hot water 24/7?

bikeman

Got a gas condensing boiler with CH by rads and an insulated hot water cylinder.

What's the cheapest way to get the water heated? Presently it's on a timer for 10mins am and 10mins pm. CH is on 1/2 hour am and 4 hours pm.

It occured to me that having the water timer match the CH timer would work out cheaper on the basis that the boiler is on for longer with the CH anyway.

Also when some of the hot water is drawn off cold water refils the tank and then has a long time to cool the remaining water. Seems logical that it would be better to reheat water as it is drawn off rather than allow it to cool the rest of the tank.

Your thoughts?

HappyMJ

Do you ever run out of hot water? 10 minutes twice a day isn't much water.

macman

Yes, the nearer you can heat the water to the time of usage, the less heat loss there is. But a properly insulated tank should stay hot all day anyway-and any heat loss there is is not really wasted, as it escapes to warm the house.
The savings would therefore be minimal.
Your 'draw off' logic is flawed, if it were true then you would leave both the heating and hot water on 24/7. Would you keep your kettle boiling constantly to avoid it cooling down?

rogerblack

There are a couple of causes of inefficiency.
Firstly, every time the boiler cycles on to heat the tank, the whole run of pipework that is outside the insulated space of the house (or if heat is not desired) is filled with hot water that is wasted. (and the boiler takes some time to heat up too)

Secondly, the heat escapes from the cylinder depending on its temperature.

This means that having the cylinder colder on average helps.

A typical tank will lose 1-2kWh a day.
Or 10-20 quid a year.

If your demand for hot water is low, check the thermostat on the cylinder is set to 65C, so that it loses less, perhaps add some more insulation.
Setting it so it heats when the central heating is on is a good idea, so the boiler is hot.
Keeping it off most of the time, so that it's not continually cycling to keep the tank hot is also a good idea.

bikeman

macman wrote: »

Your 'draw off' logic is flawed, if it were true then you would leave both the heating and hot water on 24/7. Would you keep your kettle boiling constantly to avoid it cooling down?

Actually this seems no brainer to me. If I use 50% of the tank then the influx of cold water will cool the remaining 50% probably quite quickly. It makes sense that it would use less energy to reheat it before it's all cold.

It also makes sense to heat the water at the same time as the CH is on. It's almost a free ride since no extra demand will be placed upon the boiler.

Sorry but i dont accept the comments from rogerblack re wasted energy heating pipework. Granted there will be some waste but I have a modern house, pipework length is minimal and it is all well insulated within the walls.

Thanks for all the replies.

Cardew

bikeman wrote: »

It also makes sense to heat the water at the same time as the CH is on. It's almost a free ride since no extra demand will be placed upon the boiler.

I am still not sure if this is a 'wind-up' or not! - however assuming that the question is genuine!

It is your assumption that 'no extra demand will be placed upon the boiler' that is incorrect and where your theory falls down.

You seem to be(wrongly) assuming that the boiler when operating for CH can only operate at maximum output. A boiler 'modulates' its output(turns down the wick!), or turns off, when running and produces sufficient output to supply the CH demand. If you switch on domestic hot water(DHW) during the period it is supplying CH water, then it has additionally to supply energy to heat that domestic hot water.

As an example, if the CH demand on the house was, say, 10kW the boiler would be supplying that amount of energy.

If the DHW demand is, say, 3kW then the boiler would have to supply 13kW.

The water in the Hot water tank is heated by a coil in the tank. This is just as much a load on the boiler as an extra radiator.

If your 'theory' worked you might as well turn on every radiator in the house every time the CH came on, as it would be as cheap to heat ten radiators as one radiator as the other 9 radiators would get a 'free ride' (as you put it)

creased-leach

Sorry for the mini hijack, but my ex used to argue that having the HW on at the same time as the CH used no more oil (condensor boiler.)
I could never believe that could be the case.
Are you trying to tell me I get to say I told you so?

C_Mababejive

Re draw off...the incoming water flow will likely be diffused and will naturally stratify at the bottom of the tank and will not fully mix with the existing hot water. That is to say,the hot water stays on top of the cold. Obviously there is some mixing at the interface.

rogerblack

bikeman wrote: »

Actually this seems no brainer to me. If I use 50% of the tank then the influx of cold water will cool the remaining 50% probably quite quickly. It makes sense that it would use less energy to reheat it before it's all cold.

It also makes sense to heat the water at the same time as the CH is on. It's almost a free ride since no extra demand will be placed upon the boiler.

Sorry but i dont accept the comments from rogerblack re wasted energy heating pipework. Granted there will be some waste but I have a modern house, pipework length is minimal and it is all well insulated within the walls.

The pipework loss is not however minimal in all cases.
And it is not well enough insulated in the walls that it will stay warm for an hour between boiler firings.
(It may heat the property)

If not promptly reheated, the cylinder will lose less energy over time, as the average temperature of the cylinder is lower if it is partially filled with cold (it does stratify).

The energy required to later heat it up is more than the energy required to heat it up immediately, but this is because the cylinder has cooled further.
The heat to keep the cylinder hot has to be counted too.

You use around 1-2kWh/day to keep the cylinder hot.
If it is half filled with hot water, this loss drops by a significant factor.

However, the differences, with gas, is likely to be only several pounds a year.