Heat banks (again!)

[smiffy]

Degrees are often bestowed upon intelligent idiots.

[Andy Hall]

Wowee. Well they're not going to say "By the way, John, this is not the most efficient way to run a condensing boiler", are they?

After all, they still sell conventional boilers and are quite pleased with themselves about that.

The fact is that more efficient operation by far happens at lower F&R temperatures.

A win,win situation, I'm sure.

How do you react to having your pilot lit?

.andy

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[IMM]

You are confused in many respects. Very confused. All of a sudden he is a boiler designer.

And that can be achieved by various methods.

Must be.

[IMM]

This is a classic case.

[Andy Hall]

The principles are clearly explained in the link that I posted earlier and are quite straightforward for anybody who understands the principles of latent heat and a few other simple principles of physics.

There is very little individual design left in condensing boilers on the market today with all having quoted laboratory efficiency figures within a percent or two at the most.

The question then becomes one of using the instrumentation of the boiler effectively in order to maximise its efficiency by matching to the load.

The simple and most effective one being to directly couple the boiler to the heating circuit and let it use its internal control systems properly.

The only way to match this with external systems and controls is by having a boiler which can take an analogue or PWM signal to modulate it and to then use more sophisticated external controls with more analogue or analogue equivalent sensors.

Adding on pieces with on/off controls such as thermostats, weather compensators with on/off outputs and the like is not going to do as good a job as the analogue sensors that the boiler manufacturers use in their products. The best that they can do is a very crude simulation of an analogue control system, and that is simply not going to match up.

.andy

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[IMM]

Not according to RCM: "Thermaflow is a primary water thermal storage unit that supplies mains pressure domestic hot water at good flow rates in accordance with the requirements of BS 6700. NHBC and the Water Heater Manufacturers Association Specification. The thermaflow system facilities easy installation without the requirements for building control or BBA approval and is readily incorporated into traditional or modern central heating systems. NO MORE NEED FOR COLD WATER STORAGE TANK IN YOUR ATTIC."

is a "pressurised" thermal store to assist combi's and give higher flowrates.

[IMM]

So does that web site mean the W-B boiler is pants?

It isn't.

< snip confused stuff >

[Andy Hall]

Well..... your friend Rocky seems to disagree.

From Part G3 of the Approved Documents to the Building Regulations, an Unvented Hot Water Storage System means an unvented vessel for either

a) storing domestic hot water for subsequent use, or

b) heating domestic water that passes through an integral pipe or coil

The Thermaflow seems to definitely be b)

.andy

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[Andy Hall]

Go and read it and decide for yourself.

Generically, a condensing boiler will have greater efficiency when run at lower temperatures and when the rate of heat production matches the characteristics of the load.

This is not a surprising result because the former is influenced by the degree to which latent heat can be exploited and the latter impacts on this and cycling. Having an effective control loop in place with the boiler able to sense the operating load properly is the best way to achieve that.

Actually it is.

Within the context of having the boiler in direct control of its load with its internal control system and using analogue sensing vs. having simple external sources turn it on and off, the former is clearly going to be the most effective and efficient.

Only the introduction of more sophisticated external controllers with either PWM or analogue control of boiler modulation would improve on the boiler internal control system in terms of oprating the boiler in its most effcient range.

For a normal domestic application this would be costly.

.andy

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[Pete C]

Oh well, I suppose it comes down to the cost of an extra heat exchanger and pump vs the money saved by a cheaper boiler and reduced boiler cycling.

I'd expect the heat loss when cycling is due to losses from the boiler's heat exchanger when the water isn't being heated, and shouldn't be too bad with a modulating condensing boiler using an aluminium exhanger.

cheers, Pete.

[Andy Hall]

This was my point. You don't need an extra heat exchanger or pump.

You simply connect the modulating boiler via either a diverter valve or two (or more) zone valves with one way going to heat the heatbank and the other to the radiators. The boiler will handle both in the most efficient way. It is pointless to make it any more complicated than that.

.andy

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[IMM]

And there are various ways of doing that.

A good method but there are other ways.

Not so.

Not so.

Again, not so.

[IMM]

The boiler will not have load compensation modulation control, so he is on the right track.

[IMM]

Are you still going for a heat heat bank?

[Andy Hall]

Putting a dampening element like a heatbank in the control loop is not one of them.

Adding other things will circumvent the boiler's control system and unless properly instrumented with suitable analogue or analogue equivalent sensors will not be able to improve on it.

Of course it is. Do you want me to demonstrate the behaviour of control systems to you or can you provide a properly documented reference with formulae?

You have already discussed adding in weather compensating boiler controllers and extra valves and pumps. There's at least £250-350 cost in that lot. In the context of a boiler costing £500-900, which would do the job perfectly well on its own, this is adding unnecessary cost.

Added to which, if the boiler controller is not controlling the boiler by analogue or analogue equivalent means it cannot possibly do a better job than the boiler's internal modulation controls. The simple external weather compensator boxes do not do that. They are intended as an after-market add-on for non-modulating boilers and by definition will not do as good a job as an analogue sensing internal system.

.andy

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[Andy Hall]

Not if it is a simple external box with on/off control of the boiler. If it is working, the best it can do is to cycle the boiler on and off during heat demand periods of the room thermostat to reduce or increase the average heat output. This is intended to compensate in some way the thermal behaviour of the space being heated.

With a modulating, condensing boiler, there is absolutely no point in doing this with this type of device because it simply cycles the boiler when that doesn't need to happen.

If the boiler has the ability to have an external temperature sensor connected which does give its controller an analogue reading of the outside temperature, then that is worth having. However, this type of boiler costs more, even though the sensor is cheap or even included.

.andy

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[IMM]

It is when appropriate weather compensating control is used, to ensure the heat bank, or lower section of heat bank, is maintained at the variable setpoint dictated by the compensator.

You have a boiler with a minimum control system.

You have to know what you are controlling before you apply control. You don't.

1. compenstor
2. 3-way diverter valve
3. extra cyl stat
4. cheaper simpler boiler.

All in all works out about the same, and a superior more efficient system.

You don't understand these things.

What tripe!!!

When combined with heat banks and heating a large mass of water matters are very different. This you can't understand. This is very sad.

[IMM]

When he couples it to an "integrated" heat bank matters are very different.

[Andy Hall]

The heatbank will always introduce a dampening factor into the control loop. The element that is intended to be controlled is the room temperature, not the water temperature of the heatbank. The weather compensator, if integrated with the boiler should move the operating curve of the boiler up and down to match the load.

If you switch to maintaining the temperature of part of the heatbank to maintain a set point according to the outside temperature, you introduce a double control loop.

In the piece between the heatbank and the boiler, if you use the typical separate box type of weather compensator, you will have an analogue sensor for outside and one for inside, plus an input for a switched room thermostat. The inside analogue sensor would normally be used on the boiler return, but could be used on the return from the radiators to the heatbank. Neither is ideal because connecting it to the radiator return means that the controlled device (boiler) has the damping effect of the heatbank in between which will adversely influence the control loop. Having it on the boiler return means that the temperature being monitored relates to heat use by the heatbank and not the room space.

Then you have a switched control of the boiler - cycling it on and off to attempt to maintain the temperature of the heatbank to a set point. This is inefficient because you are then cycling the boiler.

All in all, a pointless exercise.

There's no point. It's overall cheaper, better, more efficient and simpler to have a modulating boiler with monitoring of its water temperatures, connected to the radiators directly, and with a weather compensating sensor able to provide temperature information (not just on/off cycling).

It is very obvious what is being controlled. One part is the room temperature. The other part is controlling the boiler firing level most efficiently to match the room requirement. In the case of a modulating, condensing boiler, this comes by maintaining the firing rate at the minimum required rather than turning it on and off. An external temperature sensor connected directly to the boiler to provide it with external temperature info. directly will assist with comfort on the one hand by increasing output above what is suggested by room requirements when the outside temperature is falling, and reducing it when the outside temperature is rising to reduce overshoot.

In a system where the only source of heat input is a gas condensing boiler, there is no point in putting a heat bank in the middle because it simply distorts the control algorithm, and offers no advantages.

it also screws up the primary purpose of the heatbank, which is to provide an energy store to deliver large amounts of energy quickly to a heat exchanger.

Neither statement is true.

A BEM5000 costs £188

A diverter valve costs £56

You forgot the extra pump to run the radiator circuit - £56 for a Grundfos

All prices from Discounted Heating. Total cost is £311.

You can buy a Worcester Greenstar 28HE for £725 from them.

What 28kW condensing boiler you going to buy from Discounted Heating for £410 that is any good?

Oh dear. Do you know nothing about the difference between analogue or pseudo-analogue vs. switched control?

Of course they are very different and I understand the difference perfectly. The results, if using a condensing boiler as the sole source of energy, will be inferior, by definition, than the direct connection of the boiler to the radiators as the manufacturers intend and design.

The value in a heatbank is in its ability to store energy at high temperature to run a plate heat exchanger for the hot water. It is also useful in the introduction of heat from other sources such as solar.

It provides no value in a path between a modulating condensing boiler and radiators. Even with all the extra controls that you describe, the efficiency will be worsened. To suggest that coupling a switching controller with a simple boiler is an improvement is laughable.

.andy

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[Andy Hall]

Yes. Worse if the sole source of heat is the condensing boiler.

.andy

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