Cheapest way to wash up

HappyMJ

macman wrote: »

No, because it costs you approx 3 times as much to heat the hot water by electricity as by gas.
5 litres of water costs you a lot less than that differential of 300%. If you pay say the Thames Water rate of 128p per cu m (1,000l), then your 5 litres wasted is costing you about 0.6p. Or a total of £2.19pa.
It would be cheaper to use a dishwasher of course, if you discount the capital cost.

But dishwashers use electricity to heat the hot water at what you say is 3 times the cost of gas heated hot water as used in the sink....

Likestowrite

devondiver wrote: »

With respect Macman - yes, of course. But that gallon of cold water sitting in the pipe has to be replaced with hot water before the tap runs hot. When the tap is turned off the pipe is now full of hot water which slowly goes cold and is therefore wasted. There is a plumbing term for it - latent or dead areas - not sure without checking.

The term I was looking for is " a dead leg".

A quick search found this "The length of pipe between the hot water cylinder and a hot tap is known as a dead leg, because hot water left in the pipe after each use of the tap cools and is wasted. The longer the pipe, the more the waste.
Water at 60°C travelling through a 15mm copper pipe loses heat equivalent to more than 1 unit of electricity for roughly each 300mm of run a week - enough to heat about 45 litres of water.
Where a hot water supply pipe to a basin or shower would involve a dead leg of piping of more than 6m long, it is wiser to use an instantaneous heater instead." Source; http://www.readersdigest.co.uk/health-home/diy/central-heating/saving-heat

Sorry to have to drag it out but ....

Thank you --this is very interesting.
So, boiling a kettle is probably equivalent to an instantaneous heater, but then there's the problem of whether heating the water in the kettle using elecricity is more expensive than wasting the dead leg water. (And I have a lot of dead leg piping as my boiler is in a bedroom.)

I could save the water I initially run off by storing it in a big bucket and using it to water my outdoor potted plants. (or to add to the washing up bowl once the really hot water has come through) --so it wouldn't really be wasted--except I'd have paid to heat the equivalent amount because of the dead leg. Hmmm.

devondiver

Well, thank you . The fact is that an awful lot of heat and water is wasted in washing-up one way or another.

Most of the time we just run a half bowl of warm soapy water for the breakfast dishes then, so long as you scrape, wipe and maybe rinse your dishes before washing, you can make the same soapy water last for the rest of the day.

Like I said earlier, cold soapy water will still get dishes clean, it just takes a bit longer - like all reactions at lower temperature.

And before the health and safety police jump on me - I do know my food safety microbiology. So as long as dishes are properly clean, and more importantly - dry, before putting away there are no issues with this.

Likestowrite

devondiver wrote: »

Well, thank you . The fact is that an awful lot of heat and water is wasted in washing-up one way or another.

Most of the time we just run a half bowl of warm soapy water for the breakfast dishes then, so long as you scrape, wipe and maybe rinse your dishes before washing, you can make the same soapy water last for the rest of the day.

Like I said earlier, cold soapy water will still get dishes clean, it just takes a bit longer - like all reactions at lower temperature.

And before the health and safety police jump on me - I do know my food safety microbiology. So as long as dishes are properly clean, and more importantly - dry, before putting away there are no issues with this.

Thank you. So if I boil one kettle to wash up (scrape plates very clean first) and keep the soapy water for an additional washing up later, give the plates etc in the draining rack a quick rise from a jug of cold water at the end to rinse off the washing up water) that should be cheaper than running off all that dead leg water to wash up using water heated by the boiler?

Or should I run the boiler water until it's warm (saving the very cold water in a bucket to use later -- and use some of this to rinse the plates) Then use warm water to wash up (and keep the used soapy water to use again later)

HappyMJ

Likestowrite wrote: »

Thank you. So if I boil one kettle to wash up and keep the soapy water, for an additional washing up later, give the plates etc in the draining rack a quick rise from a jug of cold water at the end to rinse off the washing up water) that should be cheaper than running off all that dead leg water to wash up using water heated by the boiler?

Or should I run the boiler water until it's warm (saving the very cold water in a bucket to use later and use some of this to rinse the plates) Then use warm water to wash up (and again keep the used water to use later)

Depends on the size and power of the kettle.

A 3kW kettle is the most efficient at heating water (due to less heat lost whilst heating the water...they aren't known for being very well insulated and lose heat quickly to the room).

You could time how long it takes to fill your sink with hot water from the boiler and work out how much the gas just cost you then time how long it takes for your kettle to boil the same amount of water noting that you only need half or less as you can mix it with cold so it's not scorching hot when you put your hands in it.

It might take 1 minute (of boiler running time) to fill your sink with hot water from the tap and if you have a 30kW boiler that would therefore have used 0.5kWh of gas. A kettle might take 3 minutes to boil 2 litres of hot water and used 0.15kWh of electricity.

macman

HappyMJ wrote: »

But dishwashers use electricity to heat the hot water at what you say is 3 times the cost of gas heated hot water as used in the sink....

Yes, but modern dishwashers claim to use a fraction of the water required to wash up by hand. For example, Bosch claim just 6 litres of water per cycle on some machines.
However, I suspect that this is only entirely true if you were to fill the d/w to maximum capacity each time, which few of us will do.
I don't 'say' it's 3 times the cost. It is 3 times the cost per kWh, unless you wash up using water heated via E7.

devondiver

Likestowrite wrote: »

Thank you. So if I boil one kettle to wash up (scrape plates very clean first) and keep the soapy water for an additional washing up later, give the plates etc in the draining rack a quick rise from a jug of cold water at the end to rinse off the washing up water) that should be cheaper than running off all that dead leg water to wash up using water heated by the boiler?

Or should I run the boiler water until it's warm (saving the very cold water in a bucket to use later -- and use some of this to rinse the plates) Then use warm water to wash up (and keep the used soapy water to use again later)

It's difficult to give a definitive answer because there are several variables. I suspect if you were able to do controlled experiments, as suggested by HappyMJ, you might find that the kettle route was the cheapest. But then again, when (and if) you are paying to heat your home the 'dead leg' heat is probably not lost but helps to warm the house, probably making the gas route most cost-effective.

All I would say is - if saving pennies takes priority over time and convenience - wash up with cold or luke warm water, re-use it as much as practical and make sure your boiler thermostat is not set higher than it needs to be.

HappyMJ

macman wrote: »

Yes, but modern dishwashers claim to use a fraction of the water required to wash up by hand. For example, Bosch claim just 6 litres of water per cycle on some machines.
However, I suspect that this is only entirely true if you were to fill the d/w to maximum capacity each time, which few of us will do.
I don't 'say' it's 3 times the cost. It is 3 times the cost per kWh, unless you wash up using water heated via E7.

Compared to hand washing...not much difference...My handwashing bowl when half full of water contains 9 litres.

The economy cycle that uses 6 litres takes 185 minutes of time at 50 degrees which is only useful at cleaning normal soiled tableware...i.e they need to be rinsed clean first (using another 4 litres of water on the pre rinse cycle which they don't calculate in the effiicency figures) and it won't do the pans. Over 3 hours to get clean dishes and it uses 0.92kWh of energy to clean the dishes. Almost twice the energy of hand washing...if it takes a minute to half fill the washing up bowl with hot water using a 30kW boiler.

Personally I'd rather use the turbo cycle at 60 degrees using 8 litres of water and get the job done in 20 minutes using 0.95kWh of energy. That to me sounds so much better.

It does have a timer so it can be set to use the E7 rate at night which will reduce the cost of running it (as long as the property has E7).

victor2

Then you've got the cost of a dishwasher tablet (plus salt and rinse-aid where required) versus the amount of washing up liquid you use.
I'm happy to believe that the dishwasher does a better job and the difference in cost is not really that significant either way.

Likestowrite

devondiver wrote: »

...

Most of the time we just run a half bowl of warm soapy water for the breakfast dishes then, so long as you scrape, wipe and maybe rinse your dishes before washing, you can make the same soapy water last for the rest of the day.

Like I said earlier, cold soapy water will still get dishes clean, it just takes a bit longer - like all reactions at lower temperature.

.

Thank you. I'm trying this. Now saving the dirty dinner dishes for the next morning, boiling 1.7L kettle to wash up in a washing up bowl of warm soapy water the next the morning (starting with half a bowl and using a trickle of cold to rinse letting the trickle fill the rest of the bowl ready for the bigger things)--and then making the soapy water last for most of the day. To me this seems more economical than what I was doing before considering the dead leg issue.(And I don't have a big bucket of cold water sitting in my kitchen until I can find a use for it.)

But I'm also thinking hard about this: Not sure what kw my boiler is.
A 3kW kettle is the most efficient at heating water (due to less heat lost whilst heating the water...they aren't known for being very well insulated and lose heat quickly to the room).

You could time how long it takes to fill your sink with hot water from the boiler and work out how much the gas just cost you then time how long it takes for your kettle to boil the same amount of water noting that you only need half or less as you can mix it with cold so it's not scorching hot when you put your hands in it.

It might take 1 minute (of boiler running time) to fill your sink with hot water from the tap and if you have a 30kW boiler that would therefore have used 0.5kWh of gas. A kettle might take 3 minutes to boil 2 litres of hot water and used 0.15kWh of electricity.