Mixer valve on wet underfloor ASHP system with buffer tank and coils burried in anhydrite

[TimothyG]

Hello,

I'm hoping to avoid fitting a mixer valve in my UFH setup. I have read a few threads on this forum with some very thoughtful posts explaining why that's a very bad idea but I think my situation is different. Before going ahead, I would love to hear if there's something I haven't thought of.
My setup is as follows..

• Primary loop: ASHP running to a diverter valve which directs flow to 55 litre buffer tank when not energised and diverts to DHW when energised. 12m underground run through insulated pipe then up 6m into the attic from ASHP to buffer tank.
• 2nd loop (already working):  From the buffer tank (with own pump) to a pair of FCUs to provide heat to upstairs
• 3rd loop (planning to build):  From the buffer tank (with own pump) to UFH manifold to provide heat to downstairs
• UFH coils to be routed into existing slab with min 25mm anhydrite screed (16mm MLCP, spiral pattern, 150mm centres), then 6.5mm Impervia flooring

 

@PeterW and @Nickfromwales posted a well-argued case on the thread linked below about the need for a mixing valve to prevent the floor from getting too hot:

With a condensing boiler that could generate flow temps of 100°C if controls failed, I get that it would be possible to get the floor above the 49°C (min temp that can scald skin) in a situation where an UFH thermostat also fails. But I think my setup is sufficiently different that it doesn't need a mixer valve:

 

The ASHP is only rated to get up to 60°C. Even if you assume that it could reach 70°C in a fault situation, by the time that gets all the way from outside, through the buffer tank and down to the floor loop, it's going to have lost a good few degrees - I would think around 5°C but let's say it only dropped to 67°C. Let's assume that this happens when we're asleep so it has time to heat the slab up without us noticing that it's uncomfortably hot and turn it off.  And assume that it has time to heat the house up to 30°C (because if the house is at only 23°C then that will cool the floor more). Then with flow to coil at 67°C, the actual floor temp would be around 47°C (in a serpentine pattern could get hotter in places but I'm doing spiral).  Uncomfortable? absolutely; bad for vinyl? probably. But still below the 49°C danger level for skin and Impervia flooring can take temperatures over 60°C without damage, so in this case we would just turn the heat pump off and wait for it to be repaired.

 

Also to get that floor temp of 47°C is going to need ~200 W.m^-2.K^-1 (even with room temp as high as 30°C and ignoring heat losses to the ground).  With a pump that maxes out at 75kPa, surely it would struggle to deliver sufficient flowrate for 200 W.m^-2.K^-1 through 60mm of 16mm MLCP pipe, a manifold and a longish pipe run to get to the manifold? So in practice, even in this extreme "double failure while sleeping" case it seems like in practice the floor wouldn't even get near 47°C.

 

I get that the mixer valve also allows recirculation when all the actuators are closed. But if they are all closed, then the thermostats ought to be telling the pump not to run anyway? And if you put the pump in target pressure mode then presumably it wouldn't damage itself even if the actuators all closed and the thermostats failed and called on the pump to run?

 

So - understanding that this isn't standard practice (and is a bad idea if you have wood or vinyl flooring or a boiler as heat source), any other concerns about this setup in this case?

 

Thanks!
Tim
P.S.  Another comment I read was the danger of getting a slug of hot through when switching from DHW back to heating. I did have some concerns about that at first even for the FCUs but in practice running it, you can't even feel the air getting hotter when the system switches over so I think the 55 litre buffer tank plus the screed slab would provide plenty of damping to avoid that problem.

 

[joth]

Without reviewing the math, my setup is similar and my gut feeling is to agree with your logic.

Main questions:

What floor finish? Somethings like wood are rated only to about 30°C so you need more caution

Will you put sensor probe in the screed? With logging you can use it to empirically verify your design, and/or hook it to thermostat to emergency switch off the pump in case of overheating.

Will you be running ashp in cooling at all? And at what temp. This is the main reason you may want an electric mixing valve, to stop water below dew point creating condensation risk inside the slab. (Unlike heating, with cooling if you use a screed temp probe to shut it off if too cold you're probably already too late)

 

Fwiw I do get an annoying slug of hot water in the FCU when switching out of DHW reheat, but my schematic is weird with the buffer in series after the FCU (acting as a volumizer) so doesn't temper the impact like yours will 

 

HTH

[Temp]

23 minutes ago, TimothyG said:

With a condensing boiler that could generate flow temps of 100°C if controls failed, I get that it would be possible to get the floor above the 49°C (min temp that can scald skin) in a situation where an UFH thermostat also fails. But I think my setup is sufficiently different that it doesn't need a mixer valve:

 

You can always just use a pipe stat to turn off the UFH loop pump (or the whole system) if the flow/manifold is too hot.

[JohnMo]

I would question how are you going to get different temperatures to the different areas and a very different heat required for the FCUs and UFH, or are you planning to run at same temperature?

[TimothyG]

12 hours ago, joth said:

Without reviewing the math, my setup is similar and my gut feeling is to agree with your logic.

Main questions:

What floor finish? Somethings like wood are rated only to about 30°C so you need more caution

Will you put sensor probe in the screed? With logging you can use it to empirically verify your design, and/or hook it to thermostat to emergency switch off the pump in case of overheating.

Will you be running ashp in cooling at all? And at what temp. This is the main reason you may want an electric mixing valve, to stop water below dew point creating condensation risk inside the slab. (Unlike heating, with cooling if you use a screed temp probe to shut it off if too cold you're probably already too late)

 

Fwiw I do get an annoying slug of hot water in the FCU when switching out of DHW reheat, but my schematic is weird with the buffer in series after the FCU (acting as a volumizer) so doesn't temper the impact like yours will 

 

HTH

 

Thanks, joth.

• The floor finish will be Impervia. It's a click-fit LVT but the quirk is that the base is a stone-resin mix so it can tolerate temperatures above 60°C which is way above anything it would experience.
• Yes, definitely putting a floor sensor in the screed. I'm planning to set the thermostats to use air temperature as the primary but limit the floor temp to a max of about 31°C since otherwise it feels a bit too warm on the feet (we have electric underfloor today so am used to it)
• Yes, I hope to run in cooling.  Both Heatmiser and Danfoss have a control system that includes a dew point sensor so I would use that to make sure the floor doesn't get too cold - but the exact temperature that happens will depend on the air temp and humidity that day.  I am a little concerned that the flow temp the UFH will want (probably something like 12°C) is a good bit higher than what the FCUs would want (8°C). To avoid local cold-spots causing condensation, I'm running a spiral pattern. My gut feel is that there will be enough thermal mass in the slab that even with supply water at 8°C, it will be able to run for 30mins+ before the slab gets too cold.  I could also turn the pump speed down and thereby raise the average temperature of the cooling fluid (spiral pattern important again to avoid local cold spots). And if I still end up with the floor getting too cold in places, I can always increase the supply temp of the ASHP and let the fans run a bit faster on the FCUs - or install an electric mixing valve as you suggest (didn't know those existed! will have to research..).

Tim

[TimothyG]

Thanks, Temp. That's true - quite a bit of extra wiring to do though which I'd prefer to avoid

Tim

[TimothyG]

12 hours ago, JohnMo said:

I would question how are you going to get different temperatures to the different areas and a very different heat required for the FCUs and UFH, or are you planning to run at same temperature?

 

I'm planning to run the FCUs and underfloor at the same supply temp.  For cooling that's definitely not ideal. In the reply above I explain how I'm hoping that will work but it might not. If it doesn't it's not a big deal since the house doesn't tend to get that hot downstairs anyway so could just turn off UFH and use FCUs to cool upstairs.

 

For heating, from the numbers I've run I think they'll be pretty happy running at the same temperatures.

 

I'm starting to think that a good approach might be to leave room for an electric mixer valve, see how it goes without, and then install one if I really have to. But it would be expensive, complicated, lots more pressure drop and lots more research to do to find the right one and figure out how to control it.

 

Regarding different temperatures in different areas, I'm planning 5 loops organised into 2 zones - so one thermostat for front of house and one for back of house (they get different amount of sun so have different heating needs but left-right is pretty equal).

 

Tim

[joth]

1 hour ago, TimothyG said:

My gut feel is that there will be enough thermal mass in the slab that even with supply water at 8°C, it will be able to run for 30mins+ before the slab gets too cold.  I could also turn the pump speed down and thereby raise the average temperature of the cooling fluid (spiral pattern important again to avoid local cold spots). And if I still end up with the floor getting too cold in places, I can always increase the supply temp of the ASHP and let the fans run a bit faster on the FCUs - or install an electric mixing valve as you suggest (didn't know those existed! will have to research..).

This would worry me: while the slab itself might not get down to  8°C, the pipework will be at that temp so any air that can get into the slab around the point the pipes enter could easily develop condensation. The pipes will also be  8°C before they enter the slab and the nature of gravity is any condensation on those pipes (hopefully none but hard to be sure every one is perfectly insulated and sealed) is going to run down into the slab. This is why using slab temp to shut off the pump is helpful to avoid heating over shoot, but not so useful for cooling.

I agree focusing cooling mode on the FCUs and get them doing an excellent job will likely make the effort on UFH cooling moot. That's what I found. (And vice versa: I don't bother with FCU heating)

 

For the electric mixing valve, what ASHP are you going with? I know the Mitusbishi FTC6 includes a 3-wire output to control a mixing valve, for example, and this is what I use. (I also have a Loxone server which also has mixing valve control block, but I haven't had to take that level of control over it so far)

 

 

[TimothyG]

Thanks @joth that's super helpful.  I hadn't thought about air getting into the slab where the pipes enter.  I'll definitely be careful about that now and pay attention to construction details there. I was planning to use short runs of flexible conduit to protect the pipes. Perhaps filling any gaps with a closed-cell spray foam would be a good idea?

 

I have an Ecodan ASHP with the FTC6 controller you mention so that's really good to hear that it has built in control for an electric mixer valve!

 

I also just found this old topic which has some really useful real-world experience from @Jeremy Harris who runs underfloor cooling and finds his floor rarely gets as low as 18°C (typically around 19°C) and he's never had condensation.

This topic made me think of some other things:

• It's certainly true that dew points in the UK are typically quite low (below 18°C) but there are quite a few days in the year (typically the hot ones where the cooling duty is highest) where it can go as high as 22°C. This website has some handy data on historical dew points: http://nw3weather.co.uk/wx10.php?humtype=dew
• If your floor is bonded to the screed (e.g. tile or glued LVT) then probably the worst case is you might notice a little condensation on a particularly warm, humid day and then you'd increase the floor temp setting (assuming you avoid the condensation in the slab problems that @joth highlighted)
• But... I'm planning to use a floating floor.  This is riskier because the temperature of the screed surface will be ~3°C colder than the floor surface - so call it 15°C (assuming a typical heat transfer of ~20 W.m^-2.K^-1 and a cooling duty of 60W.m^-2 engineered wood would have a higher temperature difference since it has heat transfer of only ~10W.m^-2.K^-1 and a lower cooling duty would reduce the temperature difference). Half the days in August this year had a max dew point at/above 15°C
• and... I won't see the condensation because it'll be under the floor.
• and... even if I use a dew point sensor to set a minimum floor temp, there could be local minimums (will try to reduce this by using a spiral pattern but I don't expect that to eliminate local minimums completely)

So that gives a few options:

• Go for glued LVT instead of Impervia (and be sure to use a high temp adhesive like KE66 / Amtico HT / F48plus).
• Or just be extra cautious about dew point and allow for a bit of a buffer in the minimum floor temp (assuming the control system is sophisticated enough to allow this. Danfoss and Heatmiser both have systems with dew point sensors but don't know how much control they let you have on the settings).
• Or... bite the bullet and find an electric mixer and wire it up to the FTC6 controller and then get flow temps to the floor up around 12-13°C
• Or... give up on slab cooling!

[TimothyG]

P.S.  @Jeremy Harris - If you read this I'd love to hear about your construction details for where the pipes went into the slab.

[TimothyG]

Looks like these would be compatible with my Ecodan FTC6:

https://www.loco2heat.co.uk/products/valves/rotary-valves/mixing-diverting-valves/esbe-vrg133-mixing-valve-dn25-kvs-10-cpf-28mm

https://www.loco2heat.co.uk/products/valves/rotary-valves/actuators/vrg-series-actuators/esbe-series-ara600-3point-actuator-with-auxiliary-switch-ara652-230-v-ac

 

 

[joth]

3 hours ago, TimothyG said:

Looks like these would be compatible with my Ecodan FTC6:

https://www.loco2heat.co.uk/products/valves/rotary-valves/mixing-diverting-valves/esbe-vrg133-mixing-valve-dn25-kvs-10-cpf-28mm

https://www.loco2heat.co.uk/products/valves/rotary-valves/actuators/vrg-series-actuators/esbe-series-ara600-3point-actuator-with-auxiliary-switch-ara652-230-v-ac

 

 

 

Yup that's the one the FTC6 is pretty much designed around, think this is where I was pointed at it:

 

 

@Jeremy Harris no longer posts on here unfortunately, but his advice and learning (especially around wishing he'd provisioned for FCU upstairs) is very much what guided my design 

[TimothyG]

Thanks so much @joth - really appreciate all the guidance!

Tim

[markocosic]

*Cold* water on a *sticky* summer day of the sort where cooling is definitely wanted:

 

 

This was bad enough to drip enough to need fixing.

 

Base level fix is 9 or 13 mm armaflex "sealed" by wrapping with insulation tape.

 

I didn't have much to do so used self amalgamating tape around threaded joints and wrapped the straight bits of pipe in with insulating tape (to keep moisture away from copper that can pinhole from outside on chilled water systems in some scenarios) then the armaflex to keep outside surface above dew point and wrap that with insulating tape as the air barrier/because I bought knock-off stuff that wasn't self adhesive and it was good around 101 bends.

 

Thee slab... would probably soak up and safely redistribute any condensation IMO. 

 

It's the rest of your pipe runs that want to be insulated such that you don't get condensation under the lagging from joints etc which then drips.

[TimothyG]

Thanks, @markocosic.  Yup! Definitely planning to insulate the pipes thoroughly.  The pipework for the FCUs up in the attic was a pain and also dripped a lot!  The manifold is going to be the trickiest part..

Tim

[Temp]

On 20/09/2023 at 09:54, TimothyG said:

Thanks, Temp. That's true - quite a bit of extra wiring to do though which I'd prefer to avoid

Tim

 

Would only be something like one of these in series with the manifold loop pump. Pretty simply wiring unless I've missed something.

 

https://underfloorheatingsupply.co.uk/product/pipe-thermostat/