Heat banks (again!)

[IMM]

Re-read the thread.

[IMM]

If having a heat bank that only has one flow and return to it, when large qualities of DHW are drawn off the return will be low (only if there is no blending valve), and even lower in winter with a cold water mains temp and the CH on working against freezing temps.

[Andy Hall]

So you wouldn't know, presumably.....

I'm the one with the engineering degree. Your detailed qualification being?

As usual, you are trying to change the story as you go along.

The compensator is a term to the controller in the boiler. It looks at the outside temperature to do this - all of which is unrelated to the heatbank.

As you seem to demonstrate daily.

.andy

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[Andy Hall]

A condensing boiler will operate more efficiently at lower temperatures. Repeated cycling is going to reduce efficiency which is a second reason why they modulate down if possible rather than turning on and off.

Yes, but that's only into the cylinder. In effect, running the radiators from the heatbank is the equivalent of turning the hot tap on low. The boiler will come on and attempt to replenish the heatbank after the thermostat on it drops a few degrees below its set point. It will then bring the temperature of the heatbank up to a few degrees above that. The temperature, of course, has to be adequate for the DHW use from the heatbank. Therefore the boiler will inevitably cycle around this temperature. There is a max limit of

20-25 degrees of temperature differential across the boiler heat exchanger anyway. If you connect the boiler to the radiators directly, it can sense the return temperature from the radiators themselves rather than being switched on and off by the heatbank thermostat.

This situation could potentially be improved if the boiler has a way to take an analogue reading from the heatbank and modulate its output to match the radiator heating load for the case where the radiators are connected to the heatbank. Even then, the heatbank being in the middle will screw up the intended control algorithm because it introduces a massive dampening effect in the feedback path. Not many boilers have the ability to take a remote temperature probe for the cylinder anyway, and are reliant on a cylinder thermostat with large hysteresis.

.andy

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[Andy Hall]

That's what I mean....

.andy

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[Andy Hall]

You'll fall off if you do that....... :-)

.andy

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[IMM]

Andy, rub the crayon marks off your screen.

You are still thick.

You can't follow the thread or the explanations

Oh my God! A compensator can be a stand alone unit controlling a boiler, a valve, a step switch, etc, etc.

Good. Got it.

failed.

You don't know much about this sort of thing. A little knowledge is dangerous.

[IMM]

What tripe. A heat bank eliminates boiler cycling. One person ont his thread bought one just to do that.

Go away.....

Once again....A heat bank eliminates boiler cycling. One person ont his thread bought one just to do that.

NO!!! From the cylinder...to the boiler...called the return.

You have two stats to eliminate boiler cycling. You clearly know nothing about this sort of thing.

[Andy Hall]

I manage thanks.

Obviously. However, if it is an external unit to the boiler and controlling it, then it does so by turning it on and off and cycling it.

Reasonable to good condensing boilers have an option for an external sensor to do the weather compensation by analogue means.

.andy

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[Andy Hall]

It depends on the type of boiler, and its thermal and control characteristics. Grunff's application was to absorb a large amount of heat from an oil boiler when an old coil in cylinder approach, not surprisingly, couldn't do it.

This is not what we are discussing here, which was specifically the impact of radiators hooked up to a heatbank. If heat is being abstracted at a lower rate than the boiler can produce it will either have to modulate or cycle. If you put a heatbank in the middle, the dampening effect screws up the condensing boiler's control arrangement such that it won't be able to modulate properly.

Once again, it can under certain circumstances such as when a large amount of energy is being removed to heat the water. This could be

100kW or more. The boiler, assuming it is less than this will then come on and run at full power until it is at the set point of the heatbank.

This is not the same situation as making a lower level continuous use of heat from the heatbank to run the radiators. It is less than the boiler's capacity so it has no choice than to modulate or cycle. having the heatbank in the way, with a cylinder thermostat controlling the boiler will inevitably lead to cycling.

Yes, but the return is not from the radiators, it is from the heatbank; the storage effect of which means that the return temperature that the boiler sees will be that of the heatbank, together with its dampening effect as opposed to that directly from the radiators. They are not the same thing, and a cylinder stat has been added into the equation as well.

You can't eliminate boiler cycling with a simple thermostat, or even two of them. They have hysteresis. If you had temperature probes able to give analogue readings to the boiler and modulate it, that would be a different matter, but this is not that.

.andy

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[Steve Firth]

Well at least it lets you live with yourself.

[John Rumm]

Andy, you may as well try Latin, looks like he is as good on Control Theory as he is Physics....

[Christian McArdle]

Indeed. The idea is to promote slow cycling, perhaps using even a third or half the heat bank capacity before attempting recovery. The questions is whether the thermodynamic advantage of running at full pelt outweigh that of the lower return temperature at the less efficient low burner setting.

We are considering two systems here:

1. Radiators, low hysterysis single thermostat, modulating condensing boiler.
2. Heat bank, v.high hysterysis double thermostat, unmodulating condensing boiler.

The high hysterysis would lead to a modulating boiler become effectively non-modulating, as the heat bank will be able to absorb whatever heat is thrown at it until the off thermostat fires.

Which is better, running:

1. a burner at a low modulation rate, but with a lower return temperature to promote condensing.
2. a burner at its full rated value, with long cycling (i.e. 10-15 minutes) but with a higher return temperature.

This is all based on the understanding that modulating boilers are actually more efficient at full rated value, than at their minimum level.

Christian.

[Andy Hall]

True. The other factor is how to detect the difference between the continuous and relatively low load of the radiators vs. the large demand for DHW.

Remember that with TRVs, the radiators also present a variable load anyway - or to some extent with zones.

Kind of. If you have TRVs or zones then the heat load varies and the return temperature from the radiators will change accordingly (for a given flow temperature to them). A modulating boiler will respond to this and adjust power level to match.

This is not the case for condensing boilers. If you look at the immediate gross and net figures for efficiency for condensing boilers (not SEDBUK weighted, which confuses the issue here) you will find that efficiency figures increase at lower running temperatures as well a with matching heat output of the boiler to the load. Partial load efficency, normally measured at 30% also increases with reducing temperatures.

Take a look at

is quite a useful German tutorial on condensing boiler technology. Pages 35-41 are pertinent. Keep in mind that some of the points about condensate and chimnies have to do with local German regulations, and they talk about "forerun" and "reverse run" temperatures - means flow and return.

For a DHW system, the objective is to get as much heat as possible in the shortest time possible into the store. The heatbank does this well and runs the boiler at full tilt for this short time. That is certainly more efficient than going via an inadequate coil which results in slower transfer of heat and cycling. The objective is different, to CH though.

With CH, the objective is to match the load to the boiler, run the boiler at less than full output if you can and especially to have the flow and return temperatures as low as possible.

.andy

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[IMM]

You don't.

Go away... Cycling? On a heat bank. the compemnsator says the bottom section of the store requires 50C, the boiler comes in and makes it 50C without any boiler cycling. IF the day wr,ms up then it may only want 40C and when it is below 40C the boiler comes in and reheat that section agin to

40C.

Note:

1. 50C is a low temperature.
2. It said NO boiler cycling.

That is very nice to know. But cheaper condensers can be used and when coupled with a compensator and heat bank performance is improved all around.

[IMM]

Can't you read. The oil boiler would be cycling mainly on CH and if a lousy cylinder on DHW too.

Oh my God!! Not again!!! Can't you read!!! You put on a simple cheaper boiler (a great advantage) and allow a compensator to control the boier which when heatinhg, heats up a large volume of water so does not cycle.

Which the rad circuit pumps into from its return pipe.

You clearly do not understand. A boiler with load compensation control and an outside weather compensator controlling it. The compensator tells the boiler that it needs the lower cylinder section heated to say 45C, it heats it and the rads use this temperature water because the compesator said that is the temp you need.

At least you understand that.

failed again. Two stats can eliminate boiler cycling on a direct heat bank.

[IMM]

Which a weather compensator does with a heat bank between rads and boiler. he large volume of water being heated makes it more efficient.

[Andy Hall]

This is complete waffle and nonsense.

Are you now saying that you have separate flow into the heatbank at the top for DHW and part way down for CH, or are you suggesting having cylinder thermostats on the tank part way down?

The first of these is pointless because you might as well do the job properly and feed the radiators directly, letting the boiler detect the temperature and modulating accordingly into higher efficiency.

The second, with the involvement of a thermostat and a non-integrated weather compensator box will cause the boiler to cycle as the compensator attempts to pulse-width control it.

How is differentiation made between the DHW and CH requirements as far as the heatbank contents are concerned? For DHW operation, the need is to have as much of the heatbank at as high a temperature as possible - at least 75 degrees. Yet you talk about wanting 40-50 degrees part way down. Completely pointless.

Do you mean non-modulating types? If you are going to do this, and run them at 80 degrees out, you might as well forget having a condensing model and just get a cheap conventional model. Don't let Rocky hear about it though.... :-)

.andy

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[Andy Hall]

Sigh.

If you have an external to the boiler weather compensator, it works by pulse width modulating the switched live to the boiler.

If the run time of heat demand is long and this mechanism comes into effect, the boiler is being cycled, by definition. If the run time is short, and the thermostat stops the heat demand first then the boiler is shut down and the compensator does nothing.

.andy

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[Andy Hall]

Perfectly thanks, and at a reading age of greater than 6.

To quote:

"Quite - we have pretty poor mains pressure (just under a bar - the reservoir is about 12m above the house), and no real space problems. The reason I put in the thermal store (or heatbank as it seems to be defined here) was becuse the oil boiler was short cycling, despite all my attempts to cure it. The thermal store solved this, and provided us with a nice on demand energy store."

Please explain to me where mains water is used in a CH system other than to initially pressurise it.

Blustering will not alter that fact that you are attempting to alter the content and increase the complexity of the configuration, in a vain attempt to obfuscate the issue.

Sorry, but it won't work.

I'm glad it's not onto the floor!

I understand completely that this is nonsense. Of course you can hook this up, but it is completely pointless, because the controller, if an external device to the boiler controls it by pulse width modulating the switched live causing cycling unless the temperature rises so quickly that the thermostat gets there first. In the latter case, the compensator box will have done sweet FA.

You then have the issue that by the time you've bought this, you might just as well have bought a better boiler with integrated weather compensation giving analogue modulation of the burner.

By definition, an on/off thermostat can't eliminate cycling.

Added to this, a bunch of extra complexity is needed to make it work.

We started with a very simple arrangement of a heatbank, fed from a modulating, condensing boiler, in turn feeding a plate heat exchanger on demand of hot water. The heatbank can be maintained at a high temperature to maximise the performance. The boiler can drive this for single short periods at or close to full power. The same boiler, when the heatbank is not connected is connected directly to the radiators where it can measure return temperature directly and modulate down to low level rather than off and on.

A relatively simple and effective solution.

Now we're at several pumps and valves, multiple thermostats, weather compensation boxes and goodness knows what else. Who are you trying to kid?

.

.andy

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