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.
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.
You mean with 4G F&E tank, and you find it doesn't overflow when heated from cold?.
... many thanks for that I'll try not to do this (:->).
Egremont.
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
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.
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.
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.
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
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