thermostatic valves

[Greenbeast]

Providing a mixer tap instead of separate hot and cold taps is also an obvious way to improve matters.

i've never understood separate taps anyway!
It's fine if you want to fill the basin but if you're just running a little to wash you face/hands you have to alternately scald and freeze your hands!

[rob26440]

I also prefer mixer taps to separate taps but mixers don't provide the protection that a TMV brings.  DHW reached over 60C today.  Won't be long before it's into the 70's.  I have a Horne25 (set to max) on the main DHW feed to the taps and a smaller one in the loft on the feed to the power showers to protect the pump (rated at max 65C input). 

[EccentricAnomaly]

I presume that the new BRs don't actually make TMVs mandatory, but set a functional requirement instead. Does anybody have a link?

Scottish Regulations do, indeed, set a functional requirement.  I suppose, perhaps too cynically, it's too much to expect that English/Welsh ones will be modelled on them but:

To prevent scalding, the temperature of hot water, at point of delivery to a bath or bidet, should not exceed 48°C.

A device or system limiting water temperature should not compromise the principal means of providing protection from the risk of Legionella. It should allow flexibility in setting of a delivery temperature, up to a maximum of 48°C, in a form that is not easily altered by building users. This will allow reduction of temperature where, for example, facilities are used by those more at risk from injury, such as elderly people or unsupervised children.

Where both hot and cold water are supplied to a facility, the above may be achieved by use of a thermostatic mixing valve (TMV) or fitting complying with BS EN 1111: 1999 or BS EN 1287: 1999, fitted as close to the point of delivery as practicable. Guidance on the installation, use and maintenance of thermostatic mixing valves and fittings can be found in BRE information Paper IP 14/03 and from the Thermostatic Mixing Valve Association (TMVA).

Bit odd that they say bath and bidet but don't mention basins or showers.

[pb]

Bit odd that they say bath and bidet but don't mention basins or showers.

Apparently basins and showers (together) only account for 20% of hot water injuries.  I suppose the cost of fitting TMVs to those isn't felt to be justified by the reduction in accidents that it would bring.

See:

http://www.communities.gov.uk/documents/planningandbuilding/pdf/brac08p18.pdf

for some figures and analysis, albeit from 2008.

[noelsquibb]

The animation is good but does not show any more than the flows and thermostatic response.

The most disappointing video on the website was the one labelled 'Group Showers'   

http://www.horne.co.uk/Horne25.htm

There didn't seem to be anywhere to put in your card details ...... 

[Mudman]

yes PB that makes sense- many of the the bad injuries are when someone gets into a bath that's too hot- it's going to take a few seconds to get out at the minimum - longer than it takes to move out of a shower or remove your fingers from under a tap. My grandma did this in her 70s and could not get out very quickly - bad burns to ankles and feet (htough it could have been much worse-) and young children seem to defy physics with what they are able to do like climb over a bath edge when hardly able to walk yet (my 1 year old). horrible to hear of todlers who do this while bath is left running and the first thing the parent knows from the next room is the piercing scream - at which point it's very hard to get them straight into a cold water setting - basin too small and bath full of hot water- to limit the burns. Not that rare sadly- and just a second of inattention can cost your little one their life or looks. This is what made me fit a tmv to our bath straight away when i started looking into the risks (our water is 70 degrees from the council).

Most of us on this forum will be able to bypass a tmv if it really inconveniences us- i do find that topping up the bath when it's getting too cold to lie in takes a lot of water since fitting the tmv - but i'm all for the new buildings regs.

[Heatstat @ Horne Valves]

To answer Noel’s earlier query about using a TMV as a diverting valve, our Chief Engineer has provided the following:

The following information is a theoretical discussion about the technical operation of TMVs. It does not constitute recommendations for use of TMVs, which are designed primarily to mix hot and cold water supplies and generate safe temperature “hot” water. The purpose is to increase the understanding of the characteristics of TMVs among technically competent personnel.

TMVs could be used as flow diverters, however see the following caveats!! Obviously, the check valves at the inlets would have to be removed or they would prevent flow in the reverse direction through the inlets. The plastic cartridges would have to be broken out (we make them so that they can’t be left out accidentally during maintenance). Then comes the problem of setting the valve. Ideally, you would have a variable temperature water supply to pass through the TMV to establish the transition range where the valve diverts from one port to the other. You would need to raise and lower the temperature as the water flows to establish this. A close approximation would be to simply set the TMV in the normal way, i.e. as a mixing valve, and setting the TMV to the required “transition temperature”.

This neatly feeds back into the discussion about “differential”. The TMV will divert the flow over a temperature range. As stated elsewhere, in Horne valves, this range is lower than most, giving significantly more accurate control.

You mention the potential to damage the valve - an interesting point. If you think about it, you will realise that a TMV protects its thermostat element from overheating by mixing cold and hot water so that the element only ever senses temperatures within its operating range – the 45C you feel at the tap. However, if you put your 90C solar-heated water in the outlet of the TMV, it would flow straight onto the thermostat element, and would then get diverted out of the “cold” inlet. However, while doing this the thermostat remains continuously exposed to the water at 90C. It cannot “self-protect” as in normal use.

However, thermostat elements are generally able to withstand exposure to higher temperatures. No long term testing has been done to confirm this, or the rate of degradation of the element in particularly harsh environments such as solar heating applications. Note that Horne TMVs have integral mechanisms to permit controlled expansion of the thermostat element at higher temperatures – without this they would simply burst.

Finally, you ask if the TMV could close off the outlet in some way. A TMV operates by moving a valve disk between the hot and cold inlets. It can shut off either inlet, but not both at once. However, there is a theoretical way that you could contrive to get a reverse-acting valve (in a diverter application, as above) to close off the flow by simply blocking one of the “inlets“. Thus, blocking off the cold inlet would result in water below a certain temperature flowing out of the hot inlet. The valve would shut off the flow if the temperature rose above the transition temperature. Similarly, blocking off the hot inlet would result in water above a certain temperature flowing out of the cold inlet, and the valve closing if the temperature fell. There are a few not-terribly-obvious practical implications of this (such as, in the latter example, when the temp is too low and the valve has closed, how can it ever open again?).

[Heatstat @ Horne Valves]

To continue the discussion about the Building Regulations…

Yes, showers and basin taps (and kitchen taps – you want much hotter water there for effectively washing dishes etc) were excluded from consultation because

a)   The greatest number of scalding accidents are associated with the bath.
b)   Young children that are most at risk of scalding (toddlers) are unlikely to be able to reach the basin tap or shower controls
c)   The ability to quickly remove one’s hand from under a scalding flow from a basin tap (which may actually be an involuntary response) indicated that this was a low risk scenario.
d)   It is the size of the child relative to the bath and the subsequent partial or complete immersion in a relatively large volume of very hot water that is the issue here.  The low perception of danger also plays a part – coupled with the possibility that young children may associate the bath with (supervised) playtime fun before bed.  Very young children and the elderly/infirm/incapacitated also have much reduced mobility and so may not be able to quickly or easily remove themselves from the scalding water and therefore can suffer severely.

Going back to the comments about separate hot and cold basin taps – yes, when there is no thermostatic control, a single manual mixer is better than alternately scalding and freezing your hands….or, tongue very much in cheek, you could try our Optitherm tap – it’s both a thermostatic hot tap and a cold tap in one!   Actually, as I’ve said before, we would recommend supplying the entire bathroom - so every outlet offers scald protection - from a single DN20 mixing valve, for example our Heatstat T2, which has a low pressure drop, and utilising a manual shower mixer to mix the water down to the usual 41C showering temperature.

Regarding the animations on our website – we do not have an animation showing how a valve operates (or struggles to operate) in marginal situations, which is a rare occurrence probably only encountered by this community.  It serves to show how the valve operates under normal conditions and how it responds to changes in the incoming supply temperatures and pressures to maintain a steady mixed water temperature at the outlet and safely shut off the hot supply in the event of cold water supply reduction and failure.  Noel, I’m afraid I do not understand your comments about our shower panel animation or the ‘card details’.

Hannah

[noelsquibb]

Noel, I'm afraid I do not understand your comments about our shower panel animation or the ‘card details’.

Oh dear, I'm sorry Hannah, probably a sad male thing.

Seeing a video entitled 'group showers' might have triggered an expectation for something other than close ups of shiny levers.

The card details comment, might have been an expectation of having to pay for such a video.

I shouldn't have posted this.

I apologise.



Thank you for following up my interest in using TMV's in reverse.

On some TMV's its possible to turn the non return valves but I prefer to leave them out as the protection is not required. I think the filters are best left in place as it doesn't take much debris to stop the thermostatic parts from functioning .


Regarding high temperatures, I hadn't considered the inbuilt protection of the sensor in normal use but as your Chief Engineer observes, there is some safety margin.

A further test I can do, would be to do a reverse flow, which I do using a mixer tap, and logging the performance of the valve,  immersing it in boiling water for a decent cooking period, then reassessing its performance.

Obviously this is not a long term test but I would consider it a useful starting point.   

noel

[KLD]

Dear Hannah
Thank you for the many explanations. Sadly, I still don't get it. I mean, the part about the "marginal situations, which is a rare occurrence probably only encountered by this community."    The question remains: are the valves capable of switching the cold flow off altogether or are they not? If not, what is the minimal flow rate allowed through at a typical mains pressure, say 2.5bar?

I'm afraid neither do I understand the relation between "temperature" and "force". Following the little animation on the Horne webpage (great pain it's in realplayer format, at least for those of us who escaped from the M$ world. Quite a few in this marginal community.) it looks as if half the valve could be described as a thermostatic limiter: hot in -- thermostatic element and actuator -- outlet (similar impracticalities as described in Noel's 'reverse' scheme with one port blocked off). In physics terms, the energy needed to actuate the valve is extrated from the thermal energy of the hot flow. The amount of energy needed is minimal compared to that available, and thus you would not see a significant drop in temperature between inlet and outlet.
Thus, I can't see any reason why the thermostatic element needs to have a lower temperature then the incoming hot supply to be able to regulate that port. Now, adding the cold inlet doesn't change the mechanics of that process, does it? So, the only reason I can see for the minimum temperature differential between 'hot in' and 'mixed out' is the safety function, ie. being able to detect a pressure/ flow failure in the cold supply.

Sorry, this wasn't meant to sound so harsh, I just feel it would be good to have a complete understanding of how these valves operate. In a typical installation here (solar thermal store) the cold and the hot inlet are both connected to the same cold mains (obviously one going through the TS coil in the store first). Should the mains cold fail, there'd be zero risk of piping boiling hot water to the taps. For these applications a valve without the safety feature would make us happy ! (There you are, a market request for a new (?) product :-)

Klaus

[SimonHobson]

I'm afraid neither do I understand the relation between "temperature" and "force".

I think I can see what's missing ... I was puzzled myself until I read this thread.

As the temperature of the water changes, the temperature sensitive element will change size/shape in response. This is not a "switch", but a gradual change. Essentially, for the element to change size enough to move the valve sufficiently to fully close the hot inlet may need a good few degrees - and I think what's been suggested is that the Horne valves need only 10˚ while others may need 30˚ or more.

It also follows that the outlet temperature isn't constant. If you feed in very hot hot water, then the outlet will need to be hotter before the valve will adjust to reduce the flow rate in response. Conversely, if the hot water isn't all that hot, then the outlet temperature will need to be lower before the valve open up the hot flow in response. Eg, if it were linear (which it probably isn't) then if the mixing needs were initially for a 50:50 cold:hot, and you increase the hot water supply temp, then by the time the valve is mixing 75:25, the outlet would need to be sufficiently hotter to make the thermostat element and it's attached valve move by 25% of it's travel.

I think where the talk of differential temperature and "shut off" comes from is this. Suppose the element needs (say) 10˚ change to move between cold open/hot closed and cold closed/hot open. If the setpoint is (for ease of mental arithmetic) 50˚, then the valve would vary between cold fully open/hot fully closed with the outlet at 55˚ and cold fully closed/hot fully open at 45˚.

So what does this mean in practical terms ?
Well suppose out hot is well above 55˚ and our cold is well below 45˚. If the hot supply fails, then the outlet will go below 45˚, and with that, the cold inlet will be fully closed and there will be no flow - the valve will try to supply 100% hot but there is no supply. If the cold supply fails, then the outlet will go above 55˚, and with that the hot supply will be fully closed - the valve will try and supply 100% cold flow but again there is no supply.
In both cases, the flow will stop - and depending on the design of the valve, it will be down to a dribble or even completely stopped.

Now lets use cooler hot water - say only 52˚. We lose the cold supply, the valve will move to try and shut off the hot supply, but the temperature isn't high enough to expand the thermostat cartridge enough to move the valve over it's full stroke. We will still reduce the flow, but it won't stop completely.
The effect would be that on loss of cold supply, the flow rate would reduce, and the temperature would increase to the supply temperature. It could be argued that with such a small differential, the reduced flow and relatively small rise in water temperature isn't a huge risk - but it is a risk. The larger the differential, the bigger the risk.

What Hannah has said is that the Horne valves have a differential of only 5˚, some others I bought recently have a note to the effect that it's 15˚ for them. That means the Horne valve will fully shutoff the hot flow as long as the hot supply is 5˚ above our setpoint (55˚ in the above example), while the others (which seem to have been branded for some bathroom and kitchen company) would still allow flow unless the hot supply was at least 65˚. That IS a big difference, and effectively means that if you set your TMV at 46˚ to meet the Scottish regs quoted above, then the Horne valve would limit your hot water to 51˚ worst case*, while these others would allow a risk up to 61˚.
* Worst case, failure of cold supply, hot supply not hot enough to fully close hot side of valve.

That's the difference between a scalding risk of 2-6 mins and a few seconds (for a healthy adult) according to the figures Hannah gave earlier.

Does that help explain ? Did I get it totally wrong 

[djh]

I recently replaced my bath and now have an electronically-controlled bath filler. It came with the temperature limit preset to a nominal 45 (or was it 46). Anyway, the same as the new regs.

Totally unusable! 

One complication was that the actual limit temperature was 42 - I think this is planned so even with error tolerance it still meets regs without needing the installer to actually do anything. I think the same occurs with TMVs. So it was just about possible to fill a bath with water that wasn't too cold when you got into it, but absolutely impossible to top up the bath later.

Fortunately, being electronic, it was just a matter of twiddling a screwdriver to solve the problem.

[SimonHobson]

As the temperature of the water changes, the temperature sensitive element will change size/shape in response. This is not a "switch", but a gradual change. Essentially, for the element to change size enough to move the valve sufficiently to fully close the hot inlet may need a good few degrees - and I think what's been suggested is that the Horne valves need only 10˚ while others may need 30˚ or more.

It also follows that the outlet temperature isn't constant. If you feed in very hot hot water, then the outlet will need to be hotter before the valve will adjust to reduce the flow rate in response. Conversely, if the hot water isn't all that hot, then the outlet temperature will need to be lower before the valve open up the hot flow in response. Eg, if it were linear (which it probably isn't) then if the mixing needs were initially for a 50:50 cold:hot, and you increase the hot water supply temp, then by the time the valve is mixing 75:25, the outlet would need to be sufficiently hotter to make the thermostat element and it's attached valve move by 25% of it's travel.

...

Just to add to that earlier post, I imagine some people are wondering why the manufacturer doesn't make the thermostatic element more sensitive and reduce the differential. Without getting into any control theory, it's all to do with stability.

In theory, it would be possible to make a very sensitive element, but then it would most likely be unstable. Lets take what it probably a reasonably common scenario - you have 1 bar (header tank in the loft) on the hot water and 3 bar on the cold (reducing valve off the mains). You've been using the hot tap, and so the TMV is mixing hot and cold. As you turn off the tap, the back pressure will reach 1 bar and the hot flow will stop. Cold only now flows, and the TMV will move to the 100% hot position and flow will settle down to 'virtually nothing'.

Now you turn the tap on. The TMV is cold and on 100% hot. Hot water will flow, and the outlet will rapidly rise above the setpoint. If the element is sensitive enough, it will now completely shutoff the hot flow and supply 100% cold - and the outlet will go below the setpoint. The cycle would then repeat, each time switching between 100% hot and 100% cold supply - the problem being made worse if the element is slow to respond.

With the sort of ranges the current valves work over, long before the valve has managed to travel between fully hot and fully cold, the mix will have changed and the outlet temperature moved towards the setpoint. In the scenario above, it's still possible to get some initial instability, but it will rapidly die down.


There is also a tradeoff in response speed. A slow thermostatic element will be more stable in steady state - as it changes the valve position, the water temperature flowing over the element will change quicker than the element is trying to move the valve. In the above scenario, the valve may be on 100% hot initially, and hot water will flow out. The element will change in response, but long before it can get the valve all the way to 100% cold, the water temperature will have cooled off and it will settle somewhere in between.

Against this, a slow element will also be slow to respond to dynamic changes - so someone turning on another hot tap may well may cause your outlet to go cold for a while until the valve adjusts again.