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Heating and cooling: Combi boiler HW circuit for fast heat bank recovery?

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    • CommentAuthorbhommels
    • CommentTimeMar 21st 2018
     
    Hi All,
    With a backboiler upgrade looming, I am considering my options. The system has underfloor heating, a few radiators, and a 210l heat store for DHW. As discussed in another thread, the DHW store needs to be at 50 degrees minimum to be useful, whereas the radiators and UFH only need 30-45 degrees water.
    A system boiler could do all this, heating the heat store through the coil heat exchanger, which is a bit slow and prone to rapid cycling. Due to the large difference in output temperatures depending on who calls for heat, it will be quite hard to operate a system boiler efficiently at all times, using simple controllers.

    Since the vented heat store cylinder has direct connection bosses, I was thinking that the hot water output of a combi boiler could in principle heat the store directly, provided it is fed with with the store water. It would give super-rapid recovery rates, and since combi boilers allow different set temperatures for the CH and DHW circuits, it would make the controls a lot simpler (potentially). It would require an extra pump perhaps, and that is something I can live with.

    Any opinions on this please? Has anybody tried this? Any other ideas? Otherwise feel free to shoot me down :-)
    Bart

    PS A DHW only heat store might set alarm bells ringing, however it is heated by solar PV and provides all our DHW needs for about 7 months a year, so I am inclined not to "rip it all out and just install a combi".
    • CommentAuthorTimSmall
    • CommentTimeMar 21st 2018
     
    I've seen this arrangement documented in at least one boiler manual (Ariston?), so it should work.

    A potential problem is that when you are far from the set point, the boiler will tend to race (and run at low efficiency), but near the set point, it might short-cycle.

    If you do it with a system boiler instead, you could get something to change the set point (and limit the output too). I think this is reasonably easy on a Vaillant (at least on the one I have), since it's all been documented and/or reverse engineered.

    Has your heat store already got a coil built-in?
    • CommentAuthorbhommels
    • CommentTimeMar 22nd 2018
     
    Thanks for the reply - yes the cylinder has a coil, which is mostly used in the 5 months when the solar PV does not get the water up to temperature.
    I conveniently forgot to mention that since the cylinder and CH system have different head heights/pressures I want to avoid tying them together.
    • CommentAuthorCWatters
    • CommentTimeMar 22nd 2018
     
    Don't combi boilers detect the incoming mains pressure and complain if it falls too low?

    I've seen combi boilers used to heat a hot water cylinder but it was done using the CH output not the DHW output. In that case I think the house had the shower fed from the cylinder and the kitchen taps (and others) fed from the DHW outlet on the combi.
    • CommentAuthorArtiglio
    • CommentTimeMar 22nd 2018 edited
     
    In a previous threads there was mention of Viessman boilers , one of these i believe is in effect adapted combi which has a circulating hot water output in place of what would have been the dhw, if so this might be ideal for your purpose and would also allow for two output temperatures , one for the dhw tank and one for heating.
    • CommentAuthorTimSmall
    • CommentTimeMar 28th 2018 edited
     
    I've previously run a (stove+solar) heat store output through a combi boiler (originally an Ariston, later replaced by a Vaillant) to provide "top-up" when needed, had you considered this?

    You can remove the DHW flow restrictor from the combi (or upsize it to one from a bigger model in the same range, or add a standalone restrictor of your choice outside of the boiler) if you like (since with a smaller delta-T, a larger flow rate is possible with the same energy input).

    You could also use a thermostatic mixing valve (one without auto-low-pressure-shutoff, plumbed in "backwards") to stop the DHW passing through the combi when it's already hot enough.

    Another hack which I'd considered is to arrange for the combi boiler to always sense no-flow when the cylinder stat was over a certain value - that should be reasonably easy to do and would be an alternative to the backwards-mixing-valve thing.

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