It would be better to use an old hot water cylinder.

Where is the heat coming from that you want to dump?

Thanks for the quick responses !

The proposed build *is* using water as the heat store - maybe 240 litres, which is more than I could get into the cubby...

Otherwise, I'm not sure where I would get an old HW cylinder ? Then it might have a LEAK !
To fit in the cubby, it would have to be less than 100 cms long, laid horizontal.
I guess this would be about 100 liters ? I would have to strip the insulation off it...
If I get a new one, I'm paying for an electrical heat function that I won't be using - that's why I thought of the PEX, run off a solar collector.

Posted By: SprocketYou would do better to use water as your heat store. It has a higher heat capacity than concrete and because it is fluid it will convect so has a lower thermal resistance meaning that you can dump heat into it more rapidly than concrete. The same is also a big advantage when trying to extract heat from it.

Water has a much greater Specific Heat Capacity (water 4200J/K.kg, concrete 880K/K.kg), but it's a bit closer looked at as Thermal Density (water 4200000 J/K.m3, concrete 2112000), if the OP can find some magnetite bricks from old storage heater they manage 3893104 J/K.m3, so almost up there with water on a space for space comparison.

As to rate of heat transfer, maybe the OP wants "slow release" of heat from the cubby?

PS. Anyone interested in storing heat might be interested in where I found those Thermal Density figures. It was in a very interesting Masters Thesis
"ELECTRICAL STORAGE HEATER IN THE CONTEXT OF STORAGE TECHNOLOGIES. Author: Ignacio Becerril Romero"
http://www.esru.strath.ac.uk/Documents/MSc_2013/Becerril.pdf

Maybe you can lay your coil of PEX on the floor and then put a thick polythene bag* of water on top of it?

That way, when gently filled, the bag can fill the odd shape under the stairs (given no protruding nails) and envelop the coil of PEX.

* Water Storage Bladder?
Pillow Tank?

Many thanks for all the feedback, especially to Sprocket for the physics !

This suggestion by Crosbie got me thinking, so how does this sound to you experienced folk:

Securing (somehow…) a semi-uncoiled coil of half-inch ID PEX (L = 25 m) in an 80-liter plastic tub (dustbin) of static water. Coils oriented vertically, held in bungees.

Alternatively, use a copper coil. In which case, 10-meter coils of 14/12 mm. (Might have to be some copper in air due to roll diameter…).

Two such radiators, the coils interconnected in air, by a union.

As much floor & wall XPS as possible in the cubby; bare soffit to stairs.
(thanks to skyewright, I get the point, but space will be tight...) however, will try !

Rest of cubby volume filled with jerrycans and hi-strength poly bottles, for maximum water capacity.

Free ends of coils routed through penetrations in the crosswall, into the crawlspace, to main thermal store and circulator.

The cubby now contains (about) 200 liters of static water, with a further 50 lm x 113 cc = 5.6 liters of circulating heat if PEX), or 2.25 l (if copper).

Not much of a maths person, but reasoning like the layman I am, the volume of “involved” concrete ought to be something like: (volume of stair cage) minus (volume of air) = 0.7 cubic meters. This is for the 7-tread flight only, and does not take account of the cage walls (which will be insulated from the cubby side with 30-mm XPS).

Since concrete holds only about half the heat of water, if I have 200 liters of water at, say, 40°C, for 100% of my available heat, then 800 liters of concrete at, say, 16°C, should be able to accommodate twice my available heat, therefore the system ought to work. Would anybody be prepared to guess the likely increase in temps of my stair flight ?

Many thanks again,


gyrogear

One wonders what sort of gear you want to grow under there :-)

Posted By: gyrogearWell, nothing actually !
(I just want a warmer staircase !) (for as FREE as possible !)

Hence my emphasis on the floor insulation (on which I realise you get the point now). :)

If you are going for water then the floor is not quite so critical (but still important) because convection will take the greatest heat to the top of the water containers. If it were solid concrete then heat would be moving in the block by conduction rather than convection & conduction is just outward from the source rather than biased towards up[1]. It's the staircase (& the stair well above it?) you want to be warmer, not the ground below the cubby. :)


[1] "Heat" as such doesn't have weight! At the risk of teaching about the sucking of eggs...
"Heat rises" convection is a gravity based side effect of heat causing a fluid material (gas is "fluid" in this sense) to expand, making it less dense & resulting in denser material above sinking & forcing the lighter (warmer) material to rise.

The same logic explains why in water heat sinks at just above zero (much to the benefit of pond life) because the water starts to expand as it cools those last few degrees before zero!

End of lecture...

Posted By: gyrogearthermal mass

Running away now.
Mass has thermal properties, thermal does not have mass.

At the risk of sounding like a stuck record (thanks Dave ), learn the difference between temperature and energy.

Posted By: jamesingramhow much solar thermal (ST) you considering . In the heating season HW may be all you can do even with a large collector

well, I have a south-facing area where I could get about a hundred square foot, without making the neighbours suspicious...

gg

How much do you want to spend to get your free heat?
Have you considered PV instead of ST, it is cheaper.
Then you can get some old storage heaters and wire them in with a suitable controller/switch gear.
No plumbing to worry about, no chance of leak, no maintenance, silent and very reliable.

oops? Maybe nobody noticed, but...

My original design has *changed* - I now intend to drive the heat dump from plain old HOT *water*, from my thermodynamic DHW tank...

Reasoning: the tank is 300 liters and that is a lot of heat just "hanging around" all day.

So I thought that shunting 15 of those liters into Thermal Mass could be somewhat useful.

No way I want PV, thanks !
Nobody else has it in my part of the world (16,000 population, and I know of two houses with PV on the roof).
Round here, PV is for charging boat batteries !
(Even the cold-callers seem to have got the message)

gg

Posted By: davidfreeboroughYou can probably save more by turning down the DHW thermostat a couple of degrees or moving it higher up the tank.

Thanks, but it's not a question of saving, it's a quesion of getting *heat* into a cold area, namely the building core.

gg

Posted By: SteamyTeaDon't you want the thermal energy in the air within the building?

Yes, of course, and I have that...
It is a decent day here, I have 22.1°C in the lounge.
We have no heating on; will just run the WBS for a couple of hours this evening.
Tomorrow morning the lounge will be at 20.6° or so, like today.

However the basement "lags" - it is generally 2°C cooler than upstairs.
So I am working on a whole house fan installation, to mitigate stack effect.
Some of the cross-ventilation airstream will be UP the stairs.
I also wanted to *warm* the concrete stairs.
So borrowing off the DHW seemed like a good idea; if I can get the temps right...

gg

Thanks for the suggestion ! Would effectively consider that. Would allow me to pre-bend the matrix, then jam it to the underside like you say.

Access being pretty tight (we are looking at a half of a one-meter cube, split along the diagonal...), and to save on materials, I'm thinking of adding suitable spragging, then a "lost form board" made of 50mm XPS, onto which the concrete is thrown, to get it to slide down to the "sharp end".

This would give me an "inclined hydronic slab" about six to eight inches thick, contiguous to the stair soffit.

If 25 meters of 16mm tube, then 2.82 liters of water x TD of 20k = 54 kcal or 62 watt-hours; for 1.4 square meters... = 38 wh/m2.

Compared with my infloor radiant electric which has an installed power of 6.5 kW for 100 m2,
and which consumes (cold winter, not this one...) around 27 kWh per day (if on...).

Does this look anywhere near OK ?

gg

Thanks for the offer, but I'm probably out of area. Also, don't want any joins inside the thing: will probably end up with 3 x 25 meter rolls, to manifolds in air.

gg