Controlling Tap Temperatures with a Blending Valve

Friends, I wrote the other week about how controlling for Legionella in a domestic hot water system could lead to overly hot temperatures at hot water taps.

This presented the risk of scalding people using hot water for a day or two after the anti-legionella heating cycle had run.

The solution was to install a thermostatic blending valve on the output of the hot water cylinder.

This article is a short follow-on, showing how the blending valve behaves.

Blending Valve

Click image for a larger version. A blending valve mixes cold water with hot water from a domestic hot water tank to achieve a blended flow with a thermostatically-controlled temperature.

The Caleffi 5218 series valve I installed (data sheet as pdf) was specified to be settable for output flows between 45 °C and 65 °C, with each unit on the thermostatic control corresponding to a 2 °C change in flow temperature.

Obviously, this had to be checked!

Click image for a larger version. Testing the temperature of the tap water.

I tested the flow temperature using a thermocouple inserted in the water flow and waited for the temperature to stabilise.

As the data sheet makes clear, on first operation, there can be a short-period where the water temperature at the taps exceeds the set temperature. This seemed to be limited to about 10 to 15 seconds after which the water temperature was stable to within ±0.1 °C.

Click image for a larger version. The graph shows the measured flow temperature versus the thermostatic setting of the blending valve. The cylinder temperature was – evidently – about 56 °C, and the sensitivity of the setting was very close to its specified 2 °C per setting unit.

As the graph above shows, the valve performed exactly as specified. And although there is still a small risk of scalding due to the transient response of the valve, in practice, I think this risk is low.

Why? Because if the pipes through which the blended water is delivered are initially cold, then the over-temperature water will lose heat to the cold pipework.

I have now set the valve to a nominal 49 °C, and I propose to stop thinking about this problem. It’s a lovely day and I really want to get outside!

10 Responses to “Controlling Tap Temperatures with a Blending Valve”

  1. 171indianroad Says:

    Mix valve. Great plan.

    Have you also installed recirculation?

    • protonsforbreakfast Says:


      By re-circulation do you mean using waste water to pre-heat incoming cold water?

      It’s a great idea but I just couldn’t figure out how to implement it in my home.

      Or did you mean something else?

      Best wishes


      • 171indianroad Says:

        I mean – and this is common –

        You run a thin line from the hot water at the farthest fixture from the heater back to the inlet on the heater. Then – a small pump recirculates the water from the fixture back to the heater.

        This eliminates the wait time for hot water by effectively expanding the heater tank all the way to adjacent the fixture.

        It has the benefit of convenience and saves water and energy by eliminating the running of the faucet to get hot water.

        There are pumps that have AI and will learn when you tend to use hot water and anticipate the demand so they are not running constant. But – even running constant makes sense in most scenarios.


      • protonsforbreakfast Says:

        Bruce: Thank you. That’s a very interesting idea.

        When I get my hands on a ‘blank’ property I may try that technique. However I can’t quite see how to do that without substantial disruption given where I am now.


  2. Simon Duane Says:

    Not wishing to delay your enjoyment of the sun (it’s still grey and threatening rain in these parts), but …
    As the “proud owner” of a newly installed and nicely insulated hot water cylinder, I had to find out a bit about these mixing valve thingys myself because, as the installer explained, “we always use cylinders that have a second “solar” coil (aka heat exchanger), and solar thermal panels can make the water pretty hot, well over 60 degC.”
    Although fitting solar thermal panels would make more sense in our case than in many (because usually the fancy cylinder is an extra expense, but we already have it (conveniently placed just a few metres beneath the roof). There’s just the small matter of negotiating a planning permission hurdle. Fingers crossed, eh?
    Of course, I couldn’t bear to see a spare heat exchanger sitting idle, so I asked if the installer would please connect it, as well as the primary coil, to the replacement gas boiler (sorry!)
    Which they did, albeit in parallel rather than in series (which is what I’d had in mind). And it works a treat – if you’re going to consume gas, best do it with utmost efficiency, and questionable details like having a water cylinder at the top of the house while the boiler is at the bottom are worth thinking about. I did, and efficient exchange of heat is always good.
    It’s not only proud owners of air source heat pumps who benefit 🙂

    PS In my reading up on the valves, I found that they’re a requirement for places housing those who might be considered vulnerable. That’s in addition to all the good reasons you mentioned here and in your previous note.

    PPS That installer was helpful enough to explain that “the trouble with connecting the coils in series is you’d be looking at a few £100 extra on the necessary valves to control the flow”. Not wishing to get his back up, mid-installation, I didn’t get into an argument and assume that the valves he had in mind really were necessary…

    PPPS How is it that retired metrologists can be so taken up with such things? (I feel able to say this only because I are one too…)

    • protonsforbreakfast Says:


      Well that’s a complicated tale. And since you have time on your hands (?) here is my challenge to you.

      How efficient is your new gas boiler?

      When tuned properly they are supposed to operate at 90% efficiency or thereabouts. But I have heard many people suggesting that this rarely happens and typical operating efficiencies are much lower.

      If you could figure out how to assess that, I would be very impressed. Measure the volume of water condensed and compare with gas useage?

      All the best


  3. Simon Duane Says:

    Well, I do and I don’t (have time on my hands).
    But that is a question I’ve thought about – I’m reasonably sure it’s doable, and at a level that might even satisfy us both.
    The moment of enlightenment, on my journey towards amateur/armchair hydronics expert status, was when I started assigning (thermal) power values (in kW) to flow in pipes. Suddenly everything becomes “trivial” (as theoretical physicists less reserved than me are prone to say).
    When I realised that the HW circuit provides an easy-to-analyse setup to measure the flow rate (kg/s), and with reasonable uncertainty (because it takes a remarkably long time: 304 seconds to complete one circuit, which only makes sense if actually all the lengths of pipework hidden under the floorboards have a diameter 28 mm, not just the parts I can see at the boiler and at the cylinder. What I haven’t done yet is strap my thermistors on the flow and return pipes of the new boiler, to get the “delta T” and estimate its useful thermal power output. (I prefer this to making some inference based on condensation rate, though that would be interesting in itself. Albeit a little more complicated because, according to the installer’s description, my boiler has quite a large reservoir for condensate, more than a litre, which fills up slowly then partially empties all in one go). The last step is to read the digital gas meter (resolution 0.001 m^3 volume of gas whose official calorific value is usually published the following day, but that resolution is close to 0.01 kWh energy).
    So. The measurement is planned, but one has to savour these moments, and I’ve go a lot of other things to squeeze in, so the definitive answer is a day or three away. Probably.

    PS Fortunately, the pump (“circulator”) has a fixed speed – that component wasn’t part of the otherwise comprehensive upgrade of the system, so my previous measurement of flow rate is still relevant. (Pipework and radiators also remain – my medium term goal is to assess their capacities and compatibility with a certain alternative technology heat source that might one day replace that still very new gas boiler. I _think_ it’s possible to hack into the boiler’s internal control system, thanks to “OpenTherm”, and turn down its “set” temperature. This might amount to an empirical test of how close an actual heat pump would come to meeting our demand for heat…

    PPS This boiler is reported to achieve rather better than 90%, especially if the return temperature can be kept _well_ below that famous 55 degC. I’m looking forward to that measurement, too.

  4. Simon Duane Says:

    PPPS The slight complication is that, while the flow rate is nominally constant, the new boiler’s output isn’t (unlike that of the dinosaur it replaced). I’m unconvinced by the new one’s claimed modulation ratio of 10:1, having only seen it run at 30% maximum output, but my ever-informative installer said that it prioritises efficient combustion over everything else, and 10% looks impressive on the spec sheet but … the control system might not let that happen in practice.

    (I’m curious about modulation and heat pumps – is yours variable output? Apologies if you’ve told us before and I’ve forgotten.)

  5. Simon Duane Says:

    Just to offer a side remark to Bruce’s question (recirculation) –

    When i was offered this a few months back, I more or less dismissed it as a silly idea, likely to be wasteful of (thermal) energy: at least if the water gets cold, standing for hours in the pipe on its way to the “hot” tap, it can only lose that energy once, whereas if it is recirculated to keep it hot, then it goes on losing energy at the initially ~high rate, all the time. How wasteful. That’s why I dismissed the idea.

    Then later on, when asking myself whether we should reconsider getting a dishwasher after all, and making all the relevant estimates involved, I was amazed to work out that “hot water” is only marginally more expensive than “cold water”, of the order 15% to 20%, because the cost of the water (or rather, the cost of processing it to make it safe to drink) is, iirc, 5 or 6 times more than the cost of the gas used to heat it. Now I’m not an enthusiast of using current prices of things as the basis for environmentally sound decision making, but it was a forceful reminder that “running off the cold, while waiting for the hot to arrive” really is a waste that should not be tolerated unthinkingly.

    As it happens, making the return flow possible for us would involve significant disruption, so it can wait until there’s another reason for lifting all those floorboards, etc. So dismissing the idea was probably the right short term outcome, even if the reasoning was unsound.

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