DHW storage and Legionella Risk

Really?! I’m not familiar with Vaillant, controls but I would be very surprised if there’s no way to set the max temperature.

That sounds like a good plan, I’ve also disabled legionella on my Samsung heatpump since it insists on using the immersion even though R32 in my Samsung is able to get the tank to 55C. I heat to 45C daily since my tank gets a high turnover.

Everything about DHW is configurable: target temp, temp of water being added to reach target temp, timing of when to create it etc.

Except for the legionella run.

(And it’s impossible to create a schedule for creating hotter water on one day than others. Then one could create one’s own legionella run.)

I do find it astonishing that we are still using tanks that store potable water rather than pass the incoming cold through a coil especially when the heating circuit passes through a coil instead.

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Isn’t it a matter of heatpumps not being able to provide the necessary flow (volume per second) at the right temperature precisely when it’s requested?

Not just that, but also the number of compressor starts/stops would increase significantly. And that’s one of the primary factors determining the service life?

Unless by coil you’re referring to the immersion/resistor heating. That could work. But is wildly inefficient: COP 1.

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Nah, the suggestion here is storing hot water from the heat pump directly in the cylinder, and then having a coil that heats up potable water later on request, using the heat stored in the cylinder. I believe that phase-change thermal storage devices (e.g. Sunamp) work in a similar way.


Oh, wow, very interesting! That sounds indeed very interesting. Sounds like that would also diminish legionella risk!

I’m guessing it’s not yet widely deployed because of the additional complexity and/or because it’s fairly novel?

P.S.: I’d definitely appreciate a link or two to learn more about this! :nerd_face:

Water filled tanks used in this way (ie a body of non potable water with a coil passing through it containing potable water) are called thermal stores and are used in the domestic heating industry quite widely. The problem, when used with heat pumps, is that you really need either to heat the stored water at a temperature quite a lot higher than the desired dhw temp, or to have a much larger tank, to get adequate capacity.

From time to time there is discussion about this in the heat pump thread over on buildhub. With more modern heat pumps that can achieve flow temps of 70+C, and using a plate heat exchanger instead of a coil to heat the dhw, it might well be possible to make it work. Unfortunately the heat pump industry has pretty much adopted as a religion the idea that any existing DHW system needs to be thrown out and replaced with a new UVC based system. When heat pumps could only reach a flow temp of 50C or so, that was probably true. Now probably less so but the industry is a bit stuck in a rut IMHO (others may disagree).

I think you mean store the water at a higher temp than the CH temp.

On that basis, yes an no. If the tank has good stratification, then the CH (Rads or UFH) can draw off the bottom first and work it’s way up, blending to achieve the desired flow temp.

The advantage, is the HP can be driven at its optimum flow/return temperatures as you can control the temperature of the water flowing to it, you are not trying to do this simply on the Heat Loss from the building at the time.

However, if you did mean DHW temp, then no, my thermal store gets to about 55°C and that is plenty high enough for DHW (I’m still on a boiler).

I do mean ‘hotter than the DHW temp’ (as well as hotter than the CH temp).

Suppose you want 200l of DHW at 45C with an incoming water temp of 15C (ie you have to heat up by 30C).

To do this with a DHW tank you need to store 200l at 45C, assuming perfect stratification, or less if you store it at a higher temp.

To do this with a thermal store operating at say 55C you need 600l stored, again with perfect stratification and assuming perfect heat transfer.

Thats not to say thermal stores cant work, they definitely can provided you dont need vast amounts of hot water in a short period, or can configure the system for frequent reheat. However the heat pump ‘industry’ seems to want to design systems with 200-300l hot water tanks in order that you can have several showers and run two batch simultaneously without needing to wait or reheat. I’m not condoning this, but its a fact I think driven by the modern trend to expect essentially infinite hot water at 10-15l per minute so several members of the family can shower every morning.

So thermal stores are deprecated by the ‘industry’ and I doubt you would find an MCS installer who would countenance one.

I’m not necessarily disagreeing, but why? Because a part of the tank can cool significantly and still provide the 45°C water? I do have a big tank (long story) so it isn’t an issue, just curious on the physics.

Two basic physics principals

a) Heat will only travel from a hotter body to a cooler one (second law of thermodynamics)
b) Energy can neither be created nor destroyed (first law of thermodynamics)

Consider a storage tank with volume v which is at a temperature deltaT above another incoming body of water also of volume v. The maximum energy which can be transferred to the incoming water before the two bodies of water reach the same temperature is v c deltaT/2, at which point no more energy can transfer (c is the specific heat capacity of water, the amount of energy required to raise 1kg by 1C). This assumes complete mixing of the water in the tank, no stratification.

This energy transferred is enough to heat v litres of incoming water by deltaT/2. So you need 100l at 75 to heat 100l from 15 to 45 if you keep the storage tank completely mixed.

I think you can do a bit better than that if the stratification is ideal. I cant quite work out the complete maths (40 years ago id have written down the differential equations and solved them) but I believe it boils down to this.

Consider the situation where you have incoming water at 15C with a target outgoing temp of 45C.
Pass this through a tank with volume 100l and a thermocline (thermal gradient) from 30C at the bottom to 60C at the top. This tank can cool down to the point where its temperature is 15C at the bottom and 45C at the top, before it will stop supplying heating the incoming water to 45C. Since the tank has cooled on average by 15C it has supplied enough energy to the incoming water only to heat 50l through 30C.

Combining these two I think that the maximum energy you can extract from a tank of volume v initially at temperature T into a stream which enters at t1 and exits at t2 (so has an average temperature of (t1+t2)/2 =tavg is

v c (T-tavg)/2

The volume of water this will heat from t1 to t2 is

(v c (T-tavg/2)/(c (t2-t1))

= v (T - tavg/2)/(t1-t2)

This is a bit better than the figures above now I have (hopefully) correctly accounted for stratification, but the basic principle that you need to store more water or water at a higher temp than if you simply supply the stored water to the tap still applies.

Of course in the real world you need a little more (or higher temp) than the equation says because a perfect heat transfer doesn’t occur and/or takes too long. With a coil in a cylinder the limitation will be the rate of energy exchange once the tank temperature gets near to the incoming temperature. This can be improved by increasing the coil area but coil area detracts from tank volume so there is a limit to this (unless we invoke fractal coils which have an infinite area but occupy no volume).

@borpin Would it possible to pull all this DWH chatter out into a separate thread at all?

It seems to have taken over this Arotherm thread a little bit and seems more generic heat pump than just Vaillant? Thanks.

Hi all, interesting discussion! I’d suggest on demand hot water is where a phase change store wins out, since it can dispense energy to the outlet water without the associated temperature drop of a water store, so needs much less ‘spare’ energy and can be small. Perhaps also similar/same unit could replace the buffer in a HP install if you needed a buffer for space heating to keep your COP up? Also can charge the store from PV divert and direct solar water panels. No potable hot store = no legionella risk.

Bit worried about that 70C input though!

I am presently considering my next move here to get away from an older oil boiler for space/water heating. Presently have 4k PV and divert to charge the EV via an OpenEVSE unit. Also zoned radiator controls with their own thermostat schedules but so far no attempt to optimise oil use as prime target, more just avoiding the obvious waste of an old central stat and broken wax TRVs that you can never get right …!