thermostatic valves

[Brandon]

A vented boiler running to the store direct will remove the 10^oC differential there, and with a 55^oC valve on the return, it will deliver water as hot as it can whilst still condensing.

[Greenbeast]

Why does a TMV have any drop at all? Surely you should get hot water out at same temp as hot water in if it is below the set-point?

if only.
I'm going to put a TMV bypass in next time i drain the system down, will help in winter when we're heating the water and won't have to worry about scalding, then it can be switched over for summer

[KLD]

KLD - I am interested in your numbers.

Wookey,
Argh, you mean I should go and actually measure those temperatures then? OK, will be a few days, I'll have to make up a few DS1820s.

WRT to thermal store: I find that the temperature levels are simply raised by a few degrees, when compared to a normal cylinder. In our TS the section around the DHW coil doesn't drop below 35°C (at two thirds down the height, the lowest section around the solar coil is mostly around 25°C). So, if  your insulation is up to it (and there is no doubt about that in your case) then running the TS at 50°C top, 40°C bottom should be fine. Maybe you could run an experiment one sunny day, heat the current cylinder with the boiler in the morning (I know I know...) to say 40°C, and see whether the panels can bring it up by another 10°C under winter sunshine.

Klaus

PS AFAIU the temperature drop across the TMV is a result of the safety feature of the valve: when the cold branch fails to produce enough flow, then the whole valve shuts down, independent of whether the hot branch is too hot. In order to detect the flow capacity of the cold, the valve has to allow some minimal flow through the cold inlet at all times.

[Heatstat @ Horne Valves]

Having read through the various comments in this thread, I have a couple of comments that I hope you will find useful:

Legionella in the domestic setting is really not an issue. There are regulations and guidelines for healthcare premises and other public places because they are large buildings, usually with recirculating HW systems with multiple draw off points to basins, showers and taps.  The deadlegs need to be kept to an absolute minimum as when not in use the dead leg will cool to the ambient temperature, providing prime conditions for Legionella growth.  Infrequently used outlets are required in HTM04-01 (control of Legionella in healthcare premises) to be flushed regularly to purge the deadleg.  In hospitals, there are many immuno-suppressed patients who are obviously more vulnerable to Legionella infection than the average adult.  In the domestic setting, everything downstream of the HW store/water heater is a deadleg, and, generally due to the relative size of the building, the daily usage of the outlets will be enough to turnover the cold water deadlegs and reduce Legionella proliferation.

Regulation regarding TMVs in the domestic setting.  This has been in place in the Scottish Building Standards since May 2006 and was result of a burn and scald charity (the Scottish Burned Children's Club www.theburnsclub.org.uk) petitioning the Scottish Government to reduce scalding accidents in the home. In Scotland, in all new build and refurbished properties the temperature of the hot water to the bath must not exceed 48C.  One method to achieve this is by fitting a TMV.  A valve installed is set to 46C allowing for a 2C variation in temperature. Since then, a similar campaign to have the UK Building Regulations changed was led by Mary Creagh, Labour MP for Wakefield, and this is now due to be put in place from April this year (within Part G of the Building Regulations and is being slipped in on the energy efficiency section and not in the safety section as one might have thought).  Again, hot water to a bath in all properties should not exceed 48C in new build or refurbished properties. We suggest supplying the entire bathroom from one DN20 valve (rare that one would have the bath tap and the basin tap running at the same time in the domestic setting) - and also using this to supply mixed water to a manual mixer shower.

Extra heat input required to allow for mixing at the shower.  Most manufacturers test their TMVs for the ability to close off the hot water supply in the event of a cold water failure. This provides the scald protection that everyone knows about. However, less well known is that it is important for TMVs also to be able to shut off the cold water supply (and protect against thermal shock). The significance of this is that if the TMV cannot close off the cold water supply, then there could be a permanent cold water leakage through the TMV. This will reduce the mixed water temperature coming out of the TMV. Thus, in marginal situations, where the thermostat element is not doing any ‘work’, if you have hot water entering the valve at, say, 40C with a set temperature of 40C, you may get water coming out of the valve at 35C or something like that. The drop is due to cold water leaking into the valve past the cold water sealing face. Horne test every one of their TMVs for the ability to close off the cold water supply on a hot water failure. We are not aware of any other manufacturer who does this. This makes Horne TMVs significantly better than other manufacturers’ TMVs in such demanding applications. Horne TMVs also have a lower “differential” requirement than other manufacturers’ products. The differential is the temperature difference required between the supply temperatures and the mixed water temperature. Many TMVs required a 12K (degrees Centigrade) differential. Horne DN15 and DN20 valves require a 5K differential. Thus, for borderline applications where the hot inlet temperature approaches the required mixed water outlet temperature, a Horne TMV should outperform other TMVs.
 
Other factors can contribute to the cold water leakage through the valve. Scale, or waterborne debris can contaminate the valve faces, and prevent the TMV from shutting-off either supply. It is important to maintain any TMV in order to preserve optimum performance. In hard water areas, this is of particular significance.

I hope this helps.

Hannah

[wookey]

Thanx for that very full reply, Hannah.

The 48C bath-tap requirement seems stupid to me. If you want a nice long bath, after a while you need to put some more hot in to keep the bath temp up, and if you can't put it in at hotter than 48C then you'll need to add a lot of water to get the temp back up, which is a waste. The hotter you can put it in the better (up to around 70C where the mixing gets too uncomfortable). It might make sense where the users are inept in some way (small children, old people) but it's just pointless nanny-statism in other circumstances. <sigh>

You sau that oneht one hand you test for the ability to turn off the cold completely and tha tthe inability to do this causes the reduction in output temp compared to input temp, but then you also say that Horne TMVs have a 5C differential - i.e they too allow cold in even when the hot is only comping in at the set point. This seems contradictory to me. Are you saying that in practice it is not possible to design a valve that actually turns the cold off when none is needed? Or it is possible, but you don't (and neither does anyone else)?

I could use a TMV in summer when our tank, and thus hot water, can be stuck at 90C for days on end, but conversely in winter we don't have 5C to spare for the inefficiencies of TMVs. Is there really no better solution than a summer/winter manual bypass?

[Greenbeast]

I could use a TMV in summer when our tank, and thus hot water, can be stuck at 90C for days on end, but conversely in winter we don't have 5C to spare for the inefficiencies of TMVs. Is there really no better solution than a summer/winter manual bypass?

ditto!

[EccentricAnomaly]

What I'm planning is a thermal store containing primary (inhibited and generally horrid) water with a plate heat exchanger (PHE) to heat domestic hot water (DHW).  The DHW would be supplied directly at usable temperatures (e.g., 38°C), so no need to mix at the tap or shower, controlled by modulating the pump in the primary hot water circuit.  A separate sensor and comparator would check the PHE output temperature and directly cut off the operation of the primary circuit pump if the DHW approached the safety limit temperature (48°C).

I believe this meets the spirit and letter of the Scottish building regulations except for the requirement to keep DHW up to 55°C in the supply pipes.  I don't think this will be a problem as the house layout will be carefully chosen to keep the pipe runs very short (bathroom and kitchen adjoining different walls of the mechanical room) so the logic of:

Where both hot and cold water are supplied to a facility, the above may be achieved by use of a thermostatic mixing valve...fitted as close to the point of delivery as practicable

can be applied.

[rob26440]

I could use a TMV in summer when our tank, and thus hot water, can be stuck at 90C for days on end, but conversely in winter we don't have 5C to spare for the inefficiencies of TMVs. Is there really no better solution than a summer/winter manual bypass?


ditto!

How about a 3 port potable motorised valve on the hot input to the TMV?  One output feeds the TMV.  The other bypasses it (with an NRV in circuit?) and is connected to the blended output side of the TMV.  A suitably positioned stat on the DHW cylinder would control the motorised valve.  Might need a 2 port valve on the cold input if the TMV allows cold through if no hot.

Just a suggestion.....

[Heatstat @ Horne Valves]

There appears to be some confusion/misunderstanding over the "differential". Firstly, the "differential" is a completely separate issue from the "cold leak" issue. They are not related in any way.

The differential arises because a TMV is a self-powered analogue device. It does not behave as a switch, changing suddenly in response to temperature at a preset value. Rather, it closes off the hot supply and opens the cold supply gradually, over a temperature range. The "self-powered" aspect is important - a TMV has no external power supply, and yet it performs a highly accurate control function. (It actually uses the temperature of the water to linearly expand a thermostat element, which generates the required control actions.) A key point is that there needs to be a temperature difference to cause the thermostat to expand and initiate a control action.

Next, a matter of definition. The "differential" quoted is actually the difference required between the hot supply temperature and the mixed water temperature to ensure that the TMV will shut off the hot water supply in the event of a cold water supply failure. However, this aspect of its behaviour also has implications on the TMV's ability to close off the cold supply when the hot supply is close to the mixed water temperature.

The “differential” is just a fact of life. However, different TMVs have different differential characteristics, but the effects of the differential can be minimised. In order to minimise the practical effects of the differential, try to ensure that the hot and cold supplies are at the same pressure. If you have mains pressure cold, and tank fed hot, then it is easy to appreciate that the TMV needs to generate quite a bit of force to close off the mains pressure cold. This force comes from the temperature of the hot water expanding the thermostat. Hopefully, it is also easy to appreciate that generating more force requires a greater expansion, and therefore a higher temperature difference to generate that expansion.

Suffice to say that Horne valves are designed to have a very low differential, and they have the lowest differential that we are aware of in the market.

Also, note that a TMV will pass a flowrate based on the minimum of the two supply pressures. In other words, if you have hot water at 1 bar and cold water at 3 bar, the flowrate you will get out of the TMV is exactly the same as if you had hot and cold supplies both at 1 bar. So there is no loss of hot water flowrate associated with balancing the inlet pressures to the TMV as described.

In practice, the differential should not have a significant detrimental effect, and should only actually have any effect when the hot supply temperature is close to the desired outlet temperature.

Regarding the new regulations for TMVs in the domestic setting - it is being brought in primarily for the safety of very young children (they are more at risk of severe harm from scalding due to the increased sensitivity of their skin and also their limited perception of danger), the elderly and other at-risk individuals such as drug addicts or mentally challenged. I don't have the detail to hand, but the majority of bath scalding accidents in the UK happen in the homes of the section of society considered to be in fuel poverty - the opposite end of the spectrum from the users of this forum I suspect!  The maximum temperature setting was selected after consultation to cater for the rest of the population in order for adults to effectively top up a cooling bath without compromising on safety.  The (adult) exposure time for sustaining 3rd degree burns decreases exponentially:

44C - 6 hours;
49C - 9 minutes;
51C - 2 to 6 minutes;
52C - 2 minutes
55C - 20 to 30 seconds;
60C - 5 to 6 seconds
66C - 2 seconds
70C - 1 second.

Children and the elderly would suffer in shorter times.

With this in mind, it is worth pointing out that bathing at 46C is actually very uncomfortable  - fill the bath only from the TMV supplied hot tap and cold water will need to be added before immersion. With solar hot water temperatures reaching 90C or above in the summer months, then there is an even greater argument for their use.

I hope this explains things more fully.

regards
Hannah

[Greenbeast]

thanks for that

[noelsquibb]

Hey, Hannah Heatstat at Horne Valves,

thats a useful explanation for the differing responses I have seen with TMV's. Thank you.

Do you have a view on how TMV's might function when used in reverse i.e. as flow diverters.

So variable temperature water goes into the 'outlet' and the thermostatic response of the valve sends it out either the 'hot' port or the 'cold' port.

Im not asking you to confirm that they will do this, as the four different makes Ive tested seem happy enough.
Im interested to learn if there might be potential to damage to the valve or if it could close off  the 'outlet'  in some way, as a safety feature for normal use kicked in.

Cant be too careful 

noel

[djh]

Hannah, Thanks for your very useful posts - it's great to have this information. I'm still having trouble visualising how the 'leaks' and differential arise - are there any online resources that describe how TMVs work that include pictures or animations but are detailed enough to have a full explanation of these issues?

With regard to the new legislation, I'm with Wookey. It's the nanny state gone mad. What happens if I want to wash something in the bath or fill a bucket with hot water? I regularly fill my bath by starting with 60 C water and I also top it up with the same water whilst sitting in the bath, so this change will waste water for sure. Was there public consultation before this decision was taken or just industry and government 'experts' who know what we want? How do we object - write to our MPs or is there a better way?

[Heatstat @ Horne Valves]

djh

There is an animation of a TMV working on our website at www.horne.co.uk/Products.htm
It requires Real Player to run over the internet and although it doesn't explain the 'differential' and 'leaks' there you can visualise the element expanding in response to the higher temperature water entering via the hot port.  The expanding element pushes the 'slide valve' down, reducing the size of the hot port and the amount of hot water entering the valve.  The animation finishes with the cold water supply failing: this bathes the element in very hot water causing it to expand and push the slide valve down to seal against the hot valve face.  It is the force of this movement (the sudden expansion of the element) against the hot water pressure that the 'differential' is required for.  We are not aware of other manufacturers conducting a hot water supply failure test (exactly the same should occur but the slide valve seals on the cold valve face and stops the cold flow) and as such it may be that those valves will have a leak through the cold port.  I hope this helps you understand the difference.

Regarding the TMV regulations, I believe the consultation was to industry and various public bodies, but not to the public in general - you should be able to find some information on the government's website or the Building Regs website.  The consultation documents for the Scottish Standards are found on their website (www.sbsa.gov.uk) so I assume that the consultation document for the UK Government should also be publicly available.  I'm not sure how you would go about objecting.  It won't be a retrospective law, only required in new build and major refurb (i.e. not when replacing like for like as in when you 'redo' your existing bathroom).  I'm also not sure if refurb refers to the space (i.e. the bathroom) or the hot water services...I'll need to look into that.


Noel...I'll respond to your query later today.

Regards

Hannah

[Antman]

Thanks for the clarifications and info Hannah - very useful.

To the doubters, I can confirm that hot water delivered at 60 degC does burn and will damage skin on hands. Therefore imagine the effect on those less able to detect or act quickly on the action of immersion into such temperatures. Consider as well that 60 degC is the temperature that the regs would have you believe we should all be heating and storing our fossil-heated DHW to.....

The jobsworths at work decided 18 months ago that the recirculating HW system would be boosted to ensure that the HW at point of delivery was always 60 degC. It is impossible to hold fingers under this for more that a few seconds - and then, they emerge crimson. It is worth noting that this is usually a medical indication that damage to the skin has already been done!
The fact that our building had operated since 1960 with no problem whatsoever did not matter - but it seems that the need to prevent an hypothetical case of legionella is more important than causing actual damage to employee's skin.

As an eczema sufferer, the water temp has created a permanent issue with my hands that I am having dificulty cracking (pun intended!). I got fed up and installed a TMV on one sink on our floor so that at least some of us access to one useable hot tap.

So personally, I am all for the mandatory fittment of TMVs as part of the BRs. There have been far too many cases of HW scalding and very few are publicised. And they should certainly be fitted on any solar system installation capable of generating elevated HW temps.

Antman

[djh]

Hannah, thanks for that link. I'll play it when I get home.

Antman, I don't think anybody doubts that you can scald yourself badly with 60 degree water. I don't think anybody would argue against protection for vulnerable people but the vast majority of people are able to look out for themselves.

As regards handbasins, I don't think there's a problem limiting them to hand hot. In fact I believe the current thinking in sustainability circles is that handbasins should all be cold supply only, to save energy (that's the regulation in Frankfurt, IIRC). That's a step too far the other way, IMHO. Providing a mixer tap instead of separate hot and cold taps is also an obvious way to improve matters.

But I believe that for the bath and the kitchen sink there should be some way to override any limiting devices.

I presume that the new BRs don't actually make TMVs mandatory, but set a functional requirement instead. Does anybody have a link? I'll be interested to see how they deal with those neat, but vastly overpriced, 'butler' taps that supply boiling water on demand!