Heat Bank - space heating or not ?

If you're going to fit a heat bank to get the benefit of mains pressure hot water, no tanks, etc. etc., what are the pros and cons of heating the radiators from either the store with the necessary coil and pump or directly from the condensing boiler?

I'm wondering if I should just treat the heat bank as a cylinder and connect the rad circuit(s) directly via a zone valve or to feed them from the store

I have this suspicion that if you buffer the radiators via a heat bank you may lose some of the benefit of a condensing boiler since the return temperature will be greater than if you designed the system for radiators, opting for a lower than usual return. Yes, I know it means departing from BS, having bigger rads, adjusting for mean temperature etc. but so what if it achieves the desired effect ?

I bet this has been discussed before but I'm taking the easy option

Reply to
Jeff
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Get a larger heat bank and run at the lowest temp you canget away with for DHW. Take the rads off the heat banks and use a Grundfos Alpha pump on the rad circuit.

Advantages of Heat-Banks

- Instant high pressure hot water - When the store has reached temperature water is delivered instantly at the taps.

- Very high water flow-rate - The high-end heat-banks have a flow-rate up to

45 litres/min.

- Long efficient boiler burn - Reduces boiler on-off cycling increasing efficiency, although inefficient boiler cycling is no longer a major problem as it was with balanced flues.

- Maintains optimum boiler temperature range - using a blending valve the flow/return of the boiler can be kept to optimum maintaining greater efficiencies. The boiler operates at optimum performance.

- Combines the output of the stored water and the boiler

- Cylinder may be smaller for a similar performance - smaller cylinders than unvented cylinders.

- Cylinder at low pressure - Unlike an unvented cylinder it does not store water at high pressures.

- Fast cylinder recovery rate - When the boiler is connected directly to the heat-bank, and not via an indirect coil, the recovery rate is rapid. Although in some cases a boiler may heat the heat-bank via an indirect coil, reducing the recovery rate.

- Legionella bacteria eliminated - The Legionella bacteria cannot survive in the high temperature sealed conditions of a heat-bank.

- No scale build-up in heat-bank - Containing primary and not secondary fresh water, there is no scale build-up inside the heat-bank.

- Cold water storage eliminated - No need for cold water storage tanks.

- Solar heating storage - Water heated via solar panels may be stored in the heat-bank via a solar coil.

- Easy maintenance - If an external plate heat exchanger requires cleaning or replacing it is a matter of draining down the heat-bank, or closing isolating vales, and unscrewing the plate heat-exchanger. In some rare instances plate heat-exchangers are fitted directly inside the heat-bank preventing on-site maintenance.

- Easy to improve hot water flowrates - By simply adding additional plate heat-exchangers in parallel, hot water flow rates may be improved. Retrofit additions are possible if extra bathrooms or showers are installed.

- Stored water vessel need not be cylindrical - As no internal coil is used for hot water heat transfer the stored water vessel may be any shape, as opposed to a thermal store which has to be cylindrical for maximum efficiency. This has advantages where space is limited.

- Rads warm up period only a few minutes - The store holds enough hot water for all the rads, so rads instantly hot

- If all TVRs Close the Boiler is Not Affected - Flow is always through the boiler

Disadvantages of Heat-Banks

- The store needs be near fully temperature to supply baths - Before any hot water is drawn off, the store must be up to temperature. Many later versions use a blending valve on the return to the boiler to ensure only up to temperature water is pumped into the store by the boiler. This prevents agitation of the stored water, and aiding heat stratification within the store giving useful water at the top of the store within a shot time. The water is heat only in one pass through the boiler.

- Lower water temperatures with fast flow-rates - As with Combi boilers, fast flow-rates through the plate heat-exchanger results in lower water temperatures. This is not so pronounced with heat-banks as with thermal-stores.

- Rads may need to be larger - Rads operate at lower temp, so may be larger

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Reply to
Doctor Evil

I don't think so - though my boiler isn't condensing. With a heat bank and a programmable thermostat you could set the boiler to kick in and out at whatever is optimal for the boiler, isolating it totally from requirements like not make every radiator scalding hot or stone cold.

Reply to
Mike

If you have a condensing boiler, then it will work optimally and more efficiently at lower temperatures. Especially at this time of year and in the autumn, you can run the radiators at lower temperatures and the boiler will modulate down to match.

If you have a heat bank in the middle, in order to heat the hot water, you need to run it at 75 to 80 degrees. You can take heat out of it to run the radiator, of course, but then the problem is that it will have the effect of causing the boiler to run in bursts of full output to replenish the store. This is not as efficient as letting it run continuously or near continuously at lower power to match the radiator requirement.

Heating the space with radiators and providing the hot water are two different problems. WIth the space heating, you can achieve optimal efficiency by driving the radiators directly from the boiler. For water heating, the objective is to dump as much heat into the cylinder as quickly as possible and with efficiency as less of a concern.

In my system design I have a condensing boiler with weather compensator driving the radiators directly. THe radiator sizing has been arranged such that at the nominal -3 degree outside temperature used as part of UK heating designs, the flow temperature will be 70 degrees and return 50 degrees or less. I achieved exactly this in practice during the winter. Now when just a little heat is required in the evenings, the boiler modulates down to about 3-4 kW and the flow temperature can be around 40 degrees or so.

However, when there is hot water demand, the boiler is switched over to driving the cylinder (a fast recovery type) and winds up to full power. The concept of this is to reheat the cylinder quickly and then switch back to running the heating.

It works very well.

Reply to
Andy Hall

Using a heat bank, the temperature only be 10C above DHW temperature. IF you want 50C at the taps, then 60C store temp will do. So a condensing boiler with a 22C flow/return temp differential need only be running at 38C return. Very economical. The store then needs to be larger.

Yes, economically so.

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Reply to
Doctor Evil

With a modern boiler (and hence sophisticated load compensating control system), you will usually be better off letting it drive the rads directly. (Thermal stores are good at improving efficiency on old "all or nothing" boilers, but often just serve to c*ck up the closed loop control system on modulating boilers)

Personally I would connect the rads directly. However there is sure to be someone along shortly to contradict that. ;-)

Yup, you can get round some of the problems with extra work (blending valves etc), but it is usually far simpler to avoid creating the problem in the first place than spending lots of money fixing it.

A modern modulating boiler will run at its most efficient when it can reduce its output down to a steady low state to match the actual radiator load. If you stick an extra sodding great energy store into the equation then you are introducing extra complexity that will usually prevent the most efficient mode of operating from being reached.

Not sure I follow that bit?

It has, to death ;-) Google will have loads.

One of the difficulties is that for many years the thermal store was actually a "good thing" in radiator circuits since it improved boiler efficiency by reducing short cycling etc. Modern boilers and control systems have now rendered that common wisdom less relevant in many cases. However some folks are not so easily separated from their old ways ;-)

Reply to
John Rumm

I should have mentioned - no gas here, I'll (or probably someone else) will be fitting a condensing oil boiler. AFAIK, they don't modulate. I think it's firing or not firing.

Possibly that would make me lean towards taking the rads off the heat bank, to reduce cycling. Otherwise, during not so cold weather the boiler will be oversized and cycling regularly. Would you agree? I can see the advantage there but then I think that the heat bank will get up to temperature and take the return temp up above the optimum. Pandora says the water in the store is typically 75 degrees.

If the rads are off the boiler, I could design the system for a suitable return temperature to try and keep it condensing. But then, that only applies when the system is fully loaded, and how often is that?

Maybe I worry too much about this efficiency lark.

Reply to
Jeff

OK, you covered all the pros and cons quite comprehensively, bearing in mind I think a condensing oil boiler can only fire at the rated output, you are saying it's likely to be quite economical to let it dump a load of heat into the store and then let the rads draw it off?

Reply to
Jeff

The great thing about a thermal store/heat bank is that a simple none complicated boiler need only be used. This adds reliability, and less service changes as less to go wrong.

Not so. Modulating boilers work wekll with heat banks.

< snip misinformation >

He doesn't need a modulating boiler with sophisticated load compensation control.

Old ways? What a joke.Thermal stores never took off and obtained the status it deserves because plumbers couldn't understand them and builders kept putting in silly tanks and cylinders. It is also too new for many people, so much for "old ways". They are becoming more popular now as people realise they have more advantages than unvented cylinders which require annual services.

The store is a superb neutral point separating, the boiler, rads, DHW, solar input, etc.

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Reply to
Doctor Evil

Oil condensers are very expensive and don't offer great efficiencies over non-condensers. Yep, just get a normal heat bank.

Very wise. Get a heat bank with two cylinder stats

Yep.

It can be higher. They want lower temps to get the best out of gas condensers.

With a non-condensing oil boiler you don't want low temps.

Just have two cylinder stats to prevent boiler cycling and lag the hell out of all exposed pipes. You could fit a blending valve on the flow/return. The likes of DPS would fit one for you.

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Reply to
Doctor Evil

Only if it's a modulating boiler surely. If it can only do full or nothing I'd still suggest putting the heat bank in the middle is a better option.

Reply to
Mike

Yes

Yes. The other problem with direct connection in not so cold weather is you don't really take much heat from the boiler, but it is running flat out so the boiler thermostat operates more often than the control thermostat and you can actually feel the temperature of the radiators going down whilst waiting for the boiler to come on again. A heat store will level out this effect.

You probably won't have the boiler ever fully loaded. Thus keeping it optimal is impossible.

No. Do worry about it. But for non-modulating boilers I think a heat store is the best approach.

Reply to
Mike

If the boiler is connected directly to a store, it will be fully loaded when reheating.

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Reply to
Doctor Evil

That's what I said in the paragraph after

Reply to
Mike

He may be saying that, but it isn't the case.

Most condensing boilers can modulate downwards to match the heating load. If you connect a small load such as radiator space heating during spring and autumn then the boiler can modulate down and operate at lower temperatures which are more efficient for it as well as not needing to fire on and off.

If you do this via a heatbank, then the boiler can never enter the lower temperature ranges and probably won't modulate down either.

The technique of using a heatbank as intermediary for space heating is useful in two scenarios:

- If you have a non-condensing boiler, it will provide a "smoothing" action which will reduce the amount of cycling considerably and improve its efficiency. The non-condensing boiler is designed to run at high temperature anyway.

- If you have another source of heat such as solar and need to do combining of some sort.

Reply to
Andy Hall

Most new condensing boiler designs are modulating.

If it isn't then there could be an advantage in putting a heatbank in the middle to reduce cycling.

Reply to
Andy Hall

It is the case.

Most cannot accurately enough. To do it properly you need a boiler can modulate down very low and have load compensation control. They are "expensive" to buy and have more to go wrong.

You can have a weather compensator maintaining the bottom CH zone in the heat bank. The boiler comes in to reheat the mass of water to the settings of the compensator. Simple and effective. Cycling is vastly reduced as a mass of water cools slowly.

Let's see what wisdom we have here...

"Eliminate" cycling with another stat. The same also applies to condensing boilers.

Above condensing point. They run more efficiently when a flow/return blending valve is set to maintian the return at 59C. At 11c delta T that is

70C flow temp.

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Reply to
Doctor Evil

On temperature, not load.

Always an advantage.

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Reply to
Doctor Evil

Ah.... you didn't say that. I believe that there are modulating oil boilers on the market, but most are not.

If the boiler has fixed and high output in relation to the needs of the radiators then yes.

Bear in mind that there will be a range of temperatures from top to bottom. When you have just the radiator load, the return water temperature into the store will probably not be as low as when you are heating cold water and the return water is from the plate heat exchanger.

Remember that there isn't a Holy Grail that you get to when condensing begins. In reality what happens (if you think of it as a graph) is that efficiency increases with falling return temperature. Below the dew point where condensing begins, the *rate* of efficiency improvement increases. In other words at 53 degrees, it's not hugely better than at 55 degrees. To achieve maximum efficiency, you would need to arrange the temperature to be as low as possible.

The problem is that the scenario for the radiators is different to that for the hot water.

For the radiators, you would ideally like to let the store get almost cold and then fire up the boiler, simply because for most of the time you are using heat at a lower rate than the boiler can provide. This would result in the boiler firing up and reheating an almost cold store, which, given a fixed output boiler, is the best that you are going to get.

For the hot water, you really need to have the store as fully charged for as much of the time as possible. This is because when you make a sudden large demand such as a bath, you are using heat far faster than the boiler can produce, but of course the store can keep up while its heat lasts. If you allow the store to deplete to improve boiler efficiency, then you lose the benefit of storage and the set up becomes limited to boiler ability in terms of HW flow rates.

So really the two objectives don't coincide that well.

I suppose that one could put more sophisticated temperature sensors and control systems around the store to detect load behaviour and (for example) fire the boiler up sooner if the temperature drop is rapid, but this is becoming complex to do and the return on investment may not be there.

Probably only when you are starting the store from cold or having a large

Reply to
Andy Hall

It depends on the type of boiler.

"Accurately" is not an appropriate way to describe it. It is true that some boilers are able to modulate over a greater range than others, but that has to do with a number of factors. There are now some relatively inexpensive ones able to cover a very wide range and drop down to below 5kW.

It's a complete nonsense to suggest that a boiler with load compensation control has more to go wrong than one without. There is an additional temperature sensor outside and a more sophisticated control algorithm used in the controller electronics. this does not imply that there is more to go wrong.

You can, but it is still at variance to the required behaviour from the store when heating the DHW. One doesn't really want the behaviour of the hot water heating to change with the weather.

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Reply to
Andy Hall

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