Direct vs indirect heating of heat-bank

Hi All,

I am trying to decide whether direct or indirect heating of a heat-bank would be more suitable for my application. I am particularly interested in getting very good efficiency so am considering a condensing boiler.

I am having difficulty in understanding which option would work best with the condensing boiler. Also I'm not sure the advantages of running the heating from the heat-bank rather than off the boiler.

As far as I understand it I don't need a sealed heating system because I never intend to put radiators above the cylinder but maybe it does have other advantages?

Any help, please?

TIA, W

Reply to
W
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Oh dear here goes another 100+ Mb of bandwidth. Did you look at Google?

Reply to
John

If the boiler is a simple type with full power output or nothing then there is an advantage in using a direct heat bank because operation can be arranged to be from a single, long high power burn executed at long intervals.

An indirect one would not be significantly worse in that scenario either other than it may result in the boiler stopping rather earlier as the water in the cylinder approaches the set point and the heat transfer rate not being as great through a coil as handling the water directly. In other words there may be slightly more cycling.

It may be interesting to connect the radiators as well in that you are adding the heat storage capacity of the cylinder to that of the radiator circuit and that will also reduce cycling for this type of boiler.

However, If the boiler is a modulating type and has a low minimum output that is close to the typical heating load, there is nothing to be gained from not running the radiators directly from it because the output will modulate down as required rather than going off. Basically, when the radiator load exceeds the boiler minimum output, there won't be any cycling and the boiler will be operating in its most efficient temperature range as well.

With this type of boiler, you would arrange a diverter valve or zone valves for the heatbank and just use it for hot water. When there is a demand for heat to replenish the heatbank, the boiler will run at full tilt until that is achieved and then switch back to the heating probably at lower power. The same point applies regarding direct vs. indirect as before for the cylinder.

There are some boilers which will only work in sealed mode and for these your option has to be indirect for the heatbank if you want to DIY it. There are other benefits such as a reduction in the likelihood of air getting in and causing corrosion, easier to bleed, etc. Ed's FAQ has more on this.

Reply to
Andy Hall

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But why wouldn't you run the radiators from the direct heat bank even if the boiler could modulate down?

Reply to
Dave

If the boiler doesn't modulate and its output is substantially greater than the radiator load, it will cycle, potentially on quite a short period. Each cycle operation is an inefficient process, so the longer the burner can run, the better. Putting a heatbank in the middle with thermostats arranged to allow a reasonable part of the cylinder to cool before firing up the boiler, means that the heat for the radiators is taken out continuously at the level required, while the boiler only fires periodically and on a longer period than if the boiler directly drives the radiators.

If the boiler can modulate down, there is little point in doing this because it can be allowed to run in a much more efficient part of its range and continuously. When a modulating condensing boiler is running at full power and cycling because it is replenishing a heatbank periodically, it will not be running as efficently as if it is left to run continuously at the low end of its range.

Reply to
Andy Hall

Also, with a condensing type, there is an advantage in direct heating in that the boiler's flow temperature can be 5-10C lower, as there is no need to provide a temperature differential over the cylinder's indirect heat exchanger. Whether this advantage is enough to outweigh the advantages of sealed operation isn't at all clear, however.

Christian.

Reply to
Christian McArdle

I guess I could get away with a smaller store if the heating is taken from the boiler rather than the store?

Cheers, W

Reply to
W

One of the main reasons to consider taking radiators off the store with a modern boiler is because it allows multiple sources of heat. You can use solar, or a solid fuel range, for example.

Christian.

Reply to
Christian McArdle

Potentially, although if you also decide to go for an indirect heatbank it would be wise to make sure that you adequately size for all your likely hot water needs - i.e. ideally you don't want it to run out of heat. Although the plate heat exchanger that you will have to tranfer the heat from bank to water has a high transfer rate (which is one of the points of the heatbank idea), the coil in the cylinder won't be quite as good as direct transfer through the boiler. Added to this, your boiler won't be producing heat at the same rate that it will be being used to heat the water when you run a bath or shower.

If you are going to try to cut down on cylinder size and rely on the boiler coming on soon after you start drawing water to contribute heat back to the store, then a flow switch could be a good idea rather than just a thermostat. The point is that if you do run the store out of heat, the production rate of hot water will drop to less than a combi of equivalent rating.

Unless you are really tight on space, I think that it is better to try to design the system so that the heat bank will produce heat for enough hot water to meet your needs without relying on the boiler contribution.

You can do a rough calculation on that by measuring how much hot water you need to put into the bath or measure the shower rate and time. Then calculate how much hot water at 60 degrees that that entails taking into account that the winter temperature of cold water may be as low as 5 degrees.

The formula for resultant temperature of mixing water is

Vc[Tf - Tc] = Vh[Th - Tf]

Where upper case V=volume T=temperature and lower case h=hot, c=cold and f=final

If you start from the total volume of water at final temperature (assume 40 degrees) in the bath or shower, and you assume 60 degrees for hot, 5 degrees for cold, then given the total volume you can deduce the hot volume.

[Vf-Vh] * [Tf-Tc] = Vh*[Th-Tf]

e.g. for 150 litres of 40 degree water

[150-Vh] * 35 = Vh * 20

5250 - (35*Vh) = Vh * 20

5250 = 55 * Vh

Vh = 95.5 litres

However, the heatbank stores the water at 80 degrees (nominally) so if all of the water in the heatbank were at 80 degrees and the plate heat exchanger is able to transfer the heat at the full rate required, this would imply that you would only need to have a volume of 60/80ths or

71.6 litres.

Unfortunately things aren't quite perfect. The heat exchanger will typically be able to transfer heat at 100-200kW so isn't the main limiting factor, but the heatbank will not be storing all water at 80 degrees. On the other hand the boiler is starting to add heat back when you start to use water, and you won't always have 5 degree cold water.

The point is not to run the system close to the wire so perhaps in the above example, a 90 litre heatbank would be enough. If I were working to this requirement, I'd probably go for 120 litres and done with it on the basis of not needing to worry about it. Again, though, if you're tight on space then it may be worth looking more closely. It's obviously not an exact science, but these theoretical figures would at least give a reasonable starting point.

Reply to
Andy Hall

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