Boiling kettle vs running hot tap

Norman_Castle

From a cold boiler it takes 11 litres of water to get usable hot water at the tap so kettle for me.


If my maths is right 11 x 365 is 4.015m3. As the boiler fires as soon as the tap is turned on that's effectively the cost of heating 4m3 of unused water annually.

Raxiel

My (combi) boiler sits next to the sink, with about 1.5-2m of 15mm insulated copper pipe between the boiler output and the tap. Even though the hot water pre-heat is programmed to be on whenever the house is occupied during the day, the boiler will still let a lot of cold water flow for about half a minute before it runs hot.
I've found that for the least waste, you have to give a quick, half second blast of water from the hot tap to wake the boiler up, wait 5-6 seconds for it to fire (which isn't that long, but feels it when you're stood waiting) then turn the hot tap on - giving you very hot water within a few seconds.


Wifey usually can't be bothered with that and uses the kettle. I can never be bothered to wait for the (1.2kw) kettle to boil, so I usually use the gas.


The same on-off technique can be used to get the shower to flow hot quickly too. Wifey's solution there is to just turn it on and wander off until enough hot water to melt an ice cap has flowed down the plughole, then start her shower. :doh:

coffeehound

So four tonnes of water per household per year for each daily run up to hot water? Times by say 10m combis in the UK. What a green choice combis arent!

Raxiel you need a bigger kettle

chrisw99

Norman_Castle wrote: »

From a cold boiler it takes 11 litres of water to get usable hot water at the tap so kettle for me.


If my maths is right 11 x 365 is 4.015m3. As the boiler fires as soon as the tap is turned on that's effectively the cost of heating 4m3 of unused water annually.

You only need a trickle of water for the combi to fire up, not at full flow. Figure out the lowest flow yours takes to fire up the boiler, give it 10-15 seconds, then turn the tap properly. Should be decently hot then without wasting loads of water.

MoneyMate

Hi Raxiel, I use a similar technique with my Combi ,5 second blast of water from the hot tap to wake the boiler up, wait 5-6 seconds for it to fire then I find it comes through faster with out wasting more cool water.

Raxiel

coffeehound wrote: »

So four tonnes of water per household per year for each daily run up to hot water? Times by say 10m combis in the UK. What a green choice combis arent!

Raxiel you need a bigger kettle

Nah, the Kettle's fine when used for it's intended purpose. It's insulated like a flask, so once it's boiled (even if that takes longer than a 3kW kettle) it stays hot for a long time, just a few seconds to get back to boiling if it's been over an hour.

chrisw99 wrote: »

You only need a trickle of water for the combi to fire up, not at full flow. Figure out the lowest flow yours takes to fire up the boiler, give it 10-15 seconds, then turn the tap properly. Should be decently hot then without wasting loads of water.

I remember someone was even selling a flow restrictor you could put on the hot output that limited flow to a trickle until it reached a particular temperature. Combi-save or something like that.

chrisw99

Raxiel wrote: »

Nah, the Kettle's fine when used for it's intended purpose. It's insulated like a flask, so once it's boiled (even if that takes longer than a 3kW kettle) it stays hot for a long time, just a few seconds to get back to boiling if it's been over an hour.

Vacuum insulation is great. I just got a brand new filter coffee machine where instead of a glass jug and hot plate to keep it warm, it's a vacuum insulated steel jug. Made a pot at 7am and was surprised it was still boiling hot at 10am.

Think a vacuum insulated kettle is the next thing to get..

Helen28

What about using the preheat function on the combi boiler?


Does it save on wasted water at the cost of more gas to get hot water quicker?

My plumber told me to leave it switched off as it wasted gas.

[Deleted User]

Cheaper for me to use the tap.


2L water in a 3KW kettle takes approx 4 mins to boil.


Kettle 3000x ((4/60) /1000) =0.2kwh Electric x 0.15p = 3p


Tap + 28kw Combi already measured this other day in the Energy usage thread.
0.6kwh x 0.028p = 1.6p


There will be some hot water left in the line to your tap when you turn it off (heating the house in the winter ). How much you lose depends on how far your tap is from boiler. And some cold water wasted before the hot water reaches the tap (as above).


Above calcs depend on your tariffs obviously

ajbell

What about an old style whistling kettle on a gas hob?.
Also you can microwave water in a cup.
Personally I wait for my pv output to reach 2.5kw and flick the switch on my kettle but this does mean I can only have a cup of tea on a sunny day.