Loft header tank size

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Due to very limited loft space I want to fit the smallest possible header tank on my new hot water system. It feeds the hot taps in a 4 bed house, but not the cold ones. Can I use an 4 gallon tank or is there some Bylaw or Regulation that demands a bigger one? Does the 4 gallons refer to the total size of tank or the capacity to the nominal waterline?

I guess the risk is that a water demand of more than one tap or a fast bath tap will rapidly deplete the header leading to low pressure and maybe drawing air into the cylinder but is this 'against the law'?

A direct mains pressure system without a header is not possible in this case.
 
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a central heating feed & expansion tank was (is) known as a 10/4,ie if filled completely to the top it would hold 10 gallon, but in reality the water level was set to hold 4 gallon, i cannot imagine that a small tank will refill quickly enough to supply constant hot water to a bath etc so it depends on 2 things, the pressure of the mains refilling and the use, in old money we were told 25 gallon for hot supply
 
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Once your 4 gallon tank has drained, the flow will be limited to how fast the water flows through the approx. 5mm diameter hole in the valve body of the ballcock.

Plus the chance of airlocking the system every time....
 
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Coffin tank might work but this is a holiday house and in winter when it’s unused and unheated we may have to lightly heat the tank to keep it from freezing. The larger the surface area of tank the more volume we have to heat and the more heat loss.

Anyone recommend a suitable freeze protection heating system for the header? Could maybe add an immersion heater boss in plastic sidewall and then somehow add a water thermostat set to 5C in series with the normal thermostat, set to its lowest setting (as a safe backup). Or maybe just tweak the standard thermostat, as I see that some can be internally adjusted to make them go lower (or higher) than the normal range. This is presumably a calibration adjustment normally.

Most likely solution will be a Fortic tank with a 20l/4gall header. Can I get an screw on immersion heater boss suitable for the curved wall of these and are the headers usually copper or plastic?

We used to drain the tank over winter but are adding solar heating so can’t now drain, else the solar boils.
 
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If your really want a heater in the tank then your best bet is a heater for a fish tank, can be set pretty low. Normally I'd advise leaving the heating low and the loft hatch open, with adequate insualtion it normally does the job.
 

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