Modelling the "Chunk" Heating of a Passive Slab

[TerryE]

Nick, my only comment is that I'd avoid mixing potable water and UFH so you need a PHE somewhere, probably the one that both Jeremy and I use would be fine.  Plus a lot of insulation.  Not sure why you need 2200 Kg × 50° worth of storage though. 

 

I have this vision of having a Rocket Stove below ground level under my workshop / shed together with a couple of 1 tonne tanks and maybe 200mm EPS wrapper.  IMO, the trick with a RMH is always to use power assisted draw so that you can take the exhaust gas through cob store / exchanger well below 100°C so that you recover the latent heat of evaporation -- so the combustion is getting close to 100% efficient, and with flue gas getting over 1,200 K in the main burn chamber there are none of the usual nasty particulates left -- but it's only a dream, as the boss has the veto on such ideas One of the downsides of an otherwise superb wife.   

[SteamyTea]

57 minutes ago, TerryE said:

I'd avoid mixing potable water and UFH

It would be a stand alone system, just that the cylinders are relatively cheap, maybe second hand if I can find them.

58 minutes ago, TerryE said:

Not sure why you need 2200 Kg × 50° worth of storage though. 

Don't, just that two 200lt (400 kg) cylinders will fit under the stairs and can be plumbed in easily.

To the storage capacity for a the worse case, I would need to store 24 kWh above 30°C (ish).

Also using two cylinders allows me to only use one when the weather is not so cold.

Heat loss from the system is not such an issue as it will all be within the insulated envelope and only used when heating is needed.

 

I would have thought someone would have commented on routing out the floor to fit the pipework.

[TerryE]

Sorry I took 2, 200 as 2,200 not 2×200

[Jeremy Harris]

Rather than 2 x 200 litres of hot water, what about salvaging a few old storage heaters and adding a heat exchanger? The bricks in storage heater have a higher heat capacity than water, I think, they already have heating elements in grooves in them and it would be relatively easy to add extra insulation plus a simple loop of pipe to extract relatively low temperature water from them, I think.

 

I've mentioned this before, but a friend who converted the old Methodist chapel at Sithney many years ago, built an insulated room in the centre of the building, filled it with big rocks he collected from the beach and ran old storage heater elements through them.  He built a wind generator and that dumped all it's energy into the heating elements.  Every room had a closable grille that led to the heat store, so could be heated as required by just opening the grill and turning on a low power fan.

 

By complete coincidence, my great grandfather had been the minister at that chapel in the 1890's, and my great aunt Muriel was born in Sithney and baptised in that chapel.

[Stones]

I really do like the elegance of simplicity of being able to use a Willis heater to provide all the heat input into a house.  I talked about such an approach with my plumber when planning our build and I recall him referencing one of his customers who used an electric towel rail element to perform the same function.

 

Sadly for us, our heating requirement exceeds the circa 2500 kWh tipping point where direct electric was the most cost effective option.

[SteamyTea]

I did think about modifying some storage heaters, but the SHC is a nats under 2 kJ.kg^-1K^-1.

Thing is with water, it is easy to use, cheap to buy, all the bits are already made, and has a second hand value if the idea is a disaster.

The real problem is that I really don't need much heating, so can get away with a fan heater when I do need it.

I think last years bill was under £400.

This year there has been a large hike in the day rate for E7 customers, the night rate is not so bad still.

[jack]

Is this the Willis heater?

 

 

 

How's it plumbed in? Is it in a bypass loop?

[Jeremy Harris]

This is what a Willis heater looks like:

 

It's just an immersion heater set into a copper tube with an inlet at the bottom and an outlet at the top.   They are mounted vertically and in their original application, in Ireland, mainly, they were added to the side of an existing hot water cylinder.  They are pretty cheap, around £35.

Edited November 9, 2017 by JSHarris

[jack]

I know what a Willis heater is thanks (I seem to recall discussing whether to install one as the primary immersion way back on ebuild when doing our initial design), I'm just interested in how Terry's arranged it within his system.

 

The arrow I put in the pic above points to something vaguely the right shape, and I can't see anything else suitable in the pictures.

[Dudda]

19 minutes ago, jack said:

I know what a Willis heater is thanks (I seem to recall discussing whether to install one as the primary immersion way back on ebuild when doing our initial design), I'm just interested in how Terry's arranged it within his system.

 

The arrow I put in the pic above points to something vaguely the right shape, and I can't see anything else suitable in the pictures.

It's in between the UFH manifold and the black plate heat exchanger on the other photo. It has a white circular cap and two white stickers on it. You can just about make out 'Willis Jacket' on the top sticker. 

[jack]

Good spot, thanks!

[TerryE]

Sorry, thought that you'd see it.  Your arrow is pointing to the TMV mixing PHE heated cold + SunAmp hot into the DWH manifold box. 

 

This is what you are looking for.  Apart from the UFH pipework in the slab, this is my entire central heating system:

 

Now that I've got DS18B20 themos on 3 × out and 3 × return and on the willis  (and a couple on the DCW in and out of the PHE).  You don't need thermos in the slab.  Just run the pump for 5 mins and take the average temp of the water coming back in the returns.

[readiescards]

On 09/11/2017 at 10:22, JSHarris said:

They are mounted vertically and in their original application, in Ireland, mainly, they were added to the side of an existing hot water cylinder.  They are pretty cheap, around £35.

 

Do they have to be mounted vertically?

[joe90]

On 08/11/2017 at 22:00, SteamyTea said:

I usually think about space heating for my place this time of year.

I am rather limited in what I can do, and hate spending money.

As I have a particle board floor over polystyrene, I could router some semi-circular groves in it and fit UFH pipe and cover back over with the parquet flooring.  Then connect that to a cheap pump and hot water cylinder.

As the floor area that needs heating is probably about 20 m² and only needs about 1 kW delivered to it to heat the whole house, I would only need to store 24 kWh.

I could store that in 2, 200 lt E7 cylinders (I have room under the stairs) and run them up to 85°C.

 

Not sure how realistic that is, but probably a fairly cheap to find out and I would be left with UFH pipework if I wanted to connect to a heat pump.

 

I may have to model it.

 

 

 

ST I have a whole roll of UFH pipe left over plus lots of offcuts if you want it to play with.

[Jeremy Harris]

19 minutes ago, readiescards said:

 

Do they have to be mounted vertically?

 

 

Probably not, as long as you arrange for one pipe inlet/outlet to be vertical (to let air out) and also arrange for the pump and heater to be wired to the same circuit, so there is always flow through the heater.  Willis never designed these heaters to be used horizontally, or with a pump, they were designed originally to be used vertically with a thermosyphon, but the smaller, but similar, immersion heater in a tube that is inside a Sunamp PV is horizontal and works just fine.  It has a similar resettable over-temperature cut out as a modern immersion fitted to a Willis heater, and is arranged so that the heater can only operate when the pump is circulating water through the heating chamber.

[dpmiller]

Can't find any MIs for the Willis, but looking at the label on this example implies horizontal fitment

 

http://www.pdplumbingsupplies.com/products/rohs-external-immersion-heater-also-known-as-a-willis.html

[TerryE]

1 hour ago, dpmiller said:

looking at the label on this example implies horizontal fitment

 

In the typical installation which is external heat for an indirect HW cylinder they are fitted vertically as the thermal gradient is the primary pumping action.  In this case the thermal effect is trivial compared to the UFH pump so that can be fitted either way.

 

You can see all of the ds18b20s in this figure, plus the PHE.  The typical delta is -5°C out -> return at the manifold and 10°C return -> Willis top so the water flow is ~50:50 through the Willis and the PHE.

 

The 5°C is at the medium pump flow setting.  At high setting this drops to ~3°C. but I am happy with 5°C and very little pump noise.

 

BTW, if you want to use the out/returns to measure the slab temperatures then you need to run the pump for 5 mins first.  

 

@readiescards, I am happy to talk through all of this stuff with you, but give it 3 days or so because I've had an absolutely shitty cold for 3 weeks and still can't speak easily.  Also like Jeremy and others, I do offer an open invite to forum members to come and to see my build and to discuss any issues that they want.  Lincolnshire to Northamptonshire isn't that far.  If you want to do either, then just PM me, though can you give me your email addr because I prefer using email.   

Edited March 28, 2018 by TerryE

[TerryE]

While I think on, IIRC they have a nominal maximum rating of 6 bar, but I run my UFH loops at 1 bar.

[SteamyTea]

3 hours ago, joe90 said:

if you want it to play with

Thanks

I may well take that off your hands.

I like playing.

[Nickfromwales]

1 hour ago, SteamyTea said:

Thanks

I may well take that off your hands.

I like playing.

Factor in for aluminium spreader plates as the difference is chalk and cheese.