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Author Topic: DHW heating  (Read 1206 times)
brackwell
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« on: September 29, 2019, 02:42:33 PM »

I wonder what people believe is

1) the cheapest
2) the lowest energy

For a few yrs i have been heating my DHW purely from the excess PV production except it needs a immersion boost on about 20days a yr. However recently my Immersun solar diverter went BANG so i am back to the gas boiler ,nearly new condensing  and modulating, feeding a modern vented 50mm foam tank and a mixer shower that has only 1.5M dead leg.  

The wife has been a way for a week so it gave me the oppurtunity to monitor the energy use for me having a shower which i have in the morning and takes c 1.5mins at 8.5L/min = 12.8L and hand washing dishes in the evening using 10.5L hot water.

Assuming the cold water entering the vented tank is at 12C and the hot water is being delivered at 42C then we have a temp uplift of 30C  

The total HW used 23.3Lx 30C (temp uplift) x 0.0017 = 1.19 kwh used.

The gas input averaged out over 8 days was 3.35kwh/day  ie 35% efficient.

This finding is inline with tests done by the energy saving trust on gas condensing boilers.  Their evidence was that the very best set up using a combi boiler was c 50% efficient and a system boiler with tank was c 33% efficient. Much of the inefficiency is brought on by the stop/start losses,dead legs and leccy consumption. Note that this is at a time when there is no CH on.

So going back to the original question i think it is clear the more efficient is leccy heating particularly if it is at the time of use ie instant shower/heater.

Now as for cost, so assuming leccy at 16p/kwh and gas 4.5p/ kwh.   Useing gas would cost 3.35kwh x 4.5p/kwh = 15.1p  and using leccy 19p  ie not a lot different but it will be more once the CH is on because the boiler is already heated up for the CH.

But my point is not about what is only a few p but thinking about NET ZERO 2050 should we all be cojolued into using leccy and preferably with a HP particularly if it is not going to cost any more and of course the leccy will be coming cheaper because of the TOU (time of use) tariffs or more use of PV and Batts.

PS i am assuming the leccy grid will get to close to zero CO2

PPS forgot to mention i heat the HW to about 48C so for those heating to 60s the efficiency will be even significantly worse.

Ken

Ken

« Last Edit: September 29, 2019, 02:55:14 PM by brackwell » Logged
Stig
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« Reply #1 on: September 29, 2019, 04:16:53 PM »

And if you're got an older gas boiler / water heater that has a pilot light you'll be using 3kWhr a day on that (I measured it).  So in that case even using an immersion heater is probably the lowest energy/CO2/cost.
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brackwell
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« Reply #2 on: September 29, 2019, 04:19:19 PM »

Yes my old one was using 2.5 kwh on the pilot light but its worse than that because you also need the draught to go with it!
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Countrypaul
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« Reply #3 on: September 29, 2019, 06:17:52 PM »

Shows how much using a solar diverter can help with electric then costing effectively nil. If there were more of you it would be interesting to work out the different costs, for example shower for grown ups in the morning but baths for little ones in the evening. If multiple grown ups have showers in the morning (with no significant delay) then presumably the efficiency of the gas approach would improve.
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ecogeorge
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« Reply #4 on: September 29, 2019, 07:21:38 PM »

Make sure you replace your diversion controller asap .........
can't recommend highly enough the Solic 200 - one here ..  https://www.ebay.co.uk/i/254193676042?chn=ps&norover=1&mkevt=1&mkrid=710-134428-41853-0&mkcid=2&itemid=254193676042&targetid=519689919747&device=c&mktype=pla&googleloc=9045642&poi=&campaignid=2068027898&mkgroupid=78029382442&rlsatarget=pla-519689919747&abcId=1139106&merchantid=6995734&gclid=EAIaIQobChMIsYbg4dH25AIVicreCh1EQgjaEAQYASABEgLnQvD_BwE

Its the only device with ZERO export so every watt counts and it's British .
Have one with 2.5kw pv , 210 litre DHW -2 person household. Wood burner burning scrap pallet wood with 1kw to back boiler.
think we've boosted probably 5x in the last yr !!!
George
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Philip R
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« Reply #5 on: September 30, 2019, 12:16:45 AM »

When I installed my gas central heating, I insulated the flow and return pipes from the kitchen mounted boiler to the airing cupboard. I realised then from the drafts present between the floor and ceiling, how much heat would be lost from copper C.H. pipework.

I do a similar thing to Brackwell, except a whole tank of hot water is required for the family. I cannot do a shower with soap and shampoo in less then 8 minutes. Therefore, using gas via a condensing boiler and efficiently coiled cylinder, I believe my efficiencies are much greater than the figures mentioned earlier. I reheat my cylinder to less than 60 deg C by controlling my condensing boiler to a flow temperature not exceeding 57 deg C, thus ensuring a boiler return temperature below 54 deg C.  Because I know my zero governor gas valve controls low fire rate CO2 above 8.8%, I know that my boiler runs well into the condensing mode whilst heating the cylinder. Most other systems run the boiler flow and return temperatures much higher, (with higher firing rate, thus boilers running out of condensing mode except when initially cold.) 

I will now have to measure hot water consumption for showers and washing up, and the gas consumption on a daily basis, and try to get some meaningful figures.
However, when my electric shower broke down, and I moved the shower to the gas heated HW Cylinder, my monthly bills reduced noticeably.
Philip R
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titan
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« Reply #6 on: September 30, 2019, 09:26:47 AM »


PPS forgot to mention i heat the HW to about 48C

Ken

I think it is  recommended to heat the tank to 60C occasionally to protect against legionella
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brackwell
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« Reply #7 on: September 30, 2019, 10:05:07 AM »

Yes it is but with a copper tank and routinely changing water this risk is for all intents and purposes zero. This subject has been discussed on here a number of times over the yrs but i believe there has been no practical examples in a DOMESTIC situation.

Ken
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titan
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« Reply #8 on: September 30, 2019, 01:54:27 PM »

Because mild symptom are similar to flu I suspect a lot goes unreported    https://www.gov.uk/government/publications/legionnaires-disease-monthly-surveillance-reports-2018.  There are a number of cases of HGV drivers getting it from screen washer using plain water with the incubation temperature perfect under the bonnet. Risk is low in domestic situation but not zero.
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biff
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« Reply #9 on: September 30, 2019, 03:07:41 PM »

Legionnaires,
         Was a worry for me also. The water in our thermal store is a separate system with it's own little header tank.
 It was a very simple idea. I took out a rad and in its place I installed a thermal store. The T/S had a rad that wrapped right round the T/S internal  so the pipes that used to go to the rad were now fitted to the radiator inside the T/S.
   So now I have a T/S  that had to be filled with water by hand and an expansion tank located 450mm directly above it.. This isolated water was to be used  as a heat battery and dump load for our W/T, which was a 138vdc x 2kw immersion heater that could never be switched off.
  Instead, when the water reached a certain temperature,
 Another central heating pump kicked in and pushed the warm water in the rad inside the T/S around the central heating system, this allowed the T/S to stay out of danger from overheating.
  Yet when it was all said and done and running  beautifully  to this day, someone mentioned Legionnaires and  that gave me something to think about,, but not for long. Apparently,  anti-freeze is the answer as long as you are not going to drink it and so it was  I fed 2 ltrs into the Expansion tank and considered the problem solved.
                Biff
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An unpaid Navitron volunteer,who has been living off-grid,powered by wind and solar,each year better than the last one.
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