSHarris says, that I would get a pretty poor flow rate from the shower units as in the end the basic physics must limit the flow / temp / power curve. I can squeeze the 12Kw Stiebel model, 11Kw at the expected voltage at the end of the wire, which will give me a calculated 32deg rise at 5l/m. To get to 40 degC and assuming the linear relationship Jeremy describes 4l/min does it. So if I interlock the sink tap with the wet room lights via a solenoid valve I can perhaps get all I need from the Stiebel unit.

Edited May 30, 2017 by MikeSharp01
Typo

[Nickfromwales]

Electric shower, all day long. An unvented of 15 litres or less = no G3   

When a solution gets grossly over complicated it's usually the wrong one  

[Ferdinand]

Without doing all the sums, can you use 2 in series for the shower?

[Nickfromwales]

18 minutes ago, Ferdinand said:

Without doing all the sums, can you use 2 in series for the shower?

2 15L UVC's?

[Ferdinand]

9 minutes ago, Nickfromwales said:

2 15L UVC's?

 

Two heaters, since one would only give 34C according to the thread.

Edited May 30, 2017 by Ferdinand

[Nickfromwales]

Ok. Sorry to repeat the question, but 2 instants or 2 small UVC s? 

Assuming the former, then you'd need at least 20kw or so of electric to run them.  

[ToughButterCup]

@Nickfromwales, you've changed my mind. 9kW Electric shower. 

Now then: which one?

Any ideas?

[Gone West]

Do Sunamp make a single cell SunampPV? Expensive, but could be offset by using PV?

[Jeremy Harris]

8 hours ago, Ferdinand said:

 

Two heaters, since one would only give 34C according to the thread.

 

You run into supply capacity problems then.  Two of the 9.6 kW models can be used in series, as they accept a warm water input, but the current required would be around 83A or so.  Apart from needing a large cable, most domestic supplies are fused at 100A; some are fused at 80A, or even occasionally 60A, so there would be little headroom for any other consumption.

[ProDave]

Think out of the box.  I would seriously be looking at "caravan" technology, i.e an LPG fired Morco instant water heater a bit like the 11KW one we are using in the caravan right now.  It gives an acceptable shower (I say acceptable as I am used to a deluge from an UVC and good mains pressure and flow so have been spoiled)

 

If sticking with electric, watch the volt drop or your 9Kw shower or water heater might be quite a bit less than 9Kw in practice.

[Nickfromwales]

2 hours ago, PeterStarck said:

Do Sunamp make a single cell SunampPV? Expensive, but could be offset by using PV?

I was thinking exactly the same last night. 

This is a perfect case for a SAPV. 

[Gone West]

5 hours ago, Nickfromwales said:

I was thinking exactly the same last night. 

This is a perfect case for a SAPV. 

I looked into using SunampPV in my house but decided against it in the end. In the literature they mentioned having different numbers of cells in the SAPV and I was wondering if it would be feasible to use a single cell for a very small building.

[Jeremy Harris]

I think that one issue with just a single thermal cell would be the maximum power output.  The heat exchanger in each cell can deliver something like 15 kW, I think.  The two cells in a Sunamp PV are in parallel, so the max power is around 30 kW, around the same as many combi boilers.

 

15 kW is certainly better than an electric shower, though, by a fair margin, and there's enough energy stored in a single cell for one modest shower.

 

The downside would really be cost, as a single cell Sunamp PV would still have much of the same internal hardware as the standard two cell version.  My guess is that a single cell unit may well be around 80% of the cost of a standard 2 cell unit, even if Sunamp could be persuaded to make one.

[Nickfromwales]

10 hours ago, recoveringacademic said:

@Nickfromwales, you've changed my mind. 9kW Electric shower. 

Now then: which one?

Any ideas?

I'm not supposed to be changing your mind . Why would you choose an electric shower? For an annex / other?

[Nickfromwales]

2 hours ago, PeterStarck said:

I looked into using SunampPV in my house but decided against it in the end. In the literature they mentioned having different numbers of cells in the SAPV and I was wondering if it would be feasible to use a single cell for a very small building.

They're a good choice as you can fit them in a kitchen cupboard ( supported sufficiently of course ) and if charged by pv will be a near zero loss solution with combi performance showering. 3kw grid boost so after en evening shower you could have a short E7 top up to ensure a morning shower / wash up was guaranteed. 

These are change of £2k though, are they not?

[Jeremy Harris]

5 minutes ago, Nickfromwales said:

They're a good choice as you can fit them in a kitchen cupboard ( supported sufficiently of course ) and if charged by pv will be a near zero loss solution with combi performance showering. 3kw grid boost so after en evening shower you could have a short E7 top up to ensure a morning shower / wash up was guaranteed. 

These are change of £2k though, are they not?

 

£1700 + VAT, I think.

[Nickfromwales]

38 minutes ago, JSHarris said:

 

£1700 + VAT, I think.

?

[Triassic]

£1700+vat ouch! That's a lot to pay for some hot water. What's wrong with a 9.5kw shower and an under the sink water heater, both for £250 max!

[Nickfromwales]

3 hours ago, Triassic said:

£1700+vat ouch! That's a lot to pay for some hot water. What's wrong with a 9.5kw shower and an under the sink water heater, both for £250 max!

Nothing at all 

9.5kw though is very cold mains thirsty, so if the outhouse is sharing the main with the dwelling you may end up running it on the lowest setting to stabilise it.  

[Gone West]

13 hours ago, JSHarris said:

 

£1700 + VAT, I think.

That's for a twin cell SAPV I think. They were talking about having SAPV with different numbers of cells. Presumably if they produced a single cell SAPV it would be cheaper, but would a single cell system work effectively?

[Nickfromwales]

8 minutes ago, PeterStarck said:

That's for a twin cell SAPV I think. They were talking about having SAPV with different numbers of cells. Presumably if they produced a single cell SAPV it would be cheaper, but would a single cell system work effectively?

At that point I think sense ( financial ) would prevail, and you'd just fit a small UVC / gas multipoint heater or the aforementioned electric shower and electric under sink water heater. These would be quite efficient too with the shower in the high 90's and the electric water heater ( storage ) not far behind. In a property with Pv these would make better sense as any excess would already likely be getting consumed / stored in the main dwelling. 

Tbh, in a house with Pv, I'd run a pulsed hot return circuit from the main dwelling out to the outhouse and use what's already there. Pulsed to stave off legionella issues in the long run ( that would be considered a dead leg in a design without the hot return so no go without one ) and then a flow switch to override the pulse timer for when hot water is required. Pv would offset the losses from the hot water pipework and the pump for the hot return.