Anyone know of a supplier of fast recovery vented hot water cylinders of 250L capacity?
Loads of unvented cylinder are available at about that size, but your common or garden type seems scarce...
The background (this is partly an exercise in thinking aloud!):
Just deciding if it is cost effective to make a heat bank...
The choices for main pressure HW production seems to come down to:
COTS Heatbank like DPS Pandora I got a quote from them, and a 250L one with all the trimmings, ready to install and go, with pump and 160kW PHE on the output, a 12kW indirect coil to heat, plus 28mm blending valve, 3kW immersion, scale reduction, looks like costing about £1900
DIY Heatbank The paraphernalia around the edges could come to say £450, so much depends on the cost of a suitable tank...
Unvented  anything from £500 - £900
(and no, I don't want a combi, so let's not go there!)
Now most of the killer applications of a heat bank don't really apply in my case. I won't be integrating solar, solid fuel, or biomass etc, and heat pumps are at best only likely to match mains gas for price. So the main attraction is that in a hard water area, any scaling is going to be limited to the PHE, its a low maintenance option, and the (minimal) risks from having a pressurised cylinder are avoided. Installation is slightly easier, since there is no need for a emergency drain connection.
 Yes I know it supposed to be installed by a G3 certified bod, but they all come under building regs these days, and how many people remember to do a BN for a hot water tank swap? If one were to DIY, this is quite a cheap option. Pay for someone to do it however would probably make it the most expensive.
 DPS seemed keen on fitting a HydroFlow HS38A electronic water conditioning widget... I enquired if they found this effective, and they seemed to think that it worked in this application.
 The ideal roof slope is littered with dormers and chimneys, so nowhere to put a decent sized panel. (plus I am trying to do this with minimum capital outlay!)
 Doing the sums and planning for future requirements (i.e. at some point will have two teenagers co-resident <shudder>), I reckon on needing 400L of final temperature water from one "charge" of the store. That comes to about 230L of store temp (70 deg C) water required to achieve that (spreadsheet available if you want it).
 Best overall efficiency is going to be achieved with a weather compensating controller on the boiler. The modern integrated electronic jobbies also seem to take care of all sorts of little details like being able to programme a period of high flow temperature operation for water cylinder recharging, while running the rad zones at far more efficent and appropriate temperatures. This tends to suggest that the best mode of operation, is to recharge the store during a heating setback period, and not attempt to do it while running any heating zones. The logic for this being you don't interrupt your heating, and your rad flow temp won't have to run up to 70 all the time the HW is calling for heat.
The down side of this is that you can't really augment the size of the store by allowing for concurrent boiler recharging - hence it needs to be big enough to cope with most eventualities.
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