Shower mixer for improved COP

[Ommm]

Now I understand that if you have an unvented cylinder it's a regular hot water system and you can drive pretty much any shower.  But I wondered... an ASHP runs more efficiently at a lower temperature.  Yet thermostatic mixer showers tend not to work if they can't mix in some cold. One I looked at said it expected a 12C difference between hot in and outlet out.  So you heat the water up to 50C and then mix it back down to 38C again.  This seems crazy to me.

 

Obviously, you need a thermostatic control because sometimes you're on the legionella cycle and the water temp really is 60C. And having cooler tank water means you get less runtime compared with mixing in some cold.

 

 But is there a mixer that requires less temperature 'headroom', and so if the input is 38C it would output something like 38C and not 26C?

[ProDave]

I run ours at 48C hot water temperature and the shower mixer copes with that okay.

 

You would not want all your hot water less hot than that would you?  I arrived at 48 degrees as that is as hot as I can hold my hands in without it hurting.  You want hot water for washing up, so anything less than 48 degrees and I don't think the washing up water would be hot enough.

 

Also if you stored the hot water at say 38 degrees, you would not be diluting with cold for a shower so you would need a larger capacity hot water cylinder.

[joe90]

2 minutes ago, ProDave said:

I run ours at 48C hot water temperature and the shower mixer copes with that okay.

ditto with mine.

[Ommm]

At present we have washing up water come from the hot water cylinder which is only run when someone wants a bath. The dishwasher does 90% of the washing up, and other times there's usually a recently-boiled kettle next to the sink. So lack of hot water doesn't bother me - if it did, instant-heat taps aren't that expensive and running costs relatively minimal given the sink doesn't use a lot of hot water.

 

If you run your cylinder at 48C, how hot can the shower water go?

[SteamyTea]

Apart from legislation (possible), why bother with a thermal mixer at all?

Put your hand under the shower, if it is too hot, turn it down.

[ProDave]

9 hours ago, Ommm said:

If you run your cylinder at 48C, how hot can the shower water go?

Hotter than I can stand.  Although I can stand putting my hands in 48 degree water for washing up. that is too hot for most of my body.  The fact I can, if I wish get the shower water too hot for comfort suggests it works with a lot less headroom than the manufacturers suggest.

 

[Gone West]

10 hours ago, Ommm said:

Obviously, you need a thermostatic control because sometimes you're on the legionella cycle and the water temp really is 60C

We stored our DHW at 45C in a UVC and didn't bother with the legionella cycle. It worked fine with the thermostatic showers and the bath thermal mixer.

[A_L]

10 hours ago, Ommm said:

But is there a mixer that requires less temperature 'headroom', and so if the input is 38C it would output something like 38C and not 26C?

 

The hot to mix temp is the difference required to achieve cut off time of the hot in the event of cold supply failure. They will operate with lower hot to mix diffs.

 

Horne make quality TMV which will operate at 2-3°C diffs.

 

https://www.horne.co.uk/products/water-products/ 

[Ommm]

Thanks.  Looks like Horne only have hospital-style enclosed shower units, or mixer valves that are designed to be hidden away. But good to know low-difference mixers exist.

[markocosic]

Store at 45-48C, screw the legionella cycle, and go no thermostatic.

 

Or ask the OEM to confirm approach temperature. If less than 5C you're fine (it'll do 41-43C at the outlet which is too hot to bear) unless you have shower pipework buried in a masonry wall (so it heats the wall and is cooled en route)

 

Legionella dies from 45C+ by the way - it's just that kill time that varies. Provided that you keep the mains fed insulated cylinder sat on wood hot all the time - rather than filling a vented cylinder sat on cd concrete with legionella infested lukewarm water from an attic header tank at 37C and hoping to kill it before it gets used - you're fine. Guidance (there are no regs) caters for worst case scenario. 

[SteamyTea]

1 hour ago, markocosic said:

rather than filling a vented cylinder sat on cd concrete with legionella infested lukewarm water from an attic header tank at 37C and hoping to kill it before it gets used

Have you got evidence that this has happened, in the UK or globally?

[markocosic]

Nope! (though I'd love to see one - from a single UK/US type building with individual tank not a european "building" where one tank feeds multiple apartments (probably from a "cold" tank sat on the roof)

 

That's the worst case scenario where you're most likely to get it. Preferably a holiday home for occasional occupancy.

 

Legionella is difficult to grow, not particularly deadly unless you're already half dead or otherwise immunocompromised, and homeowners don't test though. Good luck finding single recorded case from a domestic environment.

 

Yet the legionella police still insist on storing and distributing hot water at scalding temperatures that are inefficient to produce for heat pumps. (and district heating systems) Good business in selling pointless gear to fix the problem you create with scalding hot water. Easy photos of scalded babies dropped into hot baths. Or grannies who slipped from the shock of hot shower water and clonked their head on the wall and were peeled by the hot water by the time they regained consciousness.

 

Else the legionella count in domestic cold water is so low, and the growth rate so slow, that any domestic system with reasonable "turnover" and where pipe velocities are high enough to clear the pipes will flush through anything that does appear quicker than it can take hold.

 

Don't let that stop the legionella police forcing silly storage temperatures and scaring you into lots of equipment to reduce burns though. 

 

 

 

Most recorded exposures (from plumbing rather than cooling towers) are in hot water recirculation systems that are poorly designed and poorly maintained. If you ensure that the return from these systems is >50C at all times AND all parts of the system are >50C at all times then you won't get it. Even if it's >50C for >20 hours per day that's enough. Balancing large systems (such as in hospitals, sheltered housing) sing fixed valves is a pig though and you often get cold spots.

 

Worse still are dodgy thermostatic mixer valves and dodgy mixer taps with cartridges (installed to reduce scalding risk from hot water) which allow crossover (cold water flows into the recirculation system, invisibly to the users, and creates "pockets" of cooler water that may not (due to how water is drawn off the hot circuit) show up back at the point of entry/return/reheating. Rubber hoses / washers too. (certain agents used to make EPDM are great at growing bacteria)

 

Basildon (poo hole) Hospital (even bigger poo hole) did a classic by testing their water, finding legionella, then trying to kill it by raising the temperature of the hot water system. They made it worse. How? It was in the cold water system which they helpfully raised from 25-30C up to 30-35C thanks to proximity to the now hotter pipes. Enough people died to notice. Not enough people died for the hospital to care to fix the problem though. They pour enough money down the drain to pretend to fix it but never do. Better to spend it on doctors / nurses/ medicines etc in their eyes. (possibly not wrong on an economic basis)

 

https://www.bbc.co.uk/news/uk-england-essex-23961487

https://www.echo-news.co.uk/news/local_news/9810618.10-years-on-legionella-till-a-risk-to-basildon-hospital-patients/

 

 

This book is the definitive text for those with an interest. Not much research in the area since as it's of minimal consequence and the marketing dollars all come from the tap vendors.

https://www.amazon.co.uk/Legionella-Building-Services-G-W-Brundrett/dp/0750615281

[markocosic]

US references:

 

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC183576/pdf/aem00061-0260.pdf

https://www.phcppros.com/articles/8259-legionella-growth-in-hot-water-systems

 

Too many variables though. Electric vs fossil heating should make zero difference to legionella. IMO it's likely that the electric heating folks simply didn't keep their tank hot at all times. (as I suggest with the heat pump - the heat loss is compensated by the higher COP is the gut feel)

[Ommm]

8 hours ago, markocosic said:

Store at 45-48C, screw the legionella cycle, and go no thermostatic.

 

My installer has quoted for a Mira Element EV, which is pretty bog standard thermostatic.  If I happened to want to play with this, can this or other thermostatic showers be swapped to a non-thermostatic mode?  For example by tweaking a lever or swapping cartridges?  I would still want temperature control, of course.

[markocosic]

That shower fits to exposed pipes on a standard spacing. You'd remove the whole mixer bar and replace it.

[J1mbo]

I do run legionnaires cycle with my R290 ArothermPlus once a week and heat the cylinder to 45/48C otherwise.

 

This is because the DHW cylinder is installed in an open vented configuration with cold water tank in loft.

 

Looking at the consumption data the cycle uses about 1kWhr more on average than I use on all other days. The DHW mode is set to “normal”, which is Vaillant terminology for full power (ie shortest recovery time).