A few ASHP / UFH bits of information.

[TerryE]

53 minutes ago, ProDave said:

If it can indeed be set to say 22 degrees and it will maintain that to say within half a degree, then there will be no need for a regulating UFH manifold and no need for a buffer tank.  But if it can't go that low, or it's control is not very precise then a regulating manifold will be required.

 

@ProDave Dave, this is the point that would like to challenge: the need for a regulating manifold (and therefore a buffer tank to prime it).  Let's just say that you want to circulate maintain a steady state by circulating water at 22°C which results in a heat transfer rate of ½kW into the slab.  One method of doing this is to your ASHP continuously at an O/P of 22°C, say; another is use to buffer tank and pump 3kW into it at 35°C for 30mins or whatever your hysteresis on your buffer controls dictate, and then have a "space" of 3 hrs or so whilst this is slowly bled into the slab at this constant 22°C. 

 

A third strategy could be to take your ASHP down to whatever is can run at at its lowest output temperature and run it for 30mins say heating up the slab then turn it off, but leave the recirculation going. You don't need any temperature sensors in the slab, just a temperature sensor on the return flow from the slab.  Circulate the water and wait whilst the slab (return flow) temperature falls.  When it reaches 22°C, give it another 30 mins of ASHP top up and repeat as necessary.  You don't need any fancy control of the ASHP, just a simple on off demand, as all you are controlling is the mark-space ratio.

 

The numbers might need tweaked (and trimmed down to average out the sawtooth effect) and you might need a compensating adjustment for typical ambient/trended air temperature which modifies the turn on temperature, but in principle this could all be done with a few sensors and some control logic.  No buffer tanks, no precision manifolds.  OK I am a little different from most of you guys in that I am comfortable with this sort of instrumentation and IoT devices, and I've done quite a bit of embedded development over the years, so I am a lot more comfortable in doing this in S/W than COTS control H/W, but once I've gone around the buoy a few times and (if I've) got a workable system, then the approach once documented should be transferable for others to an off-the-shelf implementation. 

 

 

[TerryE]

11 hours ago, jack said:

Nope, I don't have one and have never planned one. The only tank I have is a 250L UVC for DHW. 

 

Note to self: need to add a Strikeout icon to the editor bar! 

[Nickfromwales]

12 hours ago, TerryE said:

I've been thinking about this issue of the buffer tank.  Jeremy uses one with a precision mix-down as Nick and Neil have described above.  Jason and Dave don't; Jack has one but doesn't use it at the moment.  OK, Jeremy's approach clearly works, but I've really got to ask myself:

 

If you are using the buffer tank for heating only, then why run one at 35°C, say, when you have a humongous concrete buffer at 20°C ?

Your forgetting that @JSHarris does what I would do, and utilises the buffer as a DHW pre-heat via a PHE fed from a second set of tappings.

For a dumb buffer I agree, as long as the ashp can draw back from the UFH water, as a medium to aid defrost, then a buffer with a unified slab circuit may well be ott.

NOTE: This would be a very different argument in a regular British BR built house however, so we must remember that these 'disciplines' need to be taken as suitable for this topic alone. Just a nudge for the wider viewing audience.  

 

[ProDave]

I only raised the idea of no buffer, and no temp control on the UFH manifold as this is what the mitsubishi packaged system appears to do and on the face of it, it seemed like a good idea.

 

Another thing to consider is the big difference between a lot of you with UFH in a concrete slab with a high thermal capacity, compared to me with a timber floor and quite possibly UFH pipes in spreader plates.  That's another thing to consider when deciding which way to go. I can't see any benefit whatsoever in keeping the UFH flow going with no heat input in my case.
 

[TerryE]

Nick, thanks for the reminder to other readers that my whole discussion is only applicable to passive-class houses.  I should have emphasised this earlier. 

 

5 hours ago, Nickfromwales said:

Your forgetting that @JSHarris does what I would do, and utilises the buffer as a DHW pre-heat via a PHE fed from a second set of tappings.

 

I hadn't forgotten that, which is why I added my predicate. I've discussed this use before: my concern is that for ~8 months a year if I used a buffer tank then I would only be cycling the ASHP to keep it at 35°C say (with the associated heat losses) to save some E7 units.  Averaged over the last 4 years, we use ~240 ltr /person / per day and maybe (on average) 30-40% of that is DHW, say 240 ltr raised from 10°C to 40°C or 7.2 MJ or 2 kWh at E7 rate = ~13p/day.  (We run our dishwasher and waching machine coldfill overnight to use E7 units).

 

Using a buffer tank would save maybe half of this, but the electricity would be mostly at peak rate, so when you factor COPs vs E7:Peak and heat loss from the buffer tank, then I might be looking at a saving of £10 p.a.  I accept that the other major advantage is that doing this effectively doubles his SunAmp capacity.

 

But I've also been thinking of taking this "use the slab as a buffer tank" paradigm one step further and adding a PHE into the UFH circuit.  This way we could use the slab to preheat the main CW feed going into our SunAmp to ~20°C year-round, which will increase its effective capacity in winter by roughly a half -- for the cost of a PHE. So yes, having a bath in winter will suck a bit of heat out of the slab.  Big deal.  I plan to do the major top up using E7 electricity anyway.  The rest of the year, taking a little bit of heat from the slab is no bad thing as we are in heat excess.

 

@ProDave,  we have an MBC frame and the thermal capacity of the cellulosic filler is pretty high -- roughly half of that of the slab.  But yes, there is no point in circulating water when you don't need to.  What I am planning to do is to monitor the out and returns sides of each UFH circuit with a DS18B20 digital thermometer.  This have an absolute accuracy of ~½°C but an an adjusted relative accuracy of far better than 0.1°C.  I am monitoring all loops to help me balance my loop flows.  But yes once the slab is in steady state (and the temperature monitoring will tell my system this, then there is little point in having the circulating pump on.  

[SteamyTea]

On 02/12/2016 at 11:43, TerryE said:

But I've also been thinking of taking this "use the slab as a buffer tank"

I like this idea as after a bath, I find that I feel quite warm and would need less heating anyway.

Also made me think of something that Joe90 was saying about putting in a bit of pipework into his slab where he expects to get some solar gain (he ain't realised that it is always cloudy in Cornwall yet).

Using a multiple coil set up in a slab would also give some flexibility if things don't work according to plan i.e. you can add or remove extra heating easily.

[Ferdinand]

On 01/12/2016 at 23:18, TerryE said:

 

Note to self: need to add a Strikeout icon to the editor bar! 

 

^^^ This.

[TerryE]

On 01/12/2016 at 23:18, TerryE said:

Note to self: need to add a Strikeout icon to the editor bar! 

 

I've already added it some time ago.  First button in the second group from right when using the large edit window (PCs, Laptops, and most tablets)  Duhhh!

[Barney12]

5 hours ago, TerryE said:

 

I've already added it some time ago.  First button in the second group from right when using the large edit window (PCs, Laptops, and most tablets)  Duhhh!

 

Ooooh useful

[ProDave]

27 minutes ago, Barney12 said:

 

Ooooh useful

But you very craftily hid the strikethrough button way over there with the ^superscript and _subsctript buttons. Not with the bold, italic and underline buttons where most people would look for it.


 

[DavidFrancis]

Wasn't sure where to post this as JSH's blog on here isn't open to comment, but this thread seems fairly relevant.

 

Just in case this get overlooked by some, in his latest installment he says he has stopped trying to control the slab temperature and has found that just using a room thermostat works far better.

[Guest Alphonsox]

I remember discussing this at the time on the "other place". We never got to a good explanation as to why this was the case. I intend to set my system up to enable the temperature multiple points to be monitored and decide which to use at a latter date.

[TerryE]

David, Neil, I want to cover this in detail separately but the thermal flows in the slab make it very problematic using point monitoring in the slab as a feedback for control.   I will be using a different remote sensor:  the temperature of the UFH water return.  In JSH's case he has a Genvax which does active control of the air temperature on a far shorter time constant than the slab's so using room stats are even more problematic.  Need to finish that blog blog post one this.

[PeterW]

@TerryE I considered the same approach but I was trying to work out how to manage different temperature returns from different loops - does the return temp method not rely on having multiple zones and multiple sensors..?

[jack]

 

1 hour ago, PeterW said:

@TerryE I considered the same approach but I was trying to work out how to manage different temperature returns from different loops - does the return temp method not rely on having multiple zones and multiple sensors..?

 

Not if you only have a single "zone", as many of us running a passive slab have.

 

That said, it wouldn't much more difficult to track the temperature of the return on each loop as it comes up out of the floor.  I've had good success just using some plastic wrap to hold a one-wire sensor against a copper pipe.  How much slower will multi-layer UFH pipe be to react?  Given the time constants involved, I suspect it'll be fast enough.  

[PeterW]

I think thats my "issue" though @jack - If you control the slab by the return temp, what happens when you have say south facing areas that have warmer slabs than the cooler ones..? You want to use that heat to be redistributed across the cooler zones but if you use multiple heads to control the circuits you then lose that heat source.

 

Or I may just be over thinking this....

[jack]

Are you saying that if a loop on the sunny side gets warm, its valve will close, so the heat won't be distributed (and, even worse, the temp in that zone will continue to rise because the fluid can't get out of the loop?)  I guess that's potentially an issue if you have per-loop/zone control.  

 

With a smart enough controller (ie, not your usual thermostat), you could actually control the valve on the hot zone to open wider in this situation, so that the heat is shared/dumped more quickly with/to the other zones.  You could also potentially do some clever things like averaging across zones to see whether you actually need more heat or just need time for the hot zone to blend with the rest.

 

I must say though, if you have enough insulation under your floor, single zone control is a lot easier! 

[TerryE]

1 hour ago, jack said:

That said, it wouldn't much more difficult to track the temperature of the return on each loop as it comes up out of the floor.  I've had good success just using some plastic wrap to hold a one-wire sensor against a copper pipe.  How much slower will multi-layer UFH pipe be to react?  Given the time constants involved, I suspect it'll be fast enough.  

 

I've got three loops in a single zone and what I am proposing is to use 6 × DS18B20 sensors with a thermal gel and taped to the PEX, then insulated around each pipe + sensor.  These are positioned as the PEX leaves the slab, and I will attach a couple more at the two manifold inlet / outlets.   This might seem like overkill but the DS18B20s are cheap (IIRC, I last bought 10 in waterproof sleeves for £15) and using one-wire to connect them up to a Wemos ESP8266 module is another £10-20.

 

My main reason for this is instrumentation and calibration.  With enough data, I can correctly balance the 3 loops and also get a good handle on the transfer function of the slab.   However I also propose to use them for my control.  My measurement cycle will be something like running the pump for couple of mins whilst I collect temps.  If the temps are imbalanced then this would indicated that it might be worth running the pump to help redistribute the heat in the slab, but if they are much of a muchness then there isn't any point in wasting the electricity.

 

I am mainly doing this because I get pleasure out of this sort of project.  My ultimate control strategy will no doubt be a lot simpler, but with this data I will have an evidence-based understanding of how effective it is.  

[jack]

1 hour ago, TerryE said:

I've got three loops in a single zone and what I am proposing is to use 6 × DS18B20 sensors with a thermal gel and taped to the PEX, then insulated around each pipe + sensor.  These are positioned as the PEX leaves the slab, and I will attach a couple more at the two manifold inlet / outlets.  

 

Sounds about right.  I wouldn't get too concerned about insulation - if the sensor is held in decent contact with the pipe (+ thermal gel), even a small amount of insulation will be more than sufficient I suspect.

 

1 hour ago, TerryE said:

This might seem like overkill but the DS18B20s are cheap (IIRC, I last bought 10 in waterproof sleeves for £15) and using one-wire to connect them up to a Wemos ESP8266 module is another £10-20.  

 

Doesn't sound like overkill at all.  I have exactly those sensors (or perhaps the other type that's nearly as ubiquitous) wired into my home automation system: one in each of two positions in the slab, another two at different heights in the UVC, and two used behind switch covers upstairs and downstairs as air temperature sensors.  I've stolen one of the UVC ones now and then to temporarily attach it to the UFH manifold.  I need another couple to do slab temperature control, but it isn't a priority at the moment.

 

1 hour ago, TerryE said:

I am mainly doing this because I get pleasure out of this sort of project.

 

That's the best reason to do anything like this.  I just wish I had the time!

[TerryE]

@jack, my basic thesis is that with the thermal capacity of the slab and the time constants of the house we really don' t need a fine control.  My intent is simply to dump large chunks of heat into the slab on an occasional basis.  The exact amount per chunk isn't really that important.   This could be an inline electrical heater dumping into the slab for 3 hrs @ 3 kWh during the E7 window; it could be running the ASHP at a set 30°C, say, for 30 mins.  I doesn't really matter that much whether we have 1, 2 or  10 chunks per day; the only issue is that the chunks must add up over the day to whatever the house needs as input to maintain an environment within the target temperature tramline.  One simple strategy to ensure this is to establish a minimum back-off between chunks and trigger the next one when the flow return temperature from the slab falls below a preset threshold.

 

As to the time bit, well that's what you get when you eventually retire -- though there still don't seem to be enough hours in the day!

[le-cerveau]

Peter,  I thought the idea was to run the house as one slab, irrespective of how many loops you have in the slab!  This will then re-distribute the heat/cool around the house.  So measuring each loop is interesting but you want to control the house from the manifold temp, or as Jeremy did the room temp.  As I understand it you can't heat different parts of the hopes to different temps in a Passive type house situation, it just doesn't work as heat will migrate from the warmer parts to the cooler parts before it escapes into the outside environment.

[TerryE]

@le-cerveau, that's precisely correct: you may as well control from a single aggregate temperature. 

 

However, you also need to trim the manifold valves so that the flow-rate through each loop is roughly proportional to the area covered by that loop.  This is a one-off calibration / setting issue.  One simple way to achieve this is to balance the loop lengths within 10% say and use the same loop density on all floors, then you can just fully open all manifold valves.  The other way if you are using separate temperatures is to use the temperature curves on the 3 loops to determine if one is out of balance.

[MikeSharp01]

1 hour ago, TerryE said:

I doesn't really matter that much whether we have 1, 2 or  10 chunks per day

Not sure Terry, have you factored in the expansion / contraction of the slab, I think I worked out that across our slab (15m) the expansion per degree was around 0.2mm per Deg C (K) @ 14.5 x (10^-6 m/(m K))  {always assuming I have my powers of 10 right} so to stop / reduce movement I was sort of expecting to do my utmost to keep the slab at a constant temperature rather than cycle it. Given your wall construction (elsewhere here described) excessive slab expansion / contraction might appear in your outer leaf!

[TerryE]

@MikeSharp01 Mike, if I crank the numbers, my slab has a thermal capacity of ~23 MJ/K or 6.4 kWh/K.  Using my 9 kWh "overnight E7 chunk" example, this represents about 1½°C temperature rise or 0.2mm across our house frontage of 11.5m.  This is small beer compared to the internal expansion / contraction of the outer stone skin itself.

[jack]

4 hours ago, TerryE said:

@jack, my basic thesis is that with the thermal capacity of the slab and the time constants of the house we really don' t need a fine control.  My intent is simply to dump large chunks of heat into the slab on an occasional basis.  

 

Agree completely.  I was always surprised by suggestions that a "slab" temperature could be controlled to within a fraction of a degree C.