Central heating system under construction, incl 200L direct thermal store,
contains about 270-300L of water - quite a bit more than average, I would
think.

Would the standard 4 gallon F&E tank be adequate, or should I consider a
bigger tank - i.e. would the 4G tank be subject to frequent overflow
episodes due to normal expansion/contraction?. If so is the next size up 25
gallons?.

Thanks,
Egremont.

Somewhere out there I read the specific gravity of tap water at 80 C, say,
is 0.975 - c.f. 1. 0 at 4C. So hopefully 1L at 4C becomes 1.026L at 80C.

I'll guestimate 100L would be typical for a 'normal' system with rads +
indirect HW cylinder

So 100L over this range, 100L expands by 2.6L, 300L by 7.8L.

So 4 gallons looks adequate for a normal system, 12 gallons looks about
right for mine. 25 would be overkill, though I could set the ballcock
halfway down.

Hopefully I answered my own question? though I wouldn't know where to get a
12 gallon tank.

Egremont.

"Egremont" wrote in message
...
Central heating system under construction, incl 200L direct thermal store,
contains about 270-300L of water - quite a bit more than average, I would
think.

Would the standard 4 gallon F&E tank be adequate, or should I consider a
bigger tank - i.e. would the 4G tank be subject to frequent overflow
episodes due to normal expansion/contraction?. If so is the next size up
25
gallons?.


From experience with an even bigger system it will manage normal operation
fine but if you undertake the test of the over-temp safety thermostat on the
boiler the water boils so much that it blows out of the top of the tank and
soaks the loft.

From experience with an even bigger system it will manage normal operation
fine

You mean with 4G F&E tank, and you find it doesn't overflow when heated from
cold?.

but if you undertake the test of the over-temp safety thermostat on the
boiler the water boils so much that it blows out of the top of the tank
and
soaks the loft.

.... many thanks for that I'll try not to do this (:-).

Egremont.

So 100L over this range, 100L expands by 2.6L, 300L by 7.8L.

So 4 gallons looks adequate for a normal system, 12 gallons looks about
right for mine. 25 would be overkill, though I could set the ballcock
halfway down.

Darn it, forgot to convert untis!, 2.6L is about 1/2 Gallon, 7.8L a bit
under 2 Gallons. So a 4 Gallon tank might do for 300L? though all things
being equal it might need topping-up** more often..

** Is it the normal thing to leave supply to F&E cistern turned off, then
keep it topped-up by activating the supply gate valve periodically with
heating is on (ie water is expanded). Doing this I can see the system will
remain topped-up without the F&E tank overflowing (as would seem to be the
case if you leave supply permanently on?). Sorry I don't know much about
this stuff as I've always had gas fires up till now....

Egremont

Egremont wrote:

** Is it the normal thing to leave supply to F&E cistern turned off, then
keep it topped-up by activating the supply gate valve periodically with
heating is on (ie water is expanded). Doing this I can see the system will
remain topped-up without the F&E tank overflowing (as would seem to be the
case if you leave supply permanently on?). Sorry I don't know much about
this stuff as I've always had gas fires up till now....

Most systems leave the feed to the tank on, and rely on the float vavle
in the tank to regulate the water level.

You adjust the float valve such that it maintains the water level with
enough expansion room between the water level and the overflow to
accomodate the typical expansion.


--
Cheers,

John.

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You adjust the float valve such that it maintains the water level with
enough expansion room between the water level and the overflow to
accomodate the typical expansion.

Ah, yes, of course you do!. Though you can't put the overflow 'miles' over
the (float-valve) fill level to accomdate ~2G (if calcs are correct) of
periodic expansion in 4G tank, periodically submerging the float valve.. So
scaling up to 8-12G in my case looks sensible to me, and might help with the
high temperature blow-out scenaro mentioned. And as I'd prefer to run in
periodic top-up mode (damage limitation if leak) it'll mean less frequent
topping up.

Though I still don't know where to get 8-12G tanks.

Egremont.

Egremont wrote:

Though I still don't know where to get 8-12G tanks.

You could use a pair of 4G ones linked by a pipe at low level.

--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

You could use a pair of 4G ones linked by a pipe at low level.

True - this way I counld try with one, see what happens & add second if
neccesary.

"Egremont" wrote in message
...

From experience with an even bigger system it will manage normal
operation
fine

You mean with 4G F&E tank, and you find it doesn't overflow when heated
from
cold?.

Yes - though the ballcock is set almost to the bottom and the overflow is mm
from the top - hence the boiling problem. I would have liked to use a
larger tank but the roof-shape prohibits it. If it had been a problem I was
going to put another tank in parallel.

Yes - though the ballcock is set almost to the bottom and the overflow is
mm
from the top - hence the boiling problem. I would have liked to use a
larger tank but the roof-shape prohibits it. If it had been a problem I
was
going to put another tank in parallel.

OK thanks - the small 4G tanks must stretch further than I thought.

I found a 15G tank ("Titan" PC15R, about £20) on-line. It's 24x17x17 - I'll
go for that and be done with it, setting the float-valve & overflow about
1/4 way down if I can.

Egremont