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Water Cylinder v Thermal Store

bontwoody · 2022-03-28T17:33:50Z

I read with great interest Trystan Lea’s website on his experience with using an ASHP to heat his home and provide DHW During his video, he mentions that he was advised to use a hot water cylinder (170 lt) rather than a heat store for his DHW and was interested if anyone had a different experience. This got me thinking as I have an identical 5kW Ecodan ASHP, but heat my DHW via a heat store (450lt). My reason for doing this was to keep my options open with regard to perhaps adding a solar water heater to supplement my solar PV panels at a later date and obviating the need to do a Legionella purge cycle. Tystan heats his cylinder to about 42C twice a day, I heat my Thermal store to about 51C once a day which copes with 3 people having showers quite comfortably, although a bath depletes the store. I can’t be sure if Trystan’s system does as well or better, but I’m assuming it is the same for the purpose of comparison. I noted that Trystan does a manual Legionella purge about once per week, but read that it is advised to do one once per day. With all this information I created a spreadsheet to calculate the costs involved with a little guesswork on COPs at different temperature ranges. Cylinder 1 (HP) Cylinder 2 (HP) Cylinder 3 (Imm) Totals Heat Store Vol of water (lt) 170 170 170 450 Start Temp © 35 42 53 35 End Temp © 42 53 60 51 kJ needed 4998 7854 4998 30240 Theorectical kWh needed 1.39 2.18 1.39 8.40 Unit Elec Cost £0.30 £0.30 £0.30 £0.30 Eff of heater 375.00% 300.00% 100.00% 325.00% Actual kWh needed 0.37 0.73 1.39 2.58 Cost of heating £0.11 £0.22 £0.42 £0.78 Spec Heat Cap water (J/kg) 4200 4200 4200 4200 Weekly Cost (1 purge p/w) £1.55 £0.22 £0.42 £2.19 £5.43 Weekly Cost (1 purge p/d) £1.55 £1.53 £2.92 £6.00 I split Trystan’s heating cycle into 3 stages, 1- his standard heating twice a day, 2-his heat pump preheat for the purge and 3-the immersion top up to reach 60 C. If I haven’t made a mistake, then the two seem to come out with similar costs if the purge is done daily (£6.00 v £5.43), but much cheaper for Trystan’s system if the purge is done weekly. Hope this is of interest, I couldnt find any way of contacting Trystan, so if anyone knows how to contact him perhaps to could direct hime to this post. Thanks Mark

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Domestic Hot Water & Cylinders

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bontwoody

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Topic starter 04/04/2022 6:24 pm

@andygo

One big adavantage of the UVC that we havent mentuioned yet is smaller size and weight

I'd actually disagree with this - the 200l uvc isn't much smaller than the 277l TS it replaced - and there's other gubbins which comes with it. We've now got three expansion vessels, an exchange unit and a buffer tank crammed into the room where the store used to sit....

LOL scratch that one then

Think we need a qualified heating engineer to give us the official line on legionella?

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Derek M

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04/04/2022 6:33 pm

Posted by: @bontwoody

@andygo

One big adavantage of the UVC that we havent mentuioned yet is smaller size and weight

I'd actually disagree with this - the 200l uvc isn't much smaller than the 277l TS it replaced - and there's other gubbins which comes with it. We've now got three expansion vessels, an exchange unit and a buffer tank crammed into the room where the store used to sit....

LOL scratch that one then

Think we need a qualified heating engineer to give us the official line on legionella?

I think that I would prefer the opinion of a health professional rather than a heating engineer.

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bontwoody

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Topic starter 04/04/2022 6:43 pm

@derek-m

Lol, Im one of those and I havent got a clue! Just been scanning the web and opinions vary from one per day in industrial settings and once per week in domestic, to that the risk is so low in sealed systems as to be virtually zero. Still regulations are regulations and I thought a heating engineer might be able to shed some light on those

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Transparent

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04/04/2022 6:54 pm

Alternatively if an electrical engineer could shed some light on it, then it could be around 265nm and neatly disassemble the DNA of any lurking legionella

Save energy... recycle electrons!

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bontwoody

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Topic starter 04/04/2022 6:57 pm

@transparent

I actually have a UV lamp in my header tank, it stops the formation of "The blob" nicely

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Derek M

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04/04/2022 7:03 pm

Hi @bontwoody

From the information that I have seen, to some extent it depends upon how much hot water that you use, the bacteria is much happier fornicating in a warm tranquil environment. Most heat pump manufacturers appear to suggest raising the temperature to 60C, for at least 10 minutes, once per week.

To help save energy and reduce the cost, you could consider installing a solar PV system and electrical power diverter, which could be used to heat the water to 60C or above, most days from Spring through to Autumn. We have such a system and I swear that I can hear the bacteria screaming as they are being cooked.