Choosing a boiler - again.

[Doctor Drivel]

Wants and needs. He "wants" a direct replacement, when he "needs" to re-think and totally update his CH/water system.

[John Rumm]

You may find this more useful:

[Doctor Drivel]

Er no.At least one heat bank model has the plate "inside" the cylinder. This reduces heat loss and external space taken up by the plate and pipes.

It also says: "Another difference between Thermal Stores and Heat Banks is in how efficiently they use the amount of hot water they contain to provide DHW. This concerns stratification and mixing of water in the store, explained in the box below. The net effect is that a Heat Bank must have a higher capacity than a Thermal Store to provide the same quantity of DHW (if all other factors are equal)."

This assumes all heat banks destroy stratification in a cylinder. A modulating pump with spreader pipes in the cylinder does not destroy stratification.

[Andy Hall]

You'd never make it in sales....

[Doctor Drivel]

Also from the site:

"However in thermal stores and heat banks the water in the cylinder heats DHW indirectly through a heat exchanger so the water in the store can be the primary water shared with the boiler etc. This allows the store to recover as fast as the boiler can generate heat. Also, since there is no temperature drop between the boiler and the stored water (as there is across a primary heat exchanger) the boiler can run at a lower temperature to keep the store at its desired temperature. This allows slightly greater efficiency of operation with high-efficiency condensing boilers. "

Plate heat exchangers are far more efficient than coils. The cylinder temp can be even lower using a plate and having the boiler heat the cylinder directly.

It goes on...

"This arrangement does, however, require that the primary circuit be vented (since an unvented system would place the storage vessel under pressure, as in unvented DHW systems). In practice this means a small header tank is required above the highest point in the system i.e. above any radiators on the top floor. "

Er no. A pressurised heat bank cylinder (only at 1 bar) and system boiler can be coupled directly. The CH circuit can also be taken directly from this cylinder too improving efficiency all around. A G3 unvented ticket is not required to fit these - can be DIYed.

[Doctor Drivel]

Bright technical people never do.

[Andy Hall]

Wanna bet.....

[Dave Plowman (News)]

Dribble couldn't sell me a chilled drink in a heatwave.

[Dave Plowman (News)]

Right. I've been having trouble with the Viessmann site so will check it again using the PC. At the moment I'm tempted by the Keston Celsius 25.

[John Rumm]

That PDF did not render properly on Acrobat 5 either - not sure if it is better on later versions.

[John Rumm]

This may be the case, but does not in any way invalidate what was said before.

And what proportion of them have that?

[John Rumm]

No one mentioned coils. The passage above states "the water in the store can be the primary water shared with the boiler" - i.e. this *is* a direct cylinder with no heat exchanger at all between primary and store.

The only heat exchanger required is the one between DHW and store content (and almost certainly would be a plate HeEx).

So what exactly are you disagreeing with?

"A pressurised heat bank cylinder" is not an unpressurised heat bank then is it. So Not what was being described here.

(That's ignoring the dubious claims of efficiency gains resulting from interposing a thermal store between boiler and rads when the boiler already has the ability to load match using a proper closed loop control system)

[Doctor Drivel]

I don't sell.

[Doctor Drivel]

Please eff off as you are a troll

[Doctor Drivel]

It is saying it as that was the only configuration.

All dedicated heat banks have spreaders and/or baffles so stratification is not destroyed. Gledhill have modulating pumps which also maintain the outlet DHW temperature too.

[Doctor Drivel]

I know. The stored water temp needs to be high because of the slow and more inefficient heat transfer of immersed coils. Plates are far more efficient and can operate at lower stored water temperatures: hence using a direct boiler and a plate the stored water temp can be quite low. Insert a blending valve on the flow/return of boiler to ensure only higher temp water enters the store and the store size may be smaller and the stored water temp lower again. This really does promote condensing efficiency.

It implied that there can only be vented heat banks.

If the house is calling for say 2kW the boiler will cycle. Most boilers only go down to around 8kW, most above that, so cycling is inevitable. Only expensive boiler go down pretty low and 3kW is about the lowest (and those boilers cost). Also no centralised nuisance thermostat is needed and TRVs on all rads giving accurate local temperature control. The boiler also works with full flow through the heat exchanger - a major problem of early boiler failure is heat exchanger problems as many are operating with far too little water flow through them as TRVs close down. No too mention noise. No inefficient auto by-pass valve is needed that raises the return temperature of the boiler reducing efficiency greatly.

[John Stumbles]

What model is that?

I agree, but I don't know of any commercially available systems that control the flow to preserve stratification (a technique which is mentioned in the wiki article, incidentally). Are there any?

(I am aware that DPS - and possibly others - use spreader arrangements to restrict turbulence in the return from the PHE but that doesn't help when the system is still having to pump round at a rate sufficient for the maximum DHW demand, even if one's only demanding a dribble of HW.)

[Doctor Drivel]

Nu Heat

Gledhill, which also controls the DHW temp by pump modulation.

The spreaders and baffles do help even on full pumping speed. Speaders can be DIYed using drilled pipes and drilling out pipe stops in compression fittings. A 3-way diverter blending valve can be used sensing the DHW temp, after the plate which pumps into the store on one port and before the plate on the other. Fully open with full DHW flow and full flow into the store and reduced (pumping back on itself) if only a trickle from DHW.

[Dave Plowman (News)]

Having looked on the PC it has problems but renders OK. But doesn't really give much information. I suppose what I really need is the various installation manuals.

[:Jerry:]

Would you want him to even try, the mere thought of it turns my stomach!