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1 tank or 2?


le-cerveau

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I am moving towards finalising the design of the Heating/Cooling and DHW system.  The house will have UFH/cooling supplied from an inverter ASHP that can also cool the floor (and supply cool water to an MVHR duct cooler), heat load of 3.4kW over 419m2 from PHPP.  However we have a high DHW requirement (after long discussion on the previous forum) I have got it down to 27 L/m at ∆T of 44K with a usable capacity of ≈400L.  (5½ bath house).

I was looking at having a separate buffer tank for the UFH (200-300L) which could also pre-heat the DHW (a la Jeremy's layout, just bigger) followed by a Thermal store (300 – 500L) to supply the DHW (We have Mains Gas so boiler power).  This would separate the 2 systems (recommended by most on the forums) and allow me to run the TS at variable temperatures (50oC – 80oC) depending on required loading (low occupancy – high occupancy), whilst eliminating the Legionella problem with a UVC at low occupancy.  I would also cross connect the heat generators so the ASHP can provide low grade heat to the lower part of the TS and the boiler can supply the Buffer during ASHP high load/failure.

However I am now considering just one larger Stratified TS (750 – 1000L) to cover the whole system.  This would make plumbing and specification easier and theoretically the thermal losses from one large tank should be less than 2 smaller tank.
eg.
                AKVATERM AKVA GEO 300         
64W
                AKVATERM AKVA GEO 500         
75W
                AKVATERM AKVA GEO 750         
89W
                AKVATERM AKVA GEO 1000        96W

So the 300 + 500 gives me 139W (although the 300 would be at a lower temp so less loss).  Yes I know losses will be higher but as a guideline.

I have been looking at the Sunamp Stack but it is only available as a complete stack 40kWh and would not support the low temperature of the ASHP (requires high temp to charge > 58oC).

What would the collective experience go for with regards separate tanks and giving individual (if cross connected) systems vs one all singing (overall lower loss) tank?

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Yes,

at least 9 kW and possibly twice that although on 3 different faces (ENE, SSE, WSW) so not all generating at full capacity at the same time.

I was considering Solar Thermal, but the general advice on the forums has been to go for PV and dump excess power into TS when possible.

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I have been working on schematics and the one tank option has less control valves attached (although that is the latest design and has accumulators shown).
2 Tanks.pdf
This is the older Buffer and TS layout (Utilising coils to separate the 2 systems).

1 Tank.pdf
This is my latest TS only layout (Showing a AKVATERM AKVA GEO tank, but any suitable TS would do).

Both should work, it is just a question of which way to go.

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I am considering a very similar system. Large thermal store in garage with large solar thermal (16m2) with a preheat DHW water supply and convection heating of a normal size accumulator inside the house which is also supplied by inverter ashp and solar pv.  There will be a mix of ufh and rads in the property. ASHP and in roof thermal panels already bought from eBay. 

Tbh if had mains gas I don't think I would bother with above system, it would be the smallest system boiler I could find and a 500l uvc/thermal store

 

Edited by Alexphd1
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Planning to have it connected on a large preheat tank maybe 1000l.  I need hot water 12 months a year inc late spring to early autumn when st does contribute something.  

Edited by Alexphd1
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Agree it will contribute, but only to hot water. 

I'm considering a huge TS too, but for a DIY ( non MCS ) Pv array and my chimnea to heat it. For ST I just don't see the benefit as you have the additional capital cost to integrate it, so even if the ST kit is cheap it's still down to economics / maintenance / efficiency for me so Pv just seems a no brainer in comparison. 

I don't think you can get RHI with a TS so another negative ( and I think that's a bad deal btw as it would be better suited to a TS over an UVC in most instances :S ). 

I'll shut up now ;) 

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In my exsiting system we regularly see the bottom of our 300L UVC tank hit way above the 58 degree as heated by gas boiler with two panels.  So as a feeder tank even if the 1000l hits 30 degree it's a big help. As for components it's just a differential controller, grundfos solar pump, expansion tank and prv that needs maintained. Also keep a eye on pressure for leaks. But if I was paying normal prices for components and labour at X amount them I would agree with you, it ain't worth it. Also this would be feeding the ashp thermal store so if big thermal store heats above ashp thermal store then  St heat would be used for ufh but in reality I don't think we will get much energy from ST when we need heating in house.   As for Rhi I have totally written off inc for ashp.  

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Agree with all that :)

Not saying it won't produce hot water / heat energy, just I see it as a one-trick pony. You do have a lot of it so maybe it'll be worthwhile. What's your means for dumping excess energy when the cylinders are satisfied? 

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Wash the car with hot water:D just kidding. Was planning on building a hot tub hopefully with left over icf at a later date. Would be running this of the heat pump so haven’t figured out how but probably could use this as a heat dump.  

Edited by Alexphd1
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