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Author Topic: My HW use  (Read 5962 times)
brackwell
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« on: July 26, 2015, 11:33:01 AM »

Since April 2014 i have been heating my DHW solely by the immersion heater which i hard wired to a meter like people use for their FITS readings on PV.

Initially i had a 120L tank with 25mm insulation and the boiler connection was still attached.  With this set up the use over 170 days averaged out at 4.85kwh/day.   Water temp c50-55C

I then changed the tank to a 180L tank with 50mm insulation and a longer 24" immersion heater. Used plastic 15mm pipe direct to the shower and cut down dead leg time from 18secs to 6 secs (at 10l/min). Physically disconnected boiler pipes. Over the next 119 days the average was 3.84kwh/day

I then went on holiday but purposefully kept the immersion on to monitor tank losses. This worked out at 2.34kwh/day (= 850kwh /yr)

This all seems to add up because wife and I use about 1.2kwh/day shower + evening dish wash +v occasional bath + wife using HW hand wash.

This tank loss although large for a small HW user is in line with other peoples findings eg http://www.goodenergy.co.uk/media/BAhbBlsHOgZmIh00ZTk2YWEzZGUzNDU5YjRiMWIwMDBlMTI/i.3471_EST_SolarThermalWaterReport_Web.pdf?suffix=.pdf
 p16 fig 12   where 60% of the sample was in the range 600 to 1400 kwh/yr

Everytime i look at this kind of issue i can only conclude that just in time heating is the best.  By all means use Solar thermal or excess pv to raise the core temp of the tank and in winter/excess HW demand etc use a inline water heater of the electrical type which control temp by controlling the amount of leccy being used.

Ken

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TheFairway
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« Reply #1 on: July 26, 2015, 06:01:27 PM »

If you used an Immersun type device, is the meter that you used compatible with the method of throttling?

That said, your tank losses are inline with manufacturers published values that I have seen. In some cases, these tank losses may be partially recouped in a beneficial way, such as warming an airing cupboard.
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brackwell
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« Reply #2 on: July 26, 2015, 06:29:40 PM »

Ref https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/180950/In-situ_monitoring_of_condensing_boilers_final_report.pdfTable 13  Daily average losses associated with summer DHW production  p31

These are modern condensing gas boilers.
    

                                          Combi  KWh/day         Regular  KWh/day       Instant Electric HW      50% PV/50% instant
Gas/elec in kwh/day                     5.13                        9.89                         2.58                           3.58
Cost (Gas 4p/Elec 14p/kwh)         20.52 p                      39.56 p                   36.12 p                         25p                    
DHW out                                  2.69                          2.58                        2.58                            2.58
Tank Losses                              NA                           5.13                          NA                  say       1.0
Boiler Losses                            2.42                          2.22                          NA                              NA
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brackwell
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« Reply #3 on: July 26, 2015, 06:37:39 PM »

Fairway,

I am not using a immersun but i do not see why the production meter they use for the purpose of FITS would not measure correctly.

Well of course IF you want your cupboard heated 365 days/yr.  I personally can consider much better things to do with that energy.

Ken
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skyewright
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« Reply #4 on: July 26, 2015, 09:47:34 PM »

Everytime i look at this kind of issue i can only conclude that just in time heating is the best.
That's my conclusion too. I used a CT rather a 'proper' meter but we too have standing losses that are very high in proportion to the total DHW energy used, maybe an even bigger proportion that yours as we already have an electric shower (& that's with 50mm of foam insluation on the tank, plus a further jacket over that). The DHW tank is our lined & insulated but otherwise not heated loft space, so we aren't getting any knock on benefit from the losses. When I get around to the kitchen & bathroom refits I've been supposed to be doing for years (so many things to do...), I think we'll be going for in-line, under the sink, heaters in both kitchen & bathroom.
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Regards
David
3.91kWp PV  (17 x Moser Baer 230 and Aurora PVI-3.6-OUTD-S-UK), slope 40, WSW, Lat 57 9' (Isle of Skye)
gravelld
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« Reply #5 on: July 26, 2015, 10:15:18 PM »

Quote
in winter/excess HW demand etc use a inline water heater of the electrical type which control temp by controlling the amount of leccy being used.  
Example products? Do you have one next to the tank or multiple in each room?

I'm inclined to agree with your experience.

One thing I can't help but wonder is why tanks are fitted with such thin insulation layers. Given the temperature delta you'd think they'd have much more lagging. I suppose space may be the issue.
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brackwell
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« Reply #6 on: July 27, 2015, 09:17:41 AM »

gravelid,

http://www.cnmonline.co.uk/zip-electronic-instantaneous-water-heater-11kw.html  I believe this is actually made by a German co. and i did wonder if it was available under a different name.

http://www.stiebel-eltron-usa.com/tempra.html  This used to be available in this country not sure if it still is.

Search on  stiebel eltron dhc-e
« Last Edit: July 27, 2015, 09:26:07 AM by brackwell » Logged
skyewright
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« Reply #7 on: July 27, 2015, 09:40:33 AM »

http://www.stiebel-eltron-usa.com/tempra.html  This used to be available in this country not sure if it still is.
From: http://www.stiebel-eltron.co.uk/hot-water/information-planning/frequently-asked-questions/tempra/
Quote
Can Tempra products be used in the UK?

Unfortunately the Tempra range is only available for the American market and is not suitable for the UK due to the electrical connections. For the same reason we do not sell anything that is a direct alternative.
Sad
But there is quite a selection (that I've not looked closely at) on:
http://www.stiebel-eltron.co.uk/hot-water/products/
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Regards
David
3.91kWp PV  (17 x Moser Baer 230 and Aurora PVI-3.6-OUTD-S-UK), slope 40, WSW, Lat 57 9' (Isle of Skye)
gravelld
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« Reply #8 on: July 27, 2015, 09:55:20 AM »

What about flow rates? Electric showers are generally pretty poor in this regard. If the water is pre-heated then does this allow higher water pressure? What about if you have, say, two or three showers running at once?
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gravelld
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« Reply #9 on: July 27, 2015, 10:25:18 AM »

Do these devices need three phase supplies?

Also the modulation looks a bit disappointing. As far as I can tell, they modulate for instance from 11->14kW?

I suppose the energy usage overall may be lower.
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A.L.
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G69, Glasgow


« Reply #10 on: July 27, 2015, 10:28:41 AM »

hello,

What about flow rates? Electric showers are generally pretty poor in this regard. If the water is pre-heated then does this allow higher water pressure? What about if you have, say, two or three showers running at once?

- to heat 9l/min (reasonable shower) incoming mains at 5C to 45C needs about 25kW, using a WWHRS could reduce this by 40-50%,
if we could start with water at 20C and use WWHRS we could get this down to around 9kW

- here is somebody who has actually done it, http://www.ebuild.co.uk/blog/12/entry-232-part-thirty-five-hot-water-and-leds/


- to heat a bath supply, say 20l/min would be around 56kW, try doing that off-grid  Grin (or on for that matter!)
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gravelld
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« Reply #11 on: July 27, 2015, 10:38:49 AM »

- to heat 9l/min (reasonable shower) incoming mains at 5C to 45C needs about 25kW, using a WWHRS could reduce this by 40-50%,
if we could start with water at 20C and use WWHRS we could get this down to around 9kW
This is why I wondered whether, in a pre heat situation, you could choose between maximising flow or minimising power draw. If the water was reaching the unit at 30C, say...

Thanks for the link. I forgot jsharris had done this.
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dhaslam
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« Reply #12 on: July 27, 2015, 11:12:12 AM »

You aren't being entirely fair to stored systems.    Insulation of at least 100mm should be the norm and also  it is possible to vary the  time of heating. so that the full cylinder isn't hot for 24 hours.    The average heat loss could be  less than half that of a full hot cylinder.  Of course  solar heated systems do need to store heat overnight but usually only the top half of the cylinder is hot in the early morning, in any case losses are less when there is less solar heat gain.     
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DHW 250 litre cylinder  60 X 47mm tubes
Heating  180,000 litre straw insulated seasonal store, 90X58mm tubes + 7 sqm flat collectors, 1 kW VAWT, 3 kW heatpump plus Walltherm gasifying stove
skyewright
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« Reply #13 on: July 27, 2015, 11:21:37 AM »

You aren't being entirely fair to stored systems.    Insulation of at least 100mm should be the norm and also  it is possible to vary the  time of heating. so that the full cylinder isn't hot for 24 hours.    The average heat loss could be  less than half that of a full hot cylinder.
Very true. Given our usage I could probably significantly reduce standing losses by switching the main timed feed from the bottom element to the top one. Normally the top element is only ever used if there is an issue with the bottom one (e.g. last year the bottom one wouldn't work for about 3 weeks because of a DNO meter fault). On the couple of occasions when we've used the top element only, we've been fine for hot water.

I've also wondered about moving the (vented) tank downstairs - then any losses would be contributing to general heating (which for most of the year is way more kWh than the DHW standing losses).
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Regards
David
3.91kWp PV  (17 x Moser Baer 230 and Aurora PVI-3.6-OUTD-S-UK), slope 40, WSW, Lat 57 9' (Isle of Skye)
brackwell
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« Reply #14 on: July 27, 2015, 11:27:08 AM »

This will answer your temp uplift v flow rate  p9  http://www.advancedwater.co.uk/STOMEMO/A561-666-0122/DHC-E%208-10%20%2012%20-%20Installation%20Manual.pdf
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