On 05/02/2018 13:55, Bob Minchin wrote:
I have an un-pressurised thermal store that has a 15mm copper vertical
vent pipe at the top that goes up into the loft and hooks over the Fill
and Expansion tank.
Although it is insulated with foam, I have a hunch that I'm losing some
of the stored heat by convection up the pipe.

How much of the heat is likely to be conducted up the pipe itself as
compared by convection in the water it contains?

My gut reaction is almost nothing lost by conduction. The calculation is
not that hard, though. Estimate the cross sectional area of the copper.
Work out the temperature gradient (the difference in temperature between
the top and the bottom inside the pipe, divided by the pipe length).






I have a couple of options.

I could re-plumb using stainless steel pipe which is much less thermally
conductive. Useful if the loss is conduction through the pipe.

Why not use plastic pipe? As the plastic pipe is *much* less conductive
than copper, you'll get most of the benefit (if there is one worth
having) just by replacing a short length. The conductivity is
Copper : 400 (I suspect copper pipe is less as it's not pure copper)
Steel : 16
PVC : 0.2 (is plastic pipe made out of PVC? )
Water : 0.6

All W/(m K)


In any case, you'd need to compare the conduction through the pipe walls
with the conduction in the water, which has a much greater cross
sectional area.



I could instead reduce the bore of the pipe and hence the amount of
water contained in the pipe if the main contribution to the loss is
convection.

What size is required for safety? If smaller is okay, why did you use
15mm in the first place? If you need 15 and replace it with 10, you'll
need to run three pipe runs to get the same overflow capacity.

Convection is much harder to work out theoretically than conduction, but
I expect it's tiny. Any upward flow gets obstructed by the downward flow
- all in a 15mm pipe.





Someone has suggested that I put a U bend in the pipe run and that the
downward section of pipe run from the store will stop the convection
losses.
This sounds like it might work but I'm not 100% convinced.

Neither am I.

I wouldn't do any of this without experimenting first. Cut off the heat
inflow and measure the temperature drop in the store, both with the
expansion pipe connected and with it capped off by the tank. You'll need
to repeat this a few times in different weather conditions.



What does the team think please

You're overdoing this.