Ball park figure required - central heating header tank volume.

[julianf]

Ive looked at a bunch of calculators and still dont know - can someone with experience here give me a ball park?


System details -

Vented, header in loft.
Rayburn with back boiler (i guess that's around 30 ltrs ish?)
Thermal store (270 ltrs)
A handful of radiators (3 old style cast iron free standing ones, about 2 foot wide, and a couple of 2 foot square modern ones)
Ill probably just plumb it in 15mm as thats what i have.

Its a small house, simple system, all very basic. Indeed, we have lived here for years without any radiators.

Ive got a few foam lagged copper cylinders. Which are basically identical to the header tanks for uncontrolled heat sources, so ill lop the top off one of them.

Im just really not sure of the volume required, as the thermal store somewhat complicates things over a normal system.

 

[gaz1]

having this reference is far easier

Hot water expands at a rate of 4% so 100 ltrs = 4ltrs, most open vented heating systems will only need a 4 gallon f&e tank,( about 4 inches of water when cold)

https://fernox.com/cpt_downloads/estimate-volume-water-system/

not forgetting you can split your tanks into 2 to get them into the loft

https://www.boilerguide.co.uk/articles/water-tank-loft-replacements-removals

i presume you mean thermal store holds 270 litres not what is within the central heating pipes litre reference

and because im old school ive seen small tanks fill large 3 level property houses the main thing you have too remember is your calculating its water expansion reference

so for every 100 ltrs in your system = 4ltrs pumped back into your header tank

also remember the bigger the header tank the more water will hit that burst pipe if it ever goes on you and will still keep on feeding it until you shut off the valve to it

 

[Kayos]

A header tank is 25 litres, rarely upsize unless its a massive system. If you need expansion then set the water level low

 

[julianf]

i presume you mean thermal store holds 270 litres not what is within the central heating pipes litre reference

I dont 100% know what you are asking, but the water in the thermal store is the same as the water in the rayburn, the same as that in the rads, pipe work, and what will expand into the expansion tank.

The potable water is heated on demand by a coil in the thermal store. All the rest of all the volumes need to be managed by the expansion tank.


So, say, ive got 400ltrs to handle, that only gives me 12 ltrs needed. I think the direct hot water tanks that i have are probably 100ltr tanks, so that seems fine - and it will fit through the loft hatch without bother.

Thank you.

 

[julianf]

A header tank is 25 litres, rarely upsize unless its a massive system. If you need expansion then set the water level low

Its a tiny system with regards to radiators, but most systems dont have 100s of ltrs of thermal store attached, i guess?

 

[julianf]

To be clear, i need to handle the expansion on about 300ltrs before ive even connected any radiators at all.

 

[gaz1]

A header tank is 25 litres, rarely upsize unless its a massive system. If you need expansion then set the water level low

so you need a header tank to take 12" expansion within it and you calculate this by area size to find your size of header tank

works out to be 2-3" rise within your tank that ypu dont want to lose down the overflow pipe

we plumbers just bend the ball valve lower to except this difference

 

[gaz1]

I dont 100% know what you are asking, but the water in the thermal store is the same as the water in the rayburn, the same as that in the rads, pipe work, and what will expand into the expansion tank.

The potable water is heated on demand by a coil in the thermal store. All the rest of all the volumes need to be managed by the expansion tank.


So, say, ive got 400ltrs to handle, that only gives me 12 ltrs needed. I think the direct hot water tanks that i have are probably 100ltr tanks, so that seems fine - and it will fit through the loft hatch without bother.

Thank you.

does your central heating feed into this tank like a hot water cylinder does via a coil or does it fill the system with backup central heating water held within a thermal store

 

[Kayos]

You are over thinking it, most vented thermal stores have an integrated f and e tank, if it doesn't then use a 25l tank

Massive systems have bigger tanks to make them easier to fill, not necessarily for expansion

 

[julianf]

does your central heating feed into this tank like a hot water cylinder does via a coil or does it fill the system with backup central heating water held within a thermal store

rayburn has a direct connection to the 270l tank
tank has a direct connection to the rads and the expansion tank

if you tipped dye in any one of them, all the water would change colour

 

[julianf]

You are over thinking it, most vented thermal stores have an integrated f and e tank, if it doesn't then use a 25l tank

Massive systems have bigger tanks to make them easier to fill, not necessarily for expansion

Yeah, mine does not have an integrated expansion. This was deliberate, as i wanted the store on the ground floor, and want the rads upstairs. On the vented setup, id never be able to bleed the rads unless the expansion tank was higher.

 

[gaz1]

also depends on how high your header tank is as well

not everyone can fit the tank higher into the apex of a roof so it gives a better flow to the highest rad

sometimes if you havent got height you can adjust be weight of the water in the expansion tank

 

[Kayos]

If you are really concerned just raise the vent, I would say vents need to be external or into a soil stack but last time I gave current advice around leigionnella I was accused of scare mongering

 

[gaz1]

rayburn has a direct connection to the 270l tank
tank has a direct connection to the rads and the expansion tank

if you tipped dye in any one of them, all the water would change colour

i thought id ask as if its a coil feed its litres vs 270l held within your tank

so

270l
+ rads x litres
+ boiler and pipe litres

If you are really concerned just raise the vent, I would say vents need to be external or into a soil stack but last time I gave current advice around leigionnella I was accused of scare mongering

we know better and yes i read the post

 

[julianf]

If you are really concerned just raise the vent, I would say vents need to be external or into a soil stack but last time I gave current advice around leigionnella I was accused of scare mongering

The vent?

You mean the vent from the header tank, rather than its overflow?

I assumed it would just have a non-air tight lid on it?

Regardless, our house has a slate roof with no membrane. The roof space is as good as outside! (which is why i thought the tank with the foam lagging wouldnt be a bad idea - im well aware that no end of foam will keep something above ambient indefinitely, but id hope it will slow freezing)

 

[Kayos]

The vent?

You mean the vent from the header tank, rather than its overflow?

I assumed it would just have a non-air tight lid on it?

Regardless, our house has a slate roof with no membrane. The roof space is as good as outside! (which is why i thought the tank with the foam lagging wouldnt be a bad idea - im well aware that no end of foam will keep something above ambient indefinitely, but id hope it will slow freezing)

The vent pipe that traditionally goes over the tank, that takes up more expansion than the water.

When its cold all the pipe above the water line is empty so if you raise it right up before it drops to thd tank you massively increase the expansion volume

 

[Gareth J]

If it helps. Just taken delivery of a 400l store and 40l loft expansion tank. Don't forget metal lid and copper float ball

 

[julianf]

If it helps. Just taken delivery of a 400l store and 40l loft expansion tank. Don't forget metal lid and copper float ball

I know the ball is said to be copper.

If the tank wall ruptures and a load of hot water falls through the ceiling, that could be very bad.

In the unlikley event of the ball getting hot enough to melt, I'd probably be ok with it flushing the headder to waste with a load of cold water....

Tank will be able to withstand high temp, pipework will, ball, I'm not so sure I care so much about.

Again, if it's getting hot enough to melt plastic, I'd quite like some cold water flushing.

 

[Gareth J]

I suspect the copper ball is more to do with failing due to expansion/contraction cycles rather than catastrophic and dangerous failure.