Purpose of Gate valve on hot-water tank

Just a couple of points here..

The resistance of a gate valve is not particularly proportional to the position of the gate ie. at half open flow will not be restricted to 50% to get 50% typical gate valves would need to be about 15% open. Gate valves should not be used for balancing for this reason, they are for isolation. There are proper valves for flow control, some of which can can be used for isolation as well (double regulating valves)

The heat transfer in a cylinder coil is in not really related to the velocity of the water flowing through, except at very low flow rates: the heat transfered is all governed by the flow rate of the primary water (not it's velocity) and temperature differential, that is, the difference between the mean primary temperature and the stored water. So if 0.05 l/s is flowing through the coil and the primary flow is at 80 deg.C, the return at 60 deg.C. the mean is 70 deg.C, with the water in the cylinder at say 40 deg.C the heat transfer will be 0.05 x 30 x 4.2 (specific heat capacity of water) = 6.3 kW. So as the stored water warms up the heat flow decreases if the primary flow stays the the same, of course what also happens is that the primary return temperature increases slightly until the boiler stat turns the whole process off or the cylinder stat sends the water somewhere else.

Thermodynamics session over. If the system is working, close the cupboard door and do something else.
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