Hot water cylinder heat exchanger efficiency - reheat time?

[dennis]

That's poor, I have a 210l one and its less than 2 kWh/24 hours. Its worth it when the alternative is less than 24 l/min flow through the shower.

[John Rumm]

His question specifically mentions recovery time as being the salient point. So the "efficiency" he is interested in is that of how quickly heat can be transferred from the boiler to the tank. You may not like the word, but it should be very clear what he means (i.e. rate of energy uptake by the cylinder compared with the potential maximum rate deliverable by the boiler)

Rather meaningless, and not particularly relevant.

Yes, and generally information that is available from the cylinder manufacturer.

Not significant enough have any bearing - assuming you have some lagging on your primary loop, the cylinder may be loosing 100W, which for these purposes is negligible compared with an input rate of many kW.

you don't say...

If all you need is an indication of how long before it can run another bath etc, then its relatively easy to calculate ballpark figures IME.

For example if you take figures for a tank such as the Unistor 210, the spec from the manufacturer gives you:

Primary HE flow rate required : 23.3 lpm Heat up time to EN12897 : 28 mins Recovery (70%) : 20 mins Primary HE performance : 22.6 kW

(which alone would give the OP a good enough indication if he can find similar data for his), however, say you just had the HE transfer rate and the volume of water?

Lets say you have drawn off 150L of water and now want to reheat that amount from 10 to 60 degrees. If you do a crude calc such as:

150 x 50 x 4200 / 22,600 / 60 = time, you get about 23 mins

Now in reality, there are other factors that come into it (when the boiler starts reheating being the main one), but as an indication is pretty serviceable.

[John Rumm]

That's quite high by modern standards. My 210L unistor is rated at 1.89 kW/24h, i.e. a loss rate of just under 80W

(even the 310L version is only 2.26 kW/24h)

[harryagain]

Even a small heat loss is relevant because in many cases it's there 24/7.

[harryagain]

Dennis, as usual you haven't a clue. Go back to sleep.

[Chris J Dixon]

Yes, that is a little over half mine, which is disappointing. :-(

What insulation does yours have?

Mine is about 9 years old now. It sits, together with a pump, in the corner of my inset garage, and there isn't a lot of room to add further insulation.

Chris

[John Rumm]

Its relevant in some discussions, but not this one about recovery time.

Take the above example, let's say that ~23 mins calculated was the time including 100W of loss from the tank. Do the sums again with an effective additional 100W of input resulting from using a "perfectly lagged" cylinder, and you get 23.13 mins instead of 23.23 mins - less than 10 seconds.

[John Rumm]

Truth be told I don't know. The spec does not say, and you can't tell from looking at it since its all fully encased in an outer rigid skin over the insulation.

Yup, tricky one. Although if its an inset garage, then presumably most of the waste heat will end up in the room above it?

Mine is in the airing cupboard, so some heat loss is actually useful (and also why I did not bother lagging the flow and return pipework in there, since the losses from the cylinder itself are so low)

[Chris J Dixon]

Some of it, certainly, though the up and over metal door can't help matters. I did vaguely wonder about trying to glue some insulation onto it, but the counterbalance spring just about copes at the moment. I fear the extra mass would overload it, and trying to increase the tension looks like a task for someone who doesn't value their fingers.

At least I don't have a problem with my tools rusting. ;-)

Chris

[MJA]

That is really interesting, thanks. Is that a fully pumped system, or one with gravity circulation for the hot water cylinder?

Perhaps it is time for me to order a Raspberry Pi to use as a data logger for my system!

MJA

[Vir Campestris]

Having been using a combi for the last 6 months - I'll disagree on kitchen taps. It fires up very time you touch the tap, heats its exchanger, then goes off. Can't possibly be efficient.

Andy

[John Rumm]

My comment was not so much about efficiency as convenience. i.e most people have the boiler near the kitchen, so less wait for hot water, and also the hot water from a combi is drinkable, so it can make sense to fill the kettle / pan etc from the hot tap.

[MJA]

I should have written heat exchanger effectiveness -- efficiency was a poor choice of word and I have been chastised. I am primarily interested in reheat time with a hot water priority system.

I find it difficult to find manufacturers information. For example, Range quote reheat times of 25mins for Hercal cylinders, 20mins for Supercal cylinders and 15mins for Ultracal cylinders. I am suspicious about these figures because they are specified with 82C flow temperature at 15 litres/min (18 for Ultracal).

This is unrealistic -- a small (18Kw) boiler will produce a temperature rise of about 17C at 15 litres/min between return and flow. Hence the flow temperature will start low (10+17 = 27C) and slowly increase.

The brochure smacks too much of Hi-Fi amplifier makers giving peak music power output (PMPO) rather than continuous sine wave power into a resistive load at onset of clipping.

When it gets nearer to spring and time to do the work I will try contacting the technical departments of one or two copper cylinder manufacturers to see if they can help.

I will probably do the work in stages -- get a gas fitter in to change the boiler, then DIY the hot water cylinder change. I can see what the cylinder reheat time is like during the summer, and change it if necessary before the next heating season.

Regards,

MJA

[John Rumm]

Yeah don't worry, we knew what you meant... don't mind harry.

That assumes that the cylinder HE will extract all the heat per pass, which it won't - especially at such a low differential. (in fact if the cylinder is partly hot, you may even find the return temp to the boiler is warmer than the flow for a short while as the cylinder gives up heat to the primary water).

After a few trips through the boiler, you will be up to the expected flow temp.

Some of the cylinder specs (its worth downloading the installation manual with the technical stuff in it rather than just using the sales brochures) will have proper graphs of power inputs vs reheat time for one or more flow temps. Generally these are produced from test measurements.

The normal element of salesmanship is that many of the figures will be quoted for flow temps of 80 degrees - which are far less commonly used for modern boilers.

It its an older "normal" gravity cylinder, then obviously it will still work, although the reheat times will probably be two or three times what you would expect with a modern one with a high power HE.

I used to find my old setup (cast iron lump boiler running 80 degree flow, through a normal 900 x 450 indirect copper cylinder (max coil transfer rate about 5kW) would take over an hour for a full reheat. There was not much point running just the cylinder on its own since it could not consume the 20+ kW the boiler could produce. The new setup with a cylinder of twice the capacity, is very much quicker, simply because it can consume the full output of the boiler for most of the reheat.

I did contemplate doing the boiler swap and cylinder etc separately, but decided it was much easier to get all the changes (i.e. boiler, weather comp / controls, conversion to unvented, new cylinder, radiator juggling, extra TMVs etc) done at once to save going through all the hassle more than once.

[Andrew Gabriel]

This is fully pumped.

Yes. This system predates raspberry pi's by 10+ years, but I have been playing with a raspberry pi for the next system I do.