DPS Pandora heat bank

[Christian McArdle]

Just to let anyone who is interested know, I have just installed my DPS Pandora heat bank. Possibly anyone considering a new hot water system might be want to read on.

It is a 180 litre model with the standard plate exchanger and indirected heated by coil. Except it isn't, because the new boiler hasn't been installed yet and the old one is using a feed/expansion tank below the level of the tappings so can't be plumbed into it. It is currently electrically heated by 3kW immersion. The immersion thermostat is set to be identical to the indirect coil thermostat, so performance should be similar.

It does work exactly as expected. It easily fills an entire bath up. Flow rates are higher than my old gravity system into the bath (but not really a patch on my parent's Megaflo or the pumped gravity system in my previous house). However, at the top end of flow rate, the temperature does start to drop a little. I still need to mix cold in though, even at full pelt. However, you need to turn it down to half near the end to bring the temperature up to the TMV's set point. (If you are anything like me, you'll fill the bath with warm water, get accustomed to it and then pour in very hot only to raise it up to just sub-scalding).

The performance on a shower is absolutely superb and totally comparable to an unvented cylinder, as you would expect. Despite the much longer run to the kitchen in the new layout, I also get hot water quicker due to the extra pressure helping with the long 15mm run to that tap and the high pressure only design (I bought the tap in anticipation of the mains pressure).

So all in all, for the same purchase cost as an unvented cylinder, you do get a system that isn't much lower in performance in practice. Obviously, there are benefits to it, such as the main vessel containing static inhibited water that will eliminate scaling, the inherently greater safety of using zero head pressure in a large heated vessel, and the fact that the DHW itself is not stored and can be poured straight into the kettle or cup (assuming no phosphate dosers or ion-exchange softeners).

Christian.

[IMM]

And no pressure reducing valves and no large overflow and no yearly service, and descaling the cylinder is a DIY job, whereas descaling an unvented cylinder is major task. And expanding the system is childs play by installing other plate heat exchanger in parallel - see if you can that with an unvented cylinder.

Christian,

Sounds good. You will be installing a 25kWish boiler, so best to use the power available instead of trickle charging (re-heating) the cylinder. We still have this ingrained notion of only using the stored water and waiting for the water to be re-heated, from the old coal fire back boiler days. And depending on the heat bank system you can have 10 bar pressure, not 3.5 as on unvented cylinders.

Now what you need to do to bring the system up to top spec is:

in the hot water draw-off before the baths and showers draw-offs and after the rest, install a flow switch, which will fire the boiler immediately as there is a large demand, instead of firing when 2/3 of the thermal stores energy is exhausted. No sense in switching in the boiler if the kitchen sink tap is constantly turned on and off, which will switch in the boiler with each tap turn; this will eliminate this sort of boiler cycling.

This will combine the energy of the thermal store and the power of the boiler, in effect making the cylinder larger. You have all that boiler power laying there - USE IT. Very simple to do and just amending the heating/hot water control setup. This can be done with any cylinder heated by a boiler.

[Christian McArdle]

Well, it is limited to 6 bar. If your mains even approaches this pressure, you need a PRV in case of natural variations. I haven't measured mine, but I'm sure it isn't that high. I'm on a hill and the flow rate is about average. No banging from the taps, either when turned off.

No need. The cylinder is well overspecced, getting on for two baths with no recovery. I can wait for the cylinder stat to kick in, which it will fairly quickly, as it is right at the bottom. It is an indirect system, so I can't directly combine the boiler output without a rats nest of expensive zone valves, flow switches and additional heat exchangers. It is indirect as I need to run a pressurised primary system.

Christian.

[IMM]

Mine actually does. Which is still far higher than the 3.5 of an unvented cylinder.

Nice to have one as it acts as a regulator when the pressure is above the setpoint.

Not the right thing to do. It should be sized correctly with the combined output of the boiler taken into account.

What make of stat and what temp is is set at?

The boiler can still pump 25kW into the cylinder when draw-off large water volumes, although not directly combining its full output with the thermal stores.

[IMM]

Then you can lower the temp of the heat bank by ensuring the boiler licks in when any shower or bath draws off hot water. The boiler can be a lower temp and store a lower temp, all promoting condensing efficiency.

[Andy Hall]

It's an entirely valid thing to do.

.andy

To email, substitute .nospam with .gl

[IMM]

In Fred Dibnahs day, yes.

[Christian McArdle]

It is overspecced for five main reasons.

a) I intend to install an additional shower room at some point.

b) By not taking into account boiler output, it is useful when run from the immersion, as it is at present.

c) If the calculations had been wrong (perhaps over optimistic from manufacturer) it would cost a bomb to sort out.

d) It makes any destratification of the temperature profile less important.

e) It does not significantly impact on the space used or the purchase price.

Christian.

[IMM]

Christian,

Now you can use the boiler with a flow switch on the bath and shower pipe (the outlets with most flow), reduce the stores setpoint temp and also reduce the boilers temp when heating the store. This promotes condensing efficiency - the engineering to keep temps down. By default you may have improved efficiency.

[Andy Dingley]

It's an odd thing, but you're probably the only person where I'd take Fred Dibnah's advice on pressurised plumbing over yours....

-- Die Gotterspammerung - Junkmail of the Gods

[IMM]

That is very foolish as I know far more about pressurised plumbing than Fred. Also this point was not at all about pressurised plumbing, it was about sizing.

[Andy Hall]

Do you do after-dinner speeches about it, at all?

.andy

To email, substitute .nospam with .gl

[IMM]

My mailbox is inundated with them.