Energy Efficient Water Heating

In the UK, the vast majority of mains water supplies are chlorinated. The chlorine is there to prevent the build-up of micro-organisms in the supply pipework and it will be found in fractions of a part-per-million upwards in the rising mains that are used to feed the cold water to the home. Habitually, cold water from a rising main is immediately fit to drink except in the most dire of circumstances, therefore there's a very low risk of biological harm occurring by using this cold water straight from the tap for boiling foodstuffs or for drinking. Public water supply organisations go to great efforts to produce and maintain this biological quality.

During the boiling activity in, say, a kettle or a saucepan, the chlorine is driven off. However, any viable biology that might happen to be in the water at that time will be deactivated by the boiling process and will contribute (in a very minor way!) to the nutrient value of whatever is being boiled. Safe so far.

Now, there is a break tank usually in the loft of a UK home, that stores cold water to pressurise the hot water system. It is fed from the cold water rising main via a 'ballcock' or 'Portsmouth Valve' to maintain what the water distribution utility terms a 'class 1 air gap' between the water in the rising main and the water in the tank; the gap is to ensure that the stored water does not flow backwards into the rising main should the pressure decrease, and contaminate it with whatever is in the tank. The tank is covered with a lid, usually, that is close-fitting though not perfectly air-sealed. It is not unknown for birds, rodents, insects, arachnids etc. to find their way into these tanks. Though most are made from plastics these days, some heritage tanks are made from metals that corrode, so there will be corrosion products in the tank water from time to time. So one can expect the total dissolved solids to be higher and the biological impurity to be higher in the tank than in the incoming town mains water. Also, being less than air-tight, the chlorine that was introduced by the public utility to protect its pipes will gradually dissipate from the stored water, leaving it largely unprotected against biological activity. Bacteria reproduce like this: 1, 2, 4, 8, 16, 32, 64 etc., so it doesn't take long for the water to 'go off' slightly. Drinking this water cold is not to be recommended!

Now, this tank pressurises the hot water storage cylinder, which is a sealed vessel, and the heating system be it electric, gas, oil, etc. is run to maintain the temperature at the top of the tank contents at around 60degC or so, which will de-activate any bacteria present. So the water that enters the hot water distribution piping downstream of this tank should be largely biologically inactive. Then what happens? It cools down in the pipework when there is no flow. One bacterium survives. 1, 2, 4, 8, 16, 32, 64 etc.

Along comes Guest, who fills up a kettle or saucepan. The tap runs warm, not hot, for a while and some of the 64, some of the dead arachnids, some of that rotting storage tank and some of the decay products from that poor mistlethrush that drowned and died six months ago enter the kettle....

From an energy point of view, it doesn't matter technically whether the water is heated in the hot water tank or the kettle. The properties of water and steam are widely known and published.

From an economics point of view, it might be cheaper to heat the water most of the way, like the 60degC above, using an oil or gas furnace and top-up the temperature using an electric kettle.

From a biological point of view, who wants to drink the corrosion products of an ageing galvanised steel water tank (Fe++ and Zn++ salts in particular) and the decay products of a dead mistlethrush? Yuk! What's the economic effect of food poisoning?

So from that viewpoint it is always advisable to use the cold water supply main as the source of water for drinking and cooking. The utlimate choice, however, is Guest's!

PWSlack,

Well said!

In addition to the "nasties" that you mentioned, here in the US, residential construction built before the mid 1980's not infrequently has issues with lead leaching out of plumbing system components.

These could be in the form of lead solder, high lead content brass plumbing components, and even lead pipes. I've seen numerous instances in older buildings where the actual service connection (the pipe connecting a building to the municipal supply line buried in the street) is made of pure lead.

In fact the problem of elevated lead at the point of use has become more prevalent in some areas of the US that rely on surface reservoirs for water supply. Due to "acid rain", the natural buffering capacity of many bodies of water has been decreased or eliminated, causing the water to become "corrosive". This corrosive condition has actually dissolved away much of the layer of protective "scale" that had historically been deposited on the water system piping, allowing the corrosive water to interact with, and leach metals from the piping itself.

Some municipalities have attempted to combat this condition by adding chemicals, such as calcium orthophosphate directly to the reservoirs, in an effort to restore some of the lost buffering capacity.

Due to the longer contact time that the hot water has with a home's plumbing system components, it is more likely to have leached out more metals from the pipes. I recall several years ago NYC issuing a recommendation to only use cold water for drinking & cooking, and even went as far as to recommend that one run the tap for 30 seconds to a minute before drawing the water, in order to flush out the water that had been sitting in th building's pipes.

Good point.

Lead piping is still present in a number of heritage installations across the UK, though it has been replaced in most areas with plastics and other non-reactive materials for underground water supply. Unfortunately for Guest and the water in his/her kettle or saucepan, it doesn't matter whether (s)he draws water from the hot or the cold tap for cooking and drinking; any lead will still be there and will not be greatly affected by the vagiaries of the pumbing in any particular home. Biological issues are still the greatest concern.

Oh you live in such an ancient region.

Those header tanks are banned here on the mainland. We use pressurised vessels at ground level with no metal contact between the potable water section and the metal.

It is true that lead plumbing causes problems and it are again the poor who don't have the money to live elsewhere.

From energy point of view it makes no difference wheter U boil water from the cold tap or from the hot tap. economically it is perhaps wiser to use the cold tap: you fill up and heat your plumbing with warm water, which you don't use and which will cool down again. Warm water which has been paid for.

Those circulating systems to keep water hot in hotels and hospitals are real energy swallowers: In many cases they use more heat to keep the water and plumbing hot than they would use to heat up the water. Electrical tracing is an improvement as the size of the pipework can be reduced and the return piping is not necessary. But decent insulation is the critical factor.

Best would be to have your gas fired boiler under your sink, with half a meter piping to heat up when you want warm water. Or small electrical heaters.

With the last line I will add free energy and alternative energy sourcing to the tread:

A central boiler is interesting fro menergetic view when the heat is free. (solar/geothermal/wind)

Very good points!

For someone from such a young region,

I second this. The cleaner at my first place of employment after being promoted to tea lady decided to save time by using the hot tap and the pre-warmed hot water you describe from the header tank PW describes, and to save more time when making the tea by switching the wall-mounted electric boiler off immediately it had boiled. Most of the lads had the sh*ts, a dead bird was found in the tank and the tea lady had a severe ear-bashing. Don't do it!

Why, it is about time we chipped in some solar water heating in this forum !

Seems like everyone is talking about water quality rather than the energy it takes to heat it.

The water you heat in your water tank is fairly efficient, given that the only energy losses are that which escapes through the hot water tank insulation. If you use a gas-fired water tank, then there's heat escaping through the vent also.

When you heat water further on the stove, much of the heat you're using escapes from around the pot, so starting with warmer water is more efficient than starting with cold water.

In the big scheme of things (over a lifetime) you may save a few dollars (or whatever your currency is) by starting with warm water.

P. S. There are more efficient ways to do the same thing than the ways to which we have all grown accustomed. (It's my grammer school teachers' fault! I was told that it's improper to end a sentence with a preposititon.)

In Germany, everything is done at mains pressure, except for the Toilet cisterns, its so much safer hygiene wise.

What nobody has mentioned is Legionnaire's Disease, which is caused by breathing in water spray either from a shower or (as originally happened) water spray from a bad AC, when certain bacteria are in the water.....or water that has been in a dirty old storage tank and then not heated enough to kill everything off........if i remember correctly, the bacteria are nothing special, unless you breathe them!!

With water heating, you have to think about the energy type used, if electricity = not good, but directly heating from oil, gas or wood or wood pellets and other renewable sources (read from left to right, from bad to better!!) is better as its also cheaper!!

Pre-warming with Solar Energy saves money, even if its only 10-20°C warmer. A heat pump can improve on this further of course....

I have just taken out a small electric boiler and replaced it with a small instant water heater as getting gas heated water to our spare toilet is too complicated....they tell me I can save up to €150,- per year alone....even half that is good, the heater only cost €120,- new!!

Legionella is the reason why you should keep your water out of the zone 20°C and 55°C, below the lower limit the growth is very limited and above they start to die.

Hospitals typically heat up their hot water systems to a level +65°C, preferably above 70°C once a month. The big problem is the small tubing in the shower and the showerhead, regular removal and desifection is the only correct solution (but in many cases seen as to expensive). Also the connection tubing from the main pipework and the tap is a problem zone, somewhere there is a zone that resides at 30 to 40°C, ideal for those little buggers, and the 55°C water is not sufficient to kill them. Tracing till the tap can prevent this. (Mostly seen as to expensive)

For me Germany is part of the mainland, only decent techniques are used and the old fashioned way has been forgotten and buried.

Good info Gwen, but what did you mean when you wrote:-

Tracing till the tap can prevent this. (Mostly seen as to expensive)

I did not understand the "Tracing" part......

This thread comes near to my day to day business: electrical heat tracing. (Begleitheizung)

Tracing is sometimes used as an alternative for circulation systems, it has some benefits over circulation: the avoidance of dead legs is one of the important for hygienic reasons. and hygiene is the only valuable reason for me to install a circulation system or tracing in a building. The best way is still point hot water generation, but this is seen as the most uneconomical as the only way to do this is electrical and electricity is seen as way to expensive.

What building owners don't count or project engineers refuse to see is that the energy losses through the insulation and the need to add something to cope with it is costing the building users way more than the initial savings for the owner.

There is not only the problem that dead legs need to be heated, but the hot pipes heat up the shafts and ceilings of those buildings, heating up the cold water to a level above 20°C.

The above might sound in contrary to my business but tracing only has limited power (10-20 W/m) so everything needs to be nicely insulated or you will have cold spots. This forces the installers to deliver nicer installations than those necessary for circulation and the final result is that the energy loss is way less.

Tracing has the fantastic ability to trace and keep dead legs hot till the tap, and in a heat up cycle (supported by our HWAT-ECO controllers) the pipes can be heated up above 65°C, killing the nasty buggers, also in these dead legs.

The most eficient way to heat water in a residential or commercial setting is with a gas instantaneous heater, also known as tankless. The storage capacity is less than a gallon. There are no other energy losses to speak of. They typically last for 20-30 years.

The under-fired tank concept has been proven to be very wasteful due to the amount of energy required to heat the contents of the tank (typically 40-50 gallons in an average home) to 140 F.Adding to the problem is the huge energy losses due to poor insulation and constant flue venting.

If you want to cut your costs by 50%, get a tankless heater.

Caution: Though DIY, Lowes, etc. carry these heaters, the models they have may not be the best available. These systems must be sized properly by the manufacturer's rep. Don't trust that the discount stores know anything abou performance, design, or installation. 99% of plumbing contractors do not know anything about these systems either. I know about them, because I used to design and install them for Wendy's, Steak & Shake, Hoolihans, Holiday Inn, laundromats, apartment buildings, nursing homes, etc. I also have one in my home.

If you want the name of a few knowlegeable reps, I would be glad to refer you to them.

Solar Thermal heaters are already getting attentions in third world countries were you can preheat/heat water, food , conventional stoves are inefficient as most of heat is lost.