Storage Heaters

[AGT]

[AGT]

Fitted a short while ago, and utilising Tapo TP Link smart plug with energy monitoring.

[robl]

None of this exists yet- the house is 1920’s, dreadful. But it will have 150mm Ewi, u=1.1 windows, lots of fluff in the loft. The original plan was for the floor to be ripped out, insulated, replaced and a tepeo electric boiler installed…. Have suggested use an insulated thick ufh ‘cemfree’ slab as a heat store- this would be preferred, as it saves space. The slab would have lots of pipe embedded in it, there would be a pump, and I think three immersions in Willis heaters heating it all up. I think it’s around 40m^2 slab, 0.2m thick, so 8m^3, storing the immersions 3kWx4hr max =36kWh in the slab at 4.4kWh/degC, giving a maximum peak-peak ripple of 8C.
The lack of a suitable automated controller is a nuicance though, and a bit weird given all the so called ‘smart’ controllers there seem to be these days. We could maybe cobble something out of a Shelly and a mains contactor on a din rail, but it won’t have a ‘nice’ user interface. I’m imagining proportional control, at a basic level storing Energy=k.(Thouse-Tsetpoint). It would be nice to be able to control Tsetpoint without a computer though.

[Oldgreybeard]

I think the absence of an off-the-shelf automated control solution may well be down to a couple of factors. My experience was that it's not at all easy to do, as the only predictive data source that's readily available (via the Met Office API for the nearest Met Office station to your location) is, in my experience, just nowhere near accurate enough.

As an example, I pull data from our nearest Met Office location, using their API, and feed it into Home Assistant, purely as a forecast, nothing more. They offer a 3 hour ahead data set, that gives basic data for 15 hours ahead, roughly (right now the API is giving me forecast data for 21:00, 00:00, 03:00, 06:00 and 09:00). The temperatures they are giving for each of those data points will almost certainly be wrong, they are wrong far more often than they are right. The Met Office site is roughly 9 miles east of us.

The other factor is that coming up with an effective predictive energy input algorithm is hard. Much, much harder than I ever thought it might be. The main problem I ran into was the non-linear response time, between energy going in to a large concrete mass, and the surface temperature of that large concrete mass actually changing. I've got sensors buried all over the place in the concrete floor slab (I buried DS18B20s at various depths and locations when the floor was poured), and I just couldn't work out any sort of meaningful input energy versus output energy with respect to time characteristic that fitted what really seemed to happen.

Admittedly I used a pretty simplistic approximation for the surface temperature versus heat output with respect to room temperature, the one used by underfloor heating design engineers: Heat output (W) = (8.92*(Floor temperature(°C) - Room Temperature (°C)) ^1.1) * Effective floor surface area (m²)

Not sure that was the reason for the issues I had, I think they were mostly related to the poor quality of the forecast data and the time delay between heat energy going into the slab and the surface temperature changing (the latter being key to working out the heat output to the room).

[Countrypaul]

I have also noticed how difficult it is to predict how much heat will be required in the floor slab from our UFH. For reference we have a 70mm think S&C screed on the ground floor which required 22 tonnes of sreed to be laid. We have individual room themostats which I have set to a slightly higher than required temperature overnight (starting at 00:30 E7 start) droping back at 07:30 (end of our E7). Often if the UFH will not come back on until the following night again after stopping at 07:30. Odd imes one zone may come on (e.g. the hallway if the front door has been open for a while due to grocery delivery for example) but that is often satisfied from the thermal ore without riggering the ashp,

One (obvious to some of us) thing I have noticed though, and that is the amount of heat let out of the floor slab on apparently near identical nights/days does vary with what has been left on the floor (most noticable in the kids playroom). If the floor has been left partially covered with toys/blankets/etc. then the room can be significantly cooler than some expect in the morning, and when uncovered the floor can feel significantly warmer than they expect.

I know that outside temperature is also not the only factor affecting heat requirement, the wind can have a significant effect proably direction dependant. I am not sure whether snow as opposed to rain makes any difference if the temperature is hovering at or just over 0C.

[robl]

It sounds just too difficult to predict the energy required then, to have a simple massive slab. What about something a bit more complex?
200mm insulation
200mm slab - or is there any low carbon other heat store?
2cm of insulation, clip ufh pipes to
Bury pipes in thin screed

The above is normal I think. What I propose though, is also get pipes into the 200mm slab. Warm it up to 30C overnight with Willis heaters on a timer - they shutoff at a given return temp, it then has 1.1kWh/m^2 stored. The 2cm of insulation slows down the leakage of heat - I calc it has a 5 day time constant to leak out-dreadful. But we have pipes! With a conventional thermostat control , run both lower slab and upper screed pumps on demand, and heat can be released to the thin upper screed.

Thoughts?

[Stinsy]

It sounds just too difficult to predict the energy required then, to have a simple massive slab. What about something a bit more complex?
200mm insulation
200mm slab - or is there any low carbon other heat store?
2cm of insulation, clip ufh pipes to
Bury pipes in thin screed

The above is normal I think. What I propose though, is also get pipes into the 200mm slab. Warm it up to 30C overnight with Willis heaters on a timer - they shutoff at a given return temp, it then has 1.1kWh/m^2 stored. The 2cm of insulation slows down the leakage of heat - I calc it has a 5 day time constant to leak out-dreadful. But we have pipes! With a conventional thermostat control , run both lower slab and upper screed pumps on demand, and heat can be released to the thin upper screed.

Thoughts?

You want the UFH beneath the slab...

[Oldgreybeard]

You want the UFH beneath the slab...

Surely having the UFH pipes on the lower side rather than in the centre or closer to the upper surface increases the heat losses downwards, doesn't it?

We have 300mm of insulation under our concrete slab (which I know is a bit OTT) but we still lose around 10% or more of the heat we put into the slab down through the insulation to the cooler ground beneath. Our UFH pipes are just above the centre line in our slab, tied with cable ties to the steel reinforcement mesh, that's stood off the insulation on 50mm pillars (the slab is 100mm thick). The pipes are standard 16mm PEX UFH pipe, so the top of the pipes is not that far below the top surface (I think the steel mesh is around 6mm, so the top of the pipes will be around 28mm below the surface).

Concrete is not that bad a conductor of heat, and the sensors I've got buried in it at different depths always show that the surface is slightly cooler than the bottom or the centre with heat pump not running. The bottom and centre are always pretty close to being identical, well within the error of a DS18B20 sensor, anyway, without the heat pump heating the pipes. When the heat pump is running the centre is always warmer than the bottom or the top by a fair bit.

[openspaceman]

We have 300mm of insulation under our concrete slab (which I know is a bit OTT) but we still lose around 10% or more of the heat we put into the slab down through the insulation to the cooler ground beneath.

Doesn't this even out a bit over time? In the past I read about only insulating the perimeter of the slabs down a few feet to create a heat island. The theory being once you cut heat loss through the sides of the building the "island" of floor slab and soil reaches an equilibrium.

[Oldgreybeard]

We have 300mm of insulation under our concrete slab (which I know is a bit OTT) but we still lose around 10% or more of the heat we put into the slab down through the insulation to the cooler ground beneath.

Doesn't this even out a bit over time? In the past I read about only insulating the perimeter of the slabs down a few feet to create a heat island. The theory being once you cut heat loss through the sides of the building the "island" of floor slab and soil reaches an equilibrium.

TBH I'm not sure. One thing I regret is not having put temperature sensors in the ground under our insulation, but it was just too difficult to do when the place was still a construction site, the big machines on site would have been bound to damage them. Our insulation sits on a bed of coarse, compacted, stone, that has drains all around the edges, and I suspect we probably get some water flowing through that which shifts heat away in wet weather, despite the fairly large roof overhangs. We do have 200mm of insulation around the edge of the slab, to reduce the heat loss out that way.

I had a go at poking a long length of 10mm steel tube, with a temperature sensor epoxied into the end, under the house from the side, but the compacted stone meant I couldn't get it far enough in to get a reasonable reading. It seemed to be about the same as the deep ground temperature though, which sits at between 8°C all year around. That suggests there is probably a temperature differential of around 14°C or so between the base of the slab and the underlying ground. The UFH pipes usually run with a return temperature of around 24°C, so if they were at the bottom of the slab then with the heat pump running that temperature differential would increase by a couple of degrees. Doesn't sound much, but it would increase the heat loss through the floor by maybe 10%, and every little bit helps, I think.

I'll freely admit it is far too easy for me to get obsessive on measuring and trying to reduce losses! I think my brain is hard-wired into the "insulate, insulate, insulate!" mantra, and once you start measuring things, following the numbers can start to get a bit addictive.