Posted by: @marzipan71

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Hi @derek-m that all makes sense, thanks. The sensor is 77cm down inside the 155cm high tank, so if its seeing a 7C drop in temp from a single shower, would that mean that around half of the tank is at most 38C at that point? The Daikin manual states 'The cold water which flows out when the hot water is removed is first routed to the storage tank at the very bottom of the heat exchanger, where it cools the lower area of the storage tank down as much as possible. The readiness zone is heated by an external heat generator (condensing boiler, heat pump, solar system, electric immersion heater). Water flows through the heat exchanger for storage charging (SL-WT) from top to bottom. On its way to the top, the domestic water continuously absorbs the heat of the storage tank water. The flow direction, operating on the principle of counter-flow, and the coil-shaped heat exchanger create a pronounced temperature layering in the hot water storage tank. As high temperatures can be maintained for a very long time in the upper section of the storage tank, a high hot water output is achieved even if water is drawn off over a long period of time.'

Good idea about the shower volume test - however, in the short term, I checked our shower head and the manufacturer states there is a limiter set to 9.5l/m. Since we take 4 or 5 min showers, that suggests a shower is about 50L of hot water. I suppose what's happening is that the upper section of the storage tank is at 45C; the shower draws off 50L of water from that layer, which is replaced at the bottom of the tank with cold water that goes through the journey described above. Daikin's point in their last sentence in my reading is that hot water can be drawn off for a relatively long period of time even as cold water is topping up the volume of water in the TS due to temperature layering. The heat pump will cycle on after that first shower as it reads 38C, but there will still be a volume of water in the upper layer of the TS which is hot enough for a shower and has minimal mixing with the colder water entering at the bottom. Is that a reasonable interpretation?

I think per a discussion with @saf1973 it might be best for me to move away from reheat+scheduled to just scheduled at this point to prevent these early morning reheat cycles when we have little or no PV feeding the heat pump. Its good to know how the kit works though and the above had been puzzling me for a while! Thanks for your help, as ever.

 

 

Thinking about what actually occurs, a single shower will not use 50 litres of warm water, since it is being mixed with cold water to provide the desired temperature.

@derek-m yes good point. The total volume of water used is ca. 50l, but since we have the handle probably 80% towards max, does that mean roughly we are using 40l of water from the top layer? The water in the tank is no hotter than 48C (perhaps at the very top its 48C, even though the sensor half way down the tank says 45C?) and then there will be some temp loss as it travels across the house to the mixer valve. I don't think we have any kind of temperature limiter on our mixer valve so if we have it set all the way around to hot, I think that reflects whatever the hot water supply temp would be (minus network losses). I realise that we could probably drop the set point a little to reduce mixing but we've found 48 works across a range of uses and times of day etc.

Posted by: @marzipan71

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I don't think we have any kind of temperature limiter on our mixer valve so if we have it set all the way around to hot, I think that reflects whatever the hot water supply temp would be

Think carefully about this point.

If such a shower unit were to be fitted in a location where the DHW cylinder were heated by a log-burner, for example, then it would be very dangerous for it not to have an integral preset on the thermostatic valve.
I believe such a design has not been permitted to be installed in the UK for very many years. Perhaps a heating engineer here on the forum can comment on that?

My standard bar mixer shower has a pre-set thermal protection device hidden behind the manual temperature-setting valve on the right.
You can adjust this pre-set position manually by removing the knob.

Depending on the design of this knob, it may or may not have a button which you can press to take the temperature above the 'default'.
Others just allow 300° rotation without a button to press.

In either case, the default outlet temperature (without depressing a knob) should be 31°C to meet the UK standards.

Hi @transparent take the point yes - this is the unit we have on the showers at our place:

https://www.grohe.co.uk/en_gb/essence-single-lever-mixer-with-3-way-diverter-24169001.html

The technical information here states that the max temperature of the hot water supply is 70C:

https://cdn.cloud.grohe.com/tpi/2000/2100/2180/2189/2189585/original/2189585.pdf

There is an 'optional accessory' of a temperature limiter which I assume would be installed by a plumber to comply with UK standards - which begs the question why its an optional accessory in the UK. We live in Italy, and the spec appears identical:

https://www.grohe.it/it_it/essence-miscelatore-monocomando-a-3-vie-con-deviatore-24169001.html

Since the limiter is an optional accessory, and looks to cost 21.35 that we were never asked to pay by our plumbers, I assume its not been fitted to our installations and therefore I think my quoted text above is true. It might be a good idea if I track down some of these limiters and try to have them retrofitted. Even if its a requirement of some Italian building regs to fit a limiter there's no chance of having the plumber come back so it will be down to me.

The relevant valve is this one I think, but I don't think that's where any temperature limitation would take place:

https://www.grohe.co.uk/en_gb/grohe-rapido-smartbox-universal-rough-in-box-1-2-35600000.html

Posted by: @marzipan71

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The technical information here states that the max temperature of the hot water supply is 70C

🤣 because at that temperature the metal control lever is too hot for you to turn it off!

Human skin has a nerve reflex loop which causes the hand to jerk away from touching a surface at 60°C and above.

It's possible that this feature has become embedded in our genome by Natural Selection.
Early humans who kept getting burned would've been considered less attractive individuals for continuation of the species. 🤔 

Posted by: @marzipan71

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There is an 'optional accessory' of a temperature limiter which I assume would be installed by a plumber to comply with UK standards

I think that's the general purpose DHW temperature mixer, rather than anything directly connected to just the shower.

Here's a photo of the thermostatic mixer valve (TMV) on the DHW outlet at the top of my thermal store:

I've now found one of the UK statutory references to the requirement for a TMV.
It's in Part-G of the Building Regulations, which came into force in 2010.

Thermostatic (in-line) mixer valves are mentioned particularly in Section 3.64 onwards.

A very high proportion of plumbing/heating component parts sold in the UK are made in Italy.
I'm pretty sure the manufacturers are not making different devices for sale here.

Anything added or modified in the Building Regulations prior to Brexit will have been ratified under European design and safety standards.

The UK had considerable input to those. The manner in which BS (British Standards) specifications have been created, specified and adopted since 1901, provided an excellent base line on which the nations of the EU could move forward.

Hi @transparent yes, that all makes sense. Building regs are as far as I can see a recommendation here in Italy as the project manager in charge of the build/ renovation signs off on his own work when its complete. There's a threat of some spot-checking by the equivalent of the local council but our understanding is its rare, and we never had a visit from a building inspector or equivalent. Anyhow, we definitely don't have any (visible) kind of mixer valve on top of the thermal store/ water tank - the pipes come straight out unimpeded and then disappear into the masonry. By way of a test, I set the DHW to 55C and measured the temp of the shower water and it more or less is the same - pics below - which I suspect would be the case if I set the DHW hotter also. Its just something we will have to manage (btw I don't run the legionnaire's cycle). Thanks for your help!

Posted by: @marzipan71

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Building regs are as far as I can see a recommendation here in Italy as the project manager in charge of the build/ renovation signs off on his own work when its complete.

Yes, I think the evidence of self-certification in Italy is well supported by the consequences of seismological activity.
Take, for example, the damage to buildings in the Apennine mountain area during 2016

I will be unsurprised if a similar level of self-certification is revealed in a report concerning the devastation in Turkey and N. Syria a couple of months ago.

Why is it that us Brits seem to be so far ahead of the rest of world in appreciating the significance of Building Regulations, such that we actually attempt to implement them?! 🤔 

@transparent Turkey was far worse with regard to building codes - the ruling party granted amnesties for flouting building codes on payment of a fine and Erdogan boasted about it in election speeches.  A quote from the article:-

Buğra Gökçe, deputy secretary general of Istanbul Metropolitan Municipality, was quoted by Duvar saying that 294,165 buildings in the earthquake-affected areas had taken advantage of the amnesties.

@marzipan71 you want something like this (I have exactly this one on my cylinder output).

https://heatingpartswarehouse.co.uk/product/altecnic-tempering-valve-22mm-45-65/

typically when called a "tempering valve". these are designed as whole-circuit valves to blend everything to the target temperature on a permanent basis at a high flow rate. there are 28mm versions as well. you install it on the outlet of your cylinder. that blends ALL the hot water in your hot pipes - regardless of which outlet its going to - down to the value that you set on it. usual domestically to set it to the low end. mines set at 45. meaning that no outlet, even if turned up to full hot, will be hotter than 45. even if the water in the cylinder is at 70C.

As opposed to a "thermostatic mixing valve", which is designed to go on outlets such as showers for user comfort so that it doesn't run hotter or colder if the pressure varies when someone else turns something on. they can look almost identical so caveat emptor. but a TMV will not have the same setpoint range as a Tempering Valve.

Thanks @iancalderbank that looks like a better solution than trying to retrofit some limiter to the mixing valves - and of course, our taps in the bathrooms and kitchen would also be delivering water at tank temperature so adding a device like that seems highly sensible to limit the temp of the hot water delivered across the system.

@marzipan71 yep, also allows you to run the cylinder at a much higher temperature if you want without risk of scalding. for example if you have spare PV you can heat it much hotter - my spare PV heats it to 70C. or if you find run out of hot water "too quickly" then you can run it a bit hotter. then as it blends with more cold at the output stage, the hot in the cylinder is used less quickly so it lasts longer.

TMV's on outlets make sense for showers but not for much else in domestic situations.

What would be an ok cop to have including winter period,  im at 3.07 from march to march, that includes dhw, dont know how to work it out without it as i dont have separate metering for dhw i think.