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Discuss UFH mixing valve not restricting heat to floor in the Plumbing Jobs | The Job-board area at PlumbersForums.net

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Hi, I've installed a 2 zone kit that uses an RWC mixing valve, Thermomix with hot in, return to boiler and heat out to Ufh pump and return from floor manifold. From underfloor heating store but basically the same as a jug speedfit Ufh kit.
I thought I had a duff thermostat inside the reliance mixing valve but they supplied me a service kit and it's still not working.
The thermostat expands and contracts as it should, I tested it in a bowl of warm water, allowing the hot feed from the boiler to bypass the Ufh manifold but the inlet temp on the manifold is rising to basically as high as the boiler feed in, abou 65 degrees and the mixing valve control has no effect on any setting, I have just converted a y plan system to s plan with a separate zone for the Ufh and the return goes back close to the boiler return into the boiler. I've also fitted a bypass valve which you can hear opening when the boiler pump is on overrun and all zones have stopped calling for heat.
I'm really stumped, it's like the main system pump is overwhelming the mixing valve but it's running quietly on minimum setting.
I've no issues with the hot water or radiator zones or the boiler, and the Ufh manifold appears to be working fine and both loops heat ok, it's just there's no control of the temperature down to the 45 degrees needed for the floor.
I even tried restricting the flow to the valve by partially closing a full bore 22mm isolation valve I fitted in the supply to the Ufh zone but you have to almost shut it off completely to make any difference. There's no other parts in the mixing valve other than the spring and wax stat so nothing else I can see is wrong, I've tried the Ufh pump on different speeds, Ufh zones open or restricted, nothing makes any difference.
Any help would be great as I can't see anything wrong with the layout and components I have and I'm pulling my hair out with 50 degree water coming BACK OUT of the floor after nearly 50m of pipe run at the moment!!!
 
Of course, picture paints 1000 words... In my airing cupboard upstairs is the main boiler pump and the t off to the Ufh and zone valves for HW and CH, Zone valve for Ufh is by the Ufh manifold simply as there's space there.
Boiler return from Ufh pictured, T compression joint, it's the other side of the wall from the Ufh manifold

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Well the flow is connected to H with the in arrow and the other one (C) goes back to the boiler return through the wall so yep I'm pretty sure, sort of thing I'd do though :)
You can feel the flow pipe heat up as the boiler runs, if I hold the back of the 2 port body, and the Ufh gauge rises as that warms, but the return stays relatively cool until the Ufh return starts to come back at 40 degree ish, I was expecting the mixing valve return to boiler pipe to warm pretty quickly as the floor is already at 16 or 17 degrees
 
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That auto bypass looks like it hasn’t been wound in, so the spring will be loose inside. Wind the black knob until the red marker travels down towards the nuts.
 
The central heating main pump is upstairs in the airing cupboard by the HW tank, the flow from the boiler comes straight up to it then tees off to the trv controlled bathroom towel radiator and then a 22mm tee down to the Ufh, two more tees to the HW tank and rads 2 port valves.
Hard to see in the airing cupboard pic, but from the red pump there's 10cm or so then the bathroom rad 10mm T off, then another 15cm then the Ufh T off, which is the 2nd from right lagged pipe going down to the floor, then lastly the 22mm from the pump goes up another 10cm does a right angle bend and then T's to the two 2 port valves for HW and rads.

The bypass valve has been wound in but I wound it right out to see if it would open to reduce pressure to the Ufh, sadly no difference wound in, I had it on 0.3 bar, or fully out, less than 0.1 bar.

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Can you sketch how you’ve tapped into the heating pipework that serves the flow to the manifold. Was you aware the cylinder return is always the last tee in?
 
Can you sketch how you’ve tapped into the heating pipework that serves the flow to the manifold. Was you aware the cylinder return is always the last tee in?
I was just trying to sketch that :) hope the edited photo makes sense, yellow line is T down to Ufh 2 port downstairs. Green is 10mm to bathroom rad. Dark blue is HW tank feed through 2 port and light blue is to rads via the other 2 port there in the airing cupboard.
I don't know where the cylinder return is as its under the floor upstairs somewhere.

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Have you removed the tmv white plastic knob, to see if it’s engaging the brass spindle? Maybe try turning spindle with a spanner/ grips/ pliers
 
Have you removed the tmv white plastic knob, to see if it’s engaging the brass spindle? Maybe try turning spindle with a spanner/ grips/ pliers
I've taken that off, the tap operates fine and the stat and Spring behind it have been replaced with a new tested stat, the system was well flushed and there was nothing in the mixing valve body when I took it apart to change the stat, spotless.

Why should the Ufh return to boiler being last be a problem, I've had all 3 zones on as well as HW or Ufh only and everything works fine it's just the mixing valve doesn't seem to be restricting the hot flow from the boiler??

Attached a pic of the mixing valve adjuster and thermostat, strange little unit that allows flow from H flow in to mixer across to the C outlet when it gets to about 30 degrees

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I've taken that off, the tap operates fine and the stat and Spring behind it have been replaced with a new tested stat, the system was well flushed and there was nothing in the mixing valve body when I took it apart to change the stat, spotless.

Why should the Ufh return to boiler being last be a problem, I've had all 3 zones on as well as HW or Ufh only and everything works fine it's just the mixing valve doesn't seem to be restricting the hot flow from the boiler??

I haven’t said YET it’s a problem, I merely asked if you were aware. Over the years I have come across flow problems, because the cylinder tee wasn’t the last tee in.
It’s quite difficult to give you a solution, when there’s a hundred answers we need to know. You clearly have practical skills and commend you on that, but sometimes the tiniest things can be forgotten to be mentioned that could solve a mystery.
 
Also, how are you measuring the deltaT, are you relying on the gauges or are you using a known accurate device? Those mechanical gauges are usually pants and don’t give a true reading.
 
Sorry if I come across abrupt in any questions, i don't mean to be. I've read up a lot and absorbed lots of info from suppliers and tradespeople. I'm a rather over enthusiastic diyer but I like a challenge and have just built the entire extension bar the electrics and boiler flue change, I can almost solder neatly now too :) just signed off by building control so I'm pretty chuffed, just annoying about this flow mixing business.
I've an IR thermometer that gives a pretty much identical reading off the matt black back of the Ufh pump and the pipes to the floor feel too hot to hold when the Ufh inlet gauge gets to 60, so I'm pretty sure they are quite accurate, values also tally on boiler feed and return pipes so I'm happy it's not just a reading issue.
 
What about the ufh return pipes coming out of the ground, do they get hot too?
Yep, after 10 minutes of 60 degree heat going in the return gauge is up to over 40 degrees and the barrier pipe coming out of the floor, both loops, feels quite warm to the touch, far warmer than they should I think,it heats the floor lovely but it's doing it far too quickly and with too warm flow in
 
Well, yes, ideally you would tee back in the same order you have teed off, but I would not be pointing the finger at that item either. Although I find the airing cupboard is a bit too messy to understand without running my fingers along the pipes and pointing, I'm yet to see anything obviously wrong, so long as the flow and return are not swapped over.
The UFH pump is definitely pumping upwards, yes? (Check the arrow moulded into casting).
 
Yep, after 10 minutes of 60 degree heat going in the return gauge is up to over 40 degrees and the barrier pipe coming out of the floor, both loops, feels quite warm to the touch, far warmer than they should I think,it heats the floor lovely but it's doing it far too quickly and with too warm flow in

I haven't read the whole thread in detail, but it seems to me that the water is being circulated through the UFH emitters too fast. This means it doesn't have time to cool enough to be able to reduce the the incoming flow to the correct temperature in the mixing valve. This will give a runaway effect where for the first few minutes the water going to the UFH emitters is the right temperature but it then ramps up until the mixer is ineffective at cooling the incoming flow.

If I'm right, the remedy is to reduce the circulation by turning down the pump and/or the balancing valves on the UFH emitters.

Whoever designed the system should have calculated the circulation rate and matched it to the amount of power that the UFH needs to emit.
 
Yes, you're right there is an embossed arrow on the side of the new pump and it is going up :) the flow is right and the flow valves seem to adjust the two independent floor loops fine, one run is about 3 times the other.
It takes a while to get all of the air out of the main boiler circuit when I've drained out the mixing valve to change the stat but I know I bled the Ufh loops well and the air bleeds on the manifold work fine and the Ufh pump runs quiet and smoothly, there's a manual air bleed in the airing cupboard just after the HW tank 2 port, that traps a little air but the main trap is a taller bathroom towel radiator which was originally just a standard lower rad, changed by the previous owner. I've put an Saladin auto vent on that but it seems to stick too easily even though the system is very clean.
The original setup was a conventional y plan with a tank side flange for the showers and an always on T for the bathroom rads. the boiler is an old but surprisingly capable potterton suprima 60 that has no trouble meeting demand and its own temp dial is only ever needed at about 1/3. The main system pump is on minimum setting a is the Ufh pump.
I removed the y plan 3 port and changed to s plan 2 x 2 ports first, then when I was sure that all worked I joined up the Ufh as a Third zone. It looks a tangle in the airing cupboard but it's pretty straightforward just a small space with a few more elbows than would look nice and neat.
 
I haven't read the whole thread in detail, but it seems to me that the water is being circulated through the UFH emitters too fast. This means it doesn't have time to cool enough to be able to reduce the the incoming flow to the correct temperature in the mixing valve. This will give a runaway effect where for the first few minutes the water going to the UFH emitters is the right temperature but it then ramps up until the mixer is ineffective at cooling the incoming flow.

If I'm right, the remedy is to reduce the circulation by turning down the pump and/or the balancing valves on the UFH emitters.

Whoever designed the system should have calculated the circulation rate and matched it to the amount of power that the UFH needs to emit.
It's an underfloor heating store 20m2 kit with matched components, I've only used about 3/4 of the 100 m of barrier pipe but the 2nd smaller loop is restricted to about 1/3 the flow of the larger one. I have the Ufh pump running on the lowest setting.
When the system first runs, say first 30 minutes or maybe more from totally cold it still doesn't cool the incoming flow, the return gauge may be under 20 degrees but the input can rise to over 60 still.
I'm sure I've run it with the emitters set low but I'll try it again with both of them set to near minimum.
 
When the system first runs, say first 30 minutes or maybe more from totally cold it still doesn't cool the incoming flow, the return gauge may be under 20 degrees but the input can rise to over 60 still.
I'm sure I've run it with the emitters set low but I'll try it again with both of them set to near minimum.

Okay. If the mixing valve has equal amounts of 60° and 20°C water available it should certainly be able to regulate its outflow to ca 40°C.

Assuming you have got the flow and return connections to the valve round the right way and have replaced the cartridge perhaps there is a manufacturing defect in in the cast body of the thermostat causing it not to shut off in the incoming hot.

Ask the manufacturers to let you have a replacement to try.
 
Okay. If the mixing valve has equal amounts of 60° and 20°C water available it should certainly be able to regulate its outflow to ca 40°C.

Assuming you have got the flow and return connections to the valve round the right way and have replaced the cartridge perhaps there is a manufacturing defect in in the cast body of the thermostat causing it not to shut off in the incoming hot.

Ask the manufacturers to let you have a replacement to try.

I agree, the finger points to this if all the pipework has been installed correctly.
 
Thank you for your replies, I spoke to UFH store last week and they want me to buy a new one and then get a refund if they deem the old one to be faulty - not overly impressed at over £100. I did get through to RWC technical dept who were very helpful and sent me a service kit free of charge as I was sure the original stat was duff, but thinking about it now I may just have crushed the stat pin into the body too far in my enthusiam to get it to operate at a lower temp, this is easy to do if you remove the plastic collar on the control knob and keep turning :(
I will try RWC again but I've checked everything else over so many times, I'm sure of all my connections and the system is flowing to and from boiler and heating the manifold/floor fine it's just there's no feedback control of the UFH circuit as the water temp flowing over the stat rises.
I eventually just about figured out how the 4 port mixing valve casting works, it's not immediately obvious but it has side channels as well as the main stat/control casting.
This 'Thermomix' valve is quite widely used and the same part manufactured by RWC is used by JG Speedfit so it's not like it's some dubious import or something. I'll call RWC again and let you know what they think.
 
Gee Pal,

You're a hard nut to please!

You've done all the work yourself, altered the configuration of your heating system, added UFH and then you have a problem and you blame it on the equipment.

You have had a thermostatic repair kit supplied free of charge and then you admit to probably stuffing up the first one and now the second one.

You have had the supplier of the heating equipment offer you a full refund on the mixing assembly if the one you have is faulty.

What do you expect?
A complete replacement without them checking if the unit you installed is faulty?

As far as I can tell you are getting as good a treatment, if not better treatment, than some of us in the trade get.

I would say that you are now in the position to pay someone who has experience in heating systems to check the system over.

There's obviously a problem somewhere, but after replacing 2 thermostatic sensors and now wanting to replace the mixing block, it would seem less likely that there is equipment failure
 
yep, I may seem hard to please and I understand the suppliers position but I'm also quite technically minded and not overly impressed by their attitude in providing any level of technical support, they basically told me they never have problems with their mixing valves and to contact the manufacturer, they don't seem to know their own products very well - Unlike the RWC tech guy who was very helpful and confirmed my understanding of the operation of the valve and stat. It's hardly demanding to expect some level of customer support beyond a list of parts supplied.
I think I've explained and covered everything in my system to you guys on here and thanks for the responses, it's confirmed my thoughts and reassured me I've not overlooked something fairly obvious.
If you'd like to see the fruits of the rest of my efforts, pics attached. This is my biggest project yet and has taken me 18 months of evenings and weekends but I'm very pleased with the result and so are building control.

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One major problem and I would be worried about is that flue is very dangerous and needs turning off
 
been onto RWC again, like one of you suggested they also think restricting the flow controllers right down is worth a try.
Maybe the Grundfoss pump in the kit is just too powerful (even on low setting) for a smaller setup? It is used on the manifolds with many more loops but he thinks it's possible it's just overwhelming the return from the floor with the flow rate I've had it set at.
He's also asked for pics of my whole system to take a look over which was very good of him, now that's what I call good service and a company I'd recommend to anyone.
 
One major problem and I would be worried about is that flue is very dangerous and needs turning off
it's a bit deceptive in the picture but it's within the Gas Safe general requirements and also the manufacturer specific requirements for distance from wall, below opening window and from roof line and roof windows, this was installed by a gas safe engineer too.
 
it's a bit deceptive but it's within the Gas Safe general requirements and also the manufacturer specific requirements for distance from wall, below opening window and from roof line and roof windows, this was installed by a gas safe engineer too.

What boiler is it ?
 
So it's a min of 1m away from the window/wall?
 
So it's a min of 1m away from the window/wall?
yeah, 3 maybe 4 roof tile lengths and its a very low pitch 12.5 degree roof so not much length of tile lost in the incline, fitter was not initially sure so actually called Gas Safe to get his interpretation of the diagrams confirmed, it's close to max length of that flue allowed on that boiler with the elbows included but it actually ran better emissions when tested than the original short straight flue did - go figure that?
 
I would say it's too close
 

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