Hot water cylinder heat exchanger efficiency - reheat time?

But surely, as the hot water starts to use less heat from the boiler, the central heating pump can start up and use the extra.

Thus it may take longer than 20 minutes before the hot water is hot enough to run a second large bath, but you won't be without central heating for much longer.

One of the systems I've computerised uses a hot water cylinder, so I can give you some data from its logs. The cylinder is probably from 1990, and so not a fast recovery type, although it appears to be well insulated. The boiler is a Potterton Profile, adjusted for

Heating the cylinder up from cold, the max heat it can initially absorb is half the boiler output, which is 7kW i.e. the boiler burner (fixed power) runs at 50% duty cycle if I subtract the initial excess for heating up the boiler's low capacity cast iron heat exchanger and the direct hot water loop.

As the cylinder heats, the differential temperature will drop, and consequently the ability to absorb heat will drop too. It seems that the max heat it can absorb shortly before the cylinder stat cuts off (2 hours after starting from cold in this case) is

It looks like the cylinder requires about 8kWhr to completely heat it up (setpoint is about 58C on the cylinder stat, but I've not measured how accurately it achieves this). Of course, this doesn't heat the whole cylinder, because the cylinder stat is never positioned at the bottom - it has to be well above the bottom of the coil.

You are asking the wrong question. Efficiency is not the issue.

The words you want to google are "hot water recovery time"

Yup...

Much depends on your choice of cylinder. Old school cylinders frequently can't absorb heat faster than around 5 - 7kw - which makes them not idea in DHW priority systems. Modern part L compliant "fast recovery" ones are better and will often do twice that. Something like the unvented one I fitted can absorb at a maximum of 22kW which works quite well on a split temperature system.

This is true, however with a split temperature system you can set a much higher set point water temperature for the cylinder reheat without compromising the efficiency for running the heating. Also if the cylinder can absorb a resonable power, you will only be running at a lower efficiency toward the end of the heating cycle.

I find a full reheat of my 250L cylinder is around 35 - 40 mins.

Many cylinders will state the transfer rate of the indirect coil - so you can work out what you need from that.

You could obviously keep the old one, but it won't be well suited to a DHW priority setup, and the boiler / weather compensation won't be suited to any form of paralleled operation of the cylinder and the heating.

Having said that, depending on your DHW demands (and the size of cylinder you spec) you may find that a full reheat early in the morning before the CH kicks in, then a couple of refreshes during the day are adequate anyway, and they can be scheduled during heating setback periods.

More detail on the calcs etc here:

The difficulty there is the split temperature operation... normally the boiler and weather compensator will run the rads at a much lower flow temperature than it will use for the cylinder reheat. So if you allow the control system to run the rads during a cylinder reheat, then you will suddenly have the rads running at 70 - 80 degrees flow temp.

If you go for a cylinder that can swallow the full output of the boiler (or near enough), then you are usually better off getting that out of the way first and then switching back to heating after. A 20 min interruption to heating is not likely to be that noticeable.

To answer the original question, the efficiency is about 30%, so it's probably as cheap to use the electric immersion on economy 7 Unless you actually use stored water, install an on-demand boiler. We only use hot water for showering. Every other appliance is cold fill. A good on-demand boiler will save a lot of money; a hot tanks wastes 2-3kW overnight. You could even consider fitting on-demand to the kitchen and bath taps and keeping the cylinder for the showers.

There is no such thing as efficiency with a heat exchanger. There is only heat losses.

yes there is. I expect it's something like:

eff = ( ( heat in - heat out ) / heat in ) x 100%

or even:

eff = ( ( heat out ) / ( heat in ) ) x 100%

You are Dribble AICMFP.

Tim

Absolutely. The fact that he can't tell a Joule from a Watt is telling :)

BTW, when people quote efficiency figures on combis are they allowing for the short-cycling that happens every time you turn a hot tap on to wash your hands?

Andy

While plausible for an old cast iron lump boiler and low transfer rate cylinder, that seems a little too pessimistic for a modern boiler / cylinder combination. Although not in place yet, the SAP ratings system used to provide the SEDBUK figures will be split in future to show the system efficiency separately for heating and hot water production. From some of the examples I have seen, the hot water only performance is often about 2/3rds that of the space heating performance with efficiencies around the 60 - 70%

I presume you mean kWh? Again this is excessive for a properly insulated cylinder. However its worth noting that if you want a warm airing cupboard, then the heat is not "wasted" as such.

Not sure why you would want on demand heating for bath fill - its the application that its poorest suited to. Showers on the other hand are well suited.

Using a combi boiler and a cylinder makes sense with the cylinder being used for bath fills, and the combi for showers and kitchen taps.

Sort of, the current ratings system factors in hot water usage, but there are some assumptions made about the usage ratio of space to water heating. So in very well insulated properties the figures will somewhat optimistic.

Its worth noting that some combis are capable of condensing operation while in DHW mode, and some are not.

Nope. NOT true. There is no energy conversion.

Heat out after zero hours lapsed=100% Heat out after say 24hrs = zero. (Due to heat LOSSES). But varies depending on INSULATION

Go to the bottom of the class. Stupid boy.

Where does the heat come from harry?

Harry missing the big picture as usual.

The efficiency that matters is that of the whole system. If you use a boiler to heat a cylinder via its indirect coil, then I can guarantee that you will not convert all the energy contained in the gas, into the heat contained in the cylinder. Therefore the system efficiency is less than 100%.

Well designed coil in tank HEs are reasonably effective these days, however they are not able to transfer all of the available heat from the input side to the output side in a single pass[1]. So whichever way look at it, there will be a less than perfect system efficiency as described by Mr Streater.

I'm not sure it is. I have a Range Supercal double-lagged (which seems to mean 50 mm foam thickness) 206 litre cylinder, which, according to the label, has a maximum stored heat loss of 3.16 kWh per 24 h.

Chris

If you want, you can go all the way back to Gazprom Siberia for the energy source

But the OP is about hot water cylinders with heat exchanger. Efficiency is not relevant. There is no energy consversion, no "work" is done. Taking the instantaneous value, it must always be 100%.

The factor to consider are:- Rate of heat exchange. (Many variables.) Amount of water stored. Heat loss. (Many variables) Time taken to heat the store from income temp to desired temp. (Recovery time) Which depends upon the above factors.

This is all very difficult to calculate so one goes on the manufacturers figures and conventions/empirical methods. Somewhere there will be charts/graphs showing the performance under various conditions. Even this is only a general guide which is where the "funk factor" comes in.

Heat exchangers have an efficiency, simple coils will transfer less of the energy available than a well designed flat plate exchanger.

The energy isn't lost if its recirculated through the heat source and then back through the exchanger.