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Forum Index : Solar : water input temperature
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| isochronic Guru  Joined: 21/01/2012 Location: AustraliaPosts: 689 |
Is there a standard for water supply temperature ? I have noticed that the domestic supply is sometimes cold, sometimes almost luke warm. Of course, the colder it is the more energy it takes to heat to useful hot water - maybe a use for excess watts would be to heat input water from cold to near room temp (but still cold enough to stop bug growth), it would store the energy usefully instead of wasting it. Maybe not worth the effort/cost. |
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| Solar Mike Guru Joined: 08/02/2015 Location: New ZealandPosts: 1138 |
Hi Chronic, yes domestic city water gets colder here in winter also, looking at the incoming water temp to the HWC, it varies here between 10 - 18 Celsius. I built a heat exchanger device some years ago from copper tube and foil, placed it in a well fibreglassed timber case and placed it in the outlet of the most used shower in the house. Warm grey water from the shower outlet passes over the large surface area of the copper foil on its way to the drain, the cold incoming water supply to the shower mixer valve passes through the copper tubing and extracts some of the energy from the waste water, thus pre-heating the cold supply, this in turn means the hot tap doesn't have to be wound up as much and so saves hot water. Normally we would get 4 showers from a fully heated HWC, now we easily get 5, so its a good saving; as 90% of our hot water is heated by solar panels on the roof, its especially effective in winter when there is less sun to heat the water up. The heat exchanger preheats the water by about 8-10 degrees C. Here are some photos          The copper tubes are welded, the thin 0.25mm foil is soldered to the tubes, bit like making a solar hot water heating panel, same as how I built the ones on the roof. Cheers Mike |
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| Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406 |
Another way to do this is to run the incoming water through two normal storage water heaters connected in series. The first being only a "preheater" the second the main stored domestic hot water supply. The preheater only needs to raise the temperature slightly to make a very big difference. It can be made quite large, and because it runs at a much lower average temperature than it was designed for, any thermal insulation will be much more effective. What you use to heat it, solar thermal, solar electric, or maybe waste heat will be more efficient too, because it only needs to store a large volume of water at a relatively low temperature. Cheers, Tony. |
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| isochronic Guru Joined: 21/01/2012 Location: AustraliaPosts: 689 |
@Solar Mike - Nifty ++ !! Hadn't thought of the heat in graywater. Maybe suggest your device as a product to the hot water plumbers suppliers (?). Another source may be the heat from fridges, bit trickier though - don't want to cut the refrigerant tubes :( |
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| hotwater Senior Member  Joined: 29/08/2017 Location: United StatesPosts: 120 |
If you have PV charging batteries, I can almost guarantee you are wasting energy that could be used to preheat water as a minimum. There are ways to extract that power at power point even if you have a PWM controller. |
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| George65 Guru Joined: 18/09/2017 Location: AustraliaPosts: 308 |
I used a gas water heater with the burner removed and put a waste oil burner under instead. At first I made it a forced air burner which I had to watch so as not to boil the crap out of the water which caused the temp valve to blow off and a lot of wasted hot water. https://www.youtube.com/watch?v=4pP6jTUEJBU The next revision was to crunch some numbers and work out the fuel required and Only put that in the tank. Once the fuel was consumed, fire went out and a temp sensor cut the blower and shut everything off. Next I used a passive burner again with limited fuel supply. Light it and walk away. Not as fast or clean as the forced air burner but no power supply and impossible to over heat. I set this up as a pre heater which at the time I had not seen anyone else doing or talking about. few years later to now it seems to be a popular thing. Beauty is if I fire the burner the water goes into the regular off peak heater and I save money. If I don't do the burner for what ever reason, I still have hot water and am no worse off than before I set up the system which cost next to nothing. There are now used solar flat panel water eaters coming on various sites pretty regularly. Saw some a while ago for $25 ea guaranteed to be good and not leaking. Unfortunately someone bought the lot and took them between me leaving home and getting there. I like my oil burner option. Works in any weather and once fired you know you have double your hot water capacity and it can be totally free. I set the electric thermo a bit low and pre heat the water a bit high so as long as I light the burner consecutive days, the hot water is free. Varying inlet temp is not a big problem. I watch that and add a bit more fuel and even if the final temp is 10oC out, makes no never mind. Still have to add cold for a shower even when I turn the electric off which I tend to do when I'm outside a lot as in spring to autumn. I can easily not have the burner or electric for 3 days and still have water warm enough for a shower. There are online calcs that show the energy difference in a specific amount of water to raise it from different temps. The numbers are quite surprising. I ran some the other day and If memory serves, it was something like 5 Kw to raise 250L of water just 10oC. Well worth doing by free means if you are paying for the power or gas to do it. |
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| Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406 |
Another possibility is a large coil of that black poly pipe they use for swimming pool heaters. The temperature gain is not huge, but even a few degrees temperature gain is still something for nothing. Cheers, Tony. |
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