UFH - Wiring Requirements

[ragg987]

24 minutes ago, Nickfromwales said:

If you simply add a cheap TMV prior to the secondary pump you'll be able to store the buffer at 40oC, which will provide a much better preheat temp and virtually wipe out any low flow hysteresis and cycling caused by running 'direct'.

Not sure I fully follow the logic here. If the slab requires 1kW to heat the house and the ASHP can only modulate down to 2kW, then on average the system will need to be on 50% and off 50% of the time. This is regardless of the flow temperature - i.e. 40C or 25C - load is still 1kW.

 

The only benefit I can see is that water at 40C holds more energy than water at 25C, so the frequency of cycling will reduce in this scenario and you would get longer burns and longer intervals between burns. However the potential downside is that heating water to 40C will be more inefficient (lower COP) than heating it to 25C. 90l buffer helps.

24 minutes ago, Nickfromwales said:

PS, I'm here to pick up advice too, and am trying to forget the way I used to think and undertake jobs. This place is a goldmine of irreplaceable, invaluable and impartial knowledge. ?

Aren't we all! We do have a great community, inquisitive minds, and willingness to share and challenge. Long may it continue.

 

24 minutes ago, Nickfromwales said:

Agreed, but it is a design in its barest, crudest form imo.

And this is the primary attraction of the design for me!

Edited September 30, 2017 by ragg987
clarification

[Jeremy Harris]

13 minutes ago, ragg987 said:

You just reminded me - another reason I wanted to avoid a TMV is that it would be a single fixed flow temp setting. So if I set it for coldest day then on a milder day flow temp would be too high and system would have to cycle on and off or room temp would overshoot. Again, I am describing the theory as I understood it at the time but have learnt quite a bit about our system and the behaviour since then. At the moment I am fully at the mercy of the ASHP controller logic (which I quite like in reality - why reinvent the logic if a large company with millions to spend on R&D has already done it?).

 

 

It doesn't work like that.  All that happens is that the ASHP stays on for longer on a cold day - I don't fiddle around changing the UFH flow temp. 

 

I did spend a lot of time trying to make weather compensation work with a low heat demand house, but failed miserably.  I have around 30 variations of software I wrote to try and do closed-loop control of the slab and house temperature, before I gave up.  The problem is that the uncontrollable variables are of a similar magnitude to the controllable variable, and can only be indirectly sensed from the room temperature. 

 

For example, say the system is stable, with the slab delivering around 300 W of heat to the house (a pretty typical cool, but not cold, winter heating level).  We get two visitors arrive, and the house suddenly gets a heat input jump of around 160 to 200 W.  The room thermostat detects this when the room temperature rises 0.1 deg C above the set point, but the slab, and the water in the UFH loops, is still sitting at 24 deg C.  It therefore carries on putting out heat for ages, even with the ASHP off, until there is a new equilibrium between the heat output from the very slightly cooler slab, the heat input from the occupants and the heat loss (which won't have changed if the outside air temperature is the same). 

 

The result is that there is a temperature overshoot, and it's quite easy for this to get to 23 deg C or so, which is uncomfortably warm. 

 

This control challenge is, in my view, one of the biggest things to overcome with low energy house design.  The easiest way to limit it's impact is to make sure that the unheated inner surfaces of the house are both relatively thermally conductive and have a high heat capacity, as then they will tend to absorb some heat from the overshoot, so reducing it's magnitude.  This works reasonably well, but isn't great, as air is not a particularly good conductor of heat, and the rate of heat transfer to the walls and ceilings is not fast.

 

 

[ragg987]

4 minutes ago, JSHarris said:

For example, say the system is stable, with the slab delivering around 300 W of heat to the house (a pretty typical cool, but not cold, winter heating level).  We get two visitors arrive, and the house suddenly gets a heat input jump of around 160 to 200 W.  The room thermostat detects this when the room temperature rises 0.1 deg C above the set point, but the slab, and the water in the UFH loops, is still sitting at 24 deg C.  It therefore carries on putting out heat for ages, even with the ASHP off, until there is a new equilibrium between the heat output from the very slightly cooler slab, the heat input from the occupants and the heat loss (which won't have changed if the outside air temperature is the same).

This is indeed a challenge. In our case, I have decided it is best to keep the heating on 24x7 and let the environmental conditions determine the flow temperature.

 

In your example, the 0.1C increase in room temperature would be fed back to the Hitachi controller which would compensate by dropping the flow temperature a tiny bit, thus reducing the heat fed to the slab. Of course the room will continue to heat as the slab change will only impact the room a few hours later, however we are then reliant on the thermal mass of the house to try and steady the impact of instantaneous changes in conditions.

 

The same issue happens when the sun shines into the room on a very cold day. Solar gain is tremendous, room temperature will increase quickly. The only solution would be solar shading - I don't think any heating system based on UFH could compensate for this - we are then in the territory of air conditioning.

 

@Barney12, I am sorry I have hijacked your thread way beyond the initial question you posed.

[Nickfromwales]

Having the UFH simply running 24/7 is not how I would design it, especially in a low energy dwelling, if I understand correctly and you aren't regulating by a roomstat.

At the very least I would want a stat like Jeremy managed to find ( with 0.1oC increments ) so the heating flow would shut off with incidental temperature gain. 

If the house temp rises this way surely it would be crazy not to have a means by which the system recognises this in order to stave off further heat injection?

Fwiw, i think your 'getting away' with this because each instance is unique. I believe  your house is capable of absorbing and dissipating the heat as the HP provides it, even with the flow temperature linear to the HP hysteresis, whereas in a better / smaller / even lower energy dwelling it may be problematic. Take @JSHarris and @jack's homes, detailed similarly in terms of the requirements and equipment to maintain them, but @jack has found he would have benefitted from some UFH upstairs. 

Also we cannot take into account personal comfort levels and perceptions, so as said, these all need to be accepted as unique cases and advice considered accordingly.  

Plumbing? Piece of cake

 

[Jeremy Harris]

There's no such thing as "thermal mass", because mass isn't related to thermal stability at all in this context; what does impact on the house thermal time constant is the heat capacity and thermal conductivity of the internal surfaces and the house contents, as mentioned above.

 

The practical problem I've found, after three years of experiments, is that, because the heat capacity of the concrete slab is relatively low (the mass heat capacity of water is nearly 5 times that of concrete, for example) and the thermal conductivity of both water and concrete are broadly similar, heat transfer to the slab is relatively rapid when the UFH is running.  However, if the UFH is turned off, because the room thermostat has stopped calling for heat, the slab continues to provide heating for a long time, depending on the temperature differential between the air immediately above the slab and the temperature of the slab.  What's more, the heat transfer rate during this overshoot period is neither linear nor even, some parts of the slab will transfer heat at a greater rate than others, because of the variation in heat transfer rate caused by local effects (air flow, objects on top of the slab, floor coverings with both a differing surface emissivity and thermal conductivity, etc).

 

I got around the latter problem by running the UFH circulating pump even when the TMV or the valve controlling flow to the UFH are turned off.  Circulating water around the loops all the time acts to even out the slab temperature, lowering the temperature of the warmer spots and raising the temperature of the cooler spots.  This is extremely effective at controlling solar gain from sunshine that warms the floor, as it lowers the local floor temperature and significantly reduces the rate of heat transfer to the air.  This is particularly effective because the rate of heat transfer with temperature differential is non-linear, for a floor surface with a typical emissivity and surface convection rate it approximates to about 8.92 * Δt ^1.1 W/m².

Edited September 30, 2017 by JSHarris

[ragg987]

32 minutes ago, Nickfromwales said:

Having the UFH simply running 24/7 is not how I would design it, especially in a low energy dwelling, if I understand correctly and you aren't regulating by a roomstat.

At the very least I would want a stat like Jeremy managed to find ( with 0.1oC increments ) so the heating flow would shut off with incidental temperature gain. 

If the house temp rises this way surely it would be crazy not to have a means by which the system recognises this in order to stave off further heat injection?

Fwiw, i think your 'getting away' with this because each instance is unique. I believe  your house is capable of absorbing and dissipating the heat as the HP provides it, even with the flow temperature linear to the HP hysteresis, whereas in a better / smaller / even lower energy dwelling it may be problematic. Take @JSHarris and @jack's homes, detailed similarly in terms of the requirements and equipment to maintain them, but @jack has found he would have benefitted from some UFH upstairs. 

Also we cannot take into account personal comfort levels and perceptions, so as said, these all need to be accepted as unique cases and advice considered accordingly.  

Plumbing? Piece of cake

 

I do have a room stat, but it does not directly switch the flow on or off. The control is through feedback. The stat set temp is the target from controller perspective. It them compares actual room temp, external temp and compensation curve to decide the UFH flow temp. So all else being equal, an increase in room temp above set will reduce flow temp. In my view this is better than any on/off mechanism that a traditional stat would provide as it is fully adaptive. The Hitachi also has an option for a fall-back mechanism - if room temp goes too high above set it will stop the flow (though like Jeremy I have my circulators running on a timer to passively equalise the slab).

 

Here is another example (based on real observation last winter). External temperature drops suddenly from 10C to freezing. House starts losing heat quicker than before so HP needs to increase flow temp. The need for that increase is not immediate - house fabric has maybe 6-10h decrement delay (my estimate based on 10-12h for MBC fabric plus perhaps 1h for glazed portions). Plus slab will take maybe 3-4hrs to pass the effect of hotter water to the house. So I need to delay the ramp up of flow by perhaps about 5h so that I can pre-empt the impact of the cold snap and keep the air inside the house at a constant temperature. I have set the controller to react to external temp based on average of the last 6hrs. This has the effect of delaying the flow temp ramp up so it is gradual and the peak is delayed by a few hours. Perfect closed-loop?

 

I think we are looking at very different principles hence we are apart. I am looking at heat-input balancing heat-output based on controller providing infinite flow temp adjustment. The alternative of on/off is a different approach. Neither is right or wrong, imo.

 

I agree with your comment that my house is capable of absorbing heat as the HP provides it - that has to be the design objective, every house WILL lose heat when external temp is lower, it is just a question of degree. Background might help: 330m2 house, theoretical demand 9W/m2 (3kW if -4C outside), <0.6ACH@50Pa as tested. My HP is a 3HP unit (notional 7kW output). No UFH upstairs but option to heat upstairs only via MVHR (which we needed last winter). I expect that 99.9% of UK housing have higher heat requirements. If my dwelling were smaller I would choose a smaller HP.

49 minutes ago, JSHarris said:

There's no such thing as "thermal mass", because mass isn't related to thermal stability at all in this context; what does impact on the house thermal time constant is the heat capacity and thermal conductivity of the internal surfaces and the house contents, as mentioned above.

Agree I have been loose with the terminology, you have described it better than than I can.

[Jeremy Harris]

16 minutes ago, ragg987 said:

Background might help: 330m2 house, theoretical demand 9W/m2 (3kW if -4C outside), <0.6ACH@50Pa as tested. My HP is a 3HP unit (notional 7kW output). No UFH upstairs but option to heat upstairs only via MVHR (which we needed last winter). I expect that 99.9% of UK housing have higher heat requirements. If my dwelling were smaller I would choose a smaller HP.

 

 

That's a higher heating system demand than our house, by a fair bit.  The January UFH mean heating requirement for us is around 3.5 W/m².  That makes a big difference, as it means that most of the heat loss in our house is being made up for from incidental heat gains from the occupants, appliances, lighting, etc, and the heating system only needs to contribute a relatively small proportion of the total heating requirement.

 

That changes the control system dynamic quite significantly, and really just amplifies the point that @Nickfromwales made earlier that having no TMV isn't a "one size fits all" solution.

Edited September 30, 2017 by JSHarris

[Nickfromwales]

Good, healthy exchange . 

I now understand your setup isn't at all as crude as I first assumed, but c'mon.......when has assumption ever caused any issue ?

I lost all faith in weather compensation TBH, when I fitted a full Vaillant setup and even their own tech guys couldn't tell me how to get it to behave, so now that I know you have a fully autonomous setup I kinda get it. I would still raise the buffer temp for DHW but I think that would upset your system a bit. 

[ragg987]

Yep - enjoyed that, @Nickfromwales. Context is everything. I think all this theory is one bit, but the most important bit is the result - is the house comfortable and is heating performing efficiently? The first question was an 100% yes last winter, the second one I don't know but hope to measure this winter as I have added a meter to the ASHP.

 

I am astonished at @JSHarris's 3.5W/m2 requirement - a third of he PH standard, I thought we did well but goodness - is this your microclimate? We must have the same MBC construction with 300mm warmcell and u-value of around 0.1? Plus my house is larger so ought to have lower demand per m2. Dare I suggest your situation is not going to be so representative for us mere mortals?

Edited September 30, 2017 by ragg987
dropped a decimal

[ragg987]

The weather compensation and set-up flexibility in the ASHP is miles ahead of what I saw in my parents' Vaillant gas boiler with rads. That system tended to over-shoot or remain too cold if a lower compensation curve was selected. I like it.

[Jeremy Harris]

13 minutes ago, ragg987 said:

Yep - enjoyed that, @Nickfromwales. Context is everything. I think all this theory is one bit, but the most important bit is the result - is the house comfortable and is heating performing efficiently? The first question was an 100% yes last winter, the second one I don't know but hope to measure this winter as I have added a meter to the ASHP.

 

I am astonished at @JSHarris's 3.5W/m2 requirement - a third of he PH standard, I thought we did well but goodness - is this your microclimate? We must have the same MBC construction with 300mm warmcell and u-value of around 0.1? Plus my house is larger so ought to have lower demand per m2. Dare I suggest your situation is not going to be so representative for us mere mortals?

 

Our very worst case, when it's -10 deg C outside, is a total heat loss of just under 1700 W.  The heating requirement is lower than this, because of incidental and occupant heat input, so the practical worst case heating requirement is probably around 1300 to 1400 W.  That equates to a much higher 18 W/m² from the UFH, but it's a situation that so rarely occurs, and lasts for such a short period of time, that it has little impact on the mean heating requirement for the coldest month of the year.  In reality the temperature rarely drops below zero here (the January mean heating requirement is only around 260 W, after accounting for winter incidental heat gains) and if the outside temperature does drop below zero overnight that's usually during clear weather, so solar gain tends to be reasonable during the day, and quickly warms up our very sheltered spot, cut back into the hillside.  I've found that our outside air temperature tends to be between 2 and 3 deg C warmer than the Met Office historical mean monthly temperatures for our area, and, coupled with the much lower than typical wind speeds we get (so much lower accelerated convective heat loss) means our heating requirement seems to be much lower than I expected.

 

This very low heating requirement, coupled with the 7 kW max output ASHP (only chosen because it was readily available at the right price - 4 kW models seem rare and expensive), is a part of the reason I had control issues with the heating system initially.  The ASHP has a pretty good programmable weather compensation curve, that I did play around with a fair bit, but after a lot of testing and measurements I chose to set it to deliver a flat 40 deg C, no matter what the outside temperature, as it turned out that this setting gave the lowest overall electricity consumption.  In practice, the ASHP never runs at full power unless the buffer tank has been cooled down well below the 35 deg C that the tank stat is set to.  Most of the time the ASHP is barely ticking over, and it only seems to come on once a day, for an hour or two, then stays off.  The house seems to stay between 20.5 deg C and 21.5 deg C all the time, with around 0.5 deg C of overshoot from the set point, despite the 0.1 deg hysteresis room thermostat.  In practice this range of temperature variation feels just about OK.  I'd like to have slightly better control, but the sensitivity of the system, plus the relatively large effects from cooking, showering, having visitors etc is such that I doubt this is practical.

 

 

[ragg987]

Aha - engaging brain. Your 3.5W/m2 is your actual reading, my 9W/m2 is theoretical PHPP calculation at design temp. Apples and oranges.

[Barney12]

6 hours ago, ragg987 said:

 

 

@Barney12, I am sorry I have hijacked your thread way beyond the initial question you posed.

 

You all lost me a long time ago. I'll go and get another G@T. Monkey tonight, 47% vol  

[ragg987]

Enjoy, keeping dry for a few nights. Was a heavy one Thursday night. Somehow we moved to Tequila slammers !

[Nickfromwales]

3 bottles of Gower Gold, and a stinking kebab........living the dream. 

Enjoy your Saturday night folks, see you tomorrow for another exciting adventure. ??