To mix or not to mix - that is the question - well one of them

[MikeSharp01]

For a couple of reasons I stayed home today and have, among other things, been working on the HW system for the garden room. I hit a slight problem, well two really and they are explained in the attached image. Essentially we are using an instantaneous hot water heater like the one @JSHarris uses. to provide HW there are three HW outlets one to the kitchen sink and two in the wet room - basin and shower. I am thinking, now that I have drawn it, I won't need a mixer tap and can rely on the heater to control the water temp. In other posts we have worked out that we can get the temperature we need provided the flow is not to high. I also wondered if having a simple control scheme that prevents water being drawn by the basin taps if there is any flow to the shower, might have value. I have found the bits I need for that but not sure if I am just over thinking this. Any thoughts and / or ideas?

 

PS the drawing on the left is the physical layout from the floor ducts - under the kitchenette worktop.

Edited November 2, 2017 by MikeSharp01
PS

[Nickfromwales]

Ok. First off, never ever have a non anti-scald aka non-thermostatic shower from any instantaneous water heater........ever.......never ever. 

Ever. 

Second I wouldn't dream of not having a mixer tap on the basin. What I would do, as I have in my house, is fit a small TMV in the hot feed to cap the basin temp and it stabilises the water delivery wonderfully. 

The problem you have is asking a single water heating unit to deliver a temp range that is suitable for showering AND basin use. Typically the basin flow would be much hotter than the shower will ever need so you need to deal with the appliance which is most temp intolerant to the higher output temp, eg the shower, and design the hot water delivery around the outlet which needs the hotter water range eg the basin. 

What is feeding DHW to kitchen / utility?

Fwiw, I would not be using an electric instant unless it had a decent cold uplift pre-heater, as I think this will struggle in the colder seasons where the incoming mains gets a lot colder. 

If your dead set on this I believe @joe90 may have a steibel that is surplus to requirements, if he hadn't already sold it. 

Fwiw #2, I'd choose a Sunamp for this job and nothing else, or a small uvc. I believe life with the electric instant would not carry much luxury. If it were for wash basins only I'd go for it. Why have you decided upon this route if I may ? Annex ?

[MikeSharp01]

40 minutes ago, Nickfromwales said:

Ok. First off, never ever have a non anti-scald aka non-thermostatic shower from any instantaneous water heater........ever.......never ever. 

Ever.

Ok - does that mean I need an automatic TRV in there - I keep coming up against them needing a minimum of 60deg for them to work and I won't come close to achieving that at a decent flow rate? The Eltron is a high (ish) end unit and has good temperature control so if I set it to an achievable temperature it should deliver it safely I believe and not overshoot. The flow rates for higher temperatures would be quite low - looks like around 4L/min for 4 deg in and 45 deg out and for 4 in and 60 out only 3 L/min. So I could set the temp to say 50 and then let the TRV control it down to 41 (ish) but if you open up the flow it will get significantly cooler!

 

41 minutes ago, Nickfromwales said:

I believe life with the electric instant would not carry much luxury. If it were for wash basins only I'd go for it. Why have you decided upon this route if I may ? Annex ?

Yes its just the Garden room. It has two rooms and a wet room, sort of annez but more likely a studio. I could put an electric shower in, I don't have gas down to this building, but looking at the diversity, just one occupant perhaps most of the time, having a unit that does the whole job seemed like the sensible way forward.

 

 

[Jeremy Harris]

TRVs generally need, as a bare minimum, an inlet hot water temperature that is about 5 deg C higher than the desired output temperature.  So for 50 deg C out 55 deg C in is around the minimum.  They work better with a higher differential, and there is some variation between brands as to how low a differential temperature they will still regulate at.  In general, when the hot water inlet temperature drops below the threshold for regulation they just open up as far as they can go on the hot water side.

[Nickfromwales]

1 hour ago, MikeSharp01 said:

Ok - does that mean I need an automatic TRV in there - I keep coming up against them needing a minimum of 60deg for them to work and I won't come close to achieving that at a decent flow rate? The Eltron is a high (ish) end unit and has good temperature control so if I set it to an achievable temperature it should deliver it safely I believe and not overshoot. The flow rates for higher temperatures would be quite low - looks like around 4L/min for 4 deg in and 45 deg out and for 4 in and 60 out only 3 L/min. So I could set the temp to say 50 and then let the TRV control it down to 41 (ish) but if you open up the flow it will get significantly cooler!

 

Yes its just the Garden room. It has two rooms and a wet room, sort of annez but more likely a studio. I could put an electric shower in, I don't have gas down to this building, but looking at the diversity, just one occupant perhaps most of the time, having a unit that does the whole job seemed like the sensible way forward.

 

 

Ok, an annex.  

Fyi do you both need to edit TRV to TMV ?

Can you get an umbilical DHW supply from the house ? A hot return circuit governed by occupancy / demand would be a good solution and suffer no delay in getting hot water out of the outlets. 

Practicality needs to play a big part here but if this is a long term solution, with regular / daily usage in mind, I'd want a better lifelong solution if it were me.

At the absolute minimum I'd go for a THERMOSTATIC electric shower and a small DHW storage heater ( 15 L ) to fit under counter to service hand wash. 

Kitchen / utility ? ??

@MikeSharp01, do you have room for a small uvc ? 

[MikeSharp01]

1 hour ago, Nickfromwales said:

Can you get an umbilical DHW supply from the house ? A hot return circuit governed by occupancy / demand would be a good solution and suffer no delay in getting hot water out of the outlets.

No house there yet, possible long term but need to get the garden room up and running abut 18 months before house will be ready.

 

1 hour ago, Nickfromwales said:

Fyi do you both need to edit TRV to TMV ?

It is TMV on the drawing.

1 hour ago, Nickfromwales said:

Practicality needs to play a big part here but if this is a long term solution, with regular / daily usage in mind, I'd want a better lifelong solution if it were me.

At the absolute minimum I'd go for a THERMOSTATIC electric shower and a small DHW storage heater ( 15 L ) to fit under counter to service hand wash. 

Kitchen / utility ?

I suppose I could do this: Small DHW (15L) such as THIS for around £100 and a shower unit such as THIS  for around £100 more would work out a bit cheaper than the Stiebel option but need more infrastructure!  I will give it some thought.

 

 

Edited November 2, 2017 by MikeSharp01
Typo

[dpmiller]

is that shower actually thermostatic?

[MikeSharp01]

14 minutes ago, dpmiller said:

is that shower actually thermostatic?

Think I assumed it would be but its only for gravity fed systems anyway so its out as the feed is mains direct. Will keep looking but a quick look and it seems that I cannot get the two systems for the same cost as the Eltron unit although I will have to buy a thermostatic shower mixer tap and the spray head / bar unit.

[MikeSharp01]

Found this Bristan unit, comes in on budget as well.

[Nickfromwales]

2 hours ago, MikeSharp01 said:

Found this Bristan unit, comes in on budget as well.

Well worded, misleading sack-o-crap ? Just another plain electric with some inventive write up. They shouldn't use the word 'Thermo' as it's not a thermostatic unit in any way ?

 

This one is actually a thermostatic unit, and not a budget buster   Don't go for the larger 9.5kw as they're far more cold supply hungry, whereas the 8.5kw will tootle along, managing fluctuations with a little less noticeable dip in temp / performance. The 8.5kw will still provide a plenty adequate shower too. Most 9.5kw showers in a busy home spend their life on the medium setting anyway, as the high setting just needs too much presume and flow to keep it going flat out. 

[MikeSharp01]

13 hours ago, Nickfromwales said:

This one is actually a thermostatic unit, and not a budget buster

Yes and made in UK so that's the one I will go for along with the 15L 2kw water heater. Together and alowing for infrastructure +/- (ie two electric circuits but no hit pipework, mixer / shower hose & spray head they come in at the same cost.  I don't have room for a UVC sadly. In the main house HW will come via the Vaillant Combi unit we already discussed. Thanks for the guidance. 

[Nickfromwales]

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