How does the DHW supply in a normal house work?

1 Apr 2022
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United Kingdom
I’m having the usual constant vs timed hot water debate with myself and just need to know how the tank behaves when supplying water. As far as I understand (and I’m probably wrong), if your DHW is timed, the boiler will come on and heat your water to 60C and go off. If you then go and have a shower and use 50% of the water, cold water will immediately refill the tank and the water temperature in the cylinder will drop to somewhere between 60c and the temperature of the introduced cold water, say 35c.
Am I right in assuming that cold water will start filling the tank as you’re using it? If this is the case, won’t the temperature of the water in the tank drop rapidly as you’re having a shower?
This doesn’t seem to happen with our system at home. The temperature does drop but only quite slowly as you’re showering.
Is there a stopcock-type arrangement (like in a cistern) in the tank where water has to go below a certain level to start filling again?

Sorry for the noob questions, just trying to learn!
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Stratification is the magic word!;) Hot water rises to the top of the cylinder and "floats" on the colder water at the bottom, hot is drawn from the top of the tank, fresh cold replaces it from the bottom (often it may have a spreader baffle to prevent it rushing in too forcefully) the cold water rises as it is heated and so on. Simples!(y)
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Well I know my house, and it is not very good, I have two options to heat DHW, one is an immersion heater, with an on/off switch and built in thermostat, the other is the oil boiler with a hot coil, with the boiler there is only temperature control of the water in the hot coil, no thermostat on the tank, so my boiler in summer turns on 4 times a week, at ½ hour a time, it actually runs for 20 minutes before the boiler turns off as the hot coil can't transfer 20 kW from boiler.

It works, although debating increasing one slot to an hour to ensure water hot enough at least once a week.

The easy way out would be to fit a timer on the immersion heater so it switches on after the boiler switches off once a week to ensure water reaches the 60°C once a week, but not done that yet.

Not sure which costs the most, heating with oil or electric? There are clearly losses from the pipework, which means not a simple case of comparing cost of oil v electric. Fitting a thermostat on the tank is easy, getting the wires down two floors to the boiler is however not so easy.

Hot water use is low, shower is direct electric, and really the hot water is only required for morning wash, the dishes are washed with a dish washer which used electric to heat the water.

If my shower used the DHW then I would look at fitting a thermostat, and if I was on gas I would consider a combi boiler, gas boilers can modulate, oil boilers can be combi, but really it is a cheat, it has a small water take in the boiler, it keeps that tank warm, by switching on/off, so not intent heat, it is still stored.

Having seen the problems getting gas delivered when tied to one supplier, think I will stick with oil.

My house DHW is thermo syphon, often called C Plan, but there is also pumped transfer with Y or S plans, Y plan likely best for oil boilers as it allows thermo syphon to continue after the boiler has turned off, S plans need some built in cooling timer so water continues to circulate until boiler has cooled.

Gas modulates so boiler already cool before it turns off, if set up right, so what works best with oil may not be best for gas, and as to solid fuel, this needs very careful setting up as takes longer for boiler to cool down, often don't want to rely on pumps, and thermoplastic heater tanks can be rather dangerous.

I do have solid fuel heating ability, but not with central heating, it is a single open fire in the living room, never been lit by me, and from the wood lying around the house, would assume last owners burnt wood, problem with wood burning is storing the wood to dry, and to burn both efficiently and without particular emissions you need to store the heat, or use coke or charcoal, watching the problems with local railway trying to use alternatives to coal, not sure I want to go down solid fuel route.

However they are real boilers, not the pretend type we have in our homes, where the last thing we want is for it to actually boil the water.

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