New house with Combi boiler - hot water is SO much higher than old house with system heating, why?

MP1995

Reed_Richards said:

When I had a combi boiler there was a long pipe run between the boiler and most of my hot taps.  So every time I ran a hot tap, a lot of hot water was left in the supply pipe.  If I didn't use that tap for a while afterwards, this water would cool and so the energy used to heat it was wasted.  I'm sure that lead to reduced efficiency.

Unless the pipe ran outside the house the energy wasn't wasted. Although in summer it might be a waste, in winter it would add that heat back into the house.

JGB1955

good_thyme said:

Looking for a bit of advice here please.

 We are currently using almost twice as many units of gas than at our old house. 
Is there anything else we can do to get the gas usage for heating water under control?

Are you comparing like-with-like?  Gas is considerably cheaper than electricity. 

theoretica

Using twice as much hot water for showers and taps as you did for just taps doesn't sound out of balance to me.  Just thinking in terms of how much water is used where in the house.

theoretica

Have you double checked that no gas is used when you are away for a while - or turn the boiler off entirely for a few days?  Just in case there is a leak or something wrong with the meter.

Scot_39

A mains pressure shower if you had a cold fed electric before mey well operate at a higher flow rate - more so especially  in winter.

In winter a 9kW or so electric shower will only give a relatively small flow rate at low incoming flow temps.

I can switch my electric onto low power in height of summer and get a similar low flow at just about an acceptable temp for me.

It's not just time in the shower but flow, so total water volume and the temp you heat it to that matters.


The volume waste in pipes from combi probably not dissimilar from old cylinder (I guess you meant cylinder not cistern) to tap.  But you could have been heating it 10 or more degrees warmer.

And even a modern well insulated cylinder can lose 1-2kWh depending on size, an older cylinder more. 

And with a combi temps in high 40s enough to prevent any legionella - a cylinder 60C min recommended by HSE for cylinders.  Both likely still down mixed with cold for showering etc.

Reed_Richards

Scot_39 said:

The volume waste in pipes from combi probably not dissimilar from old cylinder (I guess you meant cylinder not cistern) to tap.  But you could have been heating it 10 or more degrees warmer.

That wasn't true for me.  My combi had replaced a cylinder at some time in the past and all the hot water pipes went from the boiler to where the cylinder had been in the centre of the house (6 metres or so of large bore pipe) then back out to the hot taps

good_thyme

Oh my goodness, so many suggestions, thank you. I'm caught up with another task for a couple of hours but I'll read and learn later this evening, thank you again

Scot_39

Reed_Richards said:

Scot_39 said:

The volume waste in pipes from combi probably not dissimilar from old cylinder (I guess you meant cylinder not cistern) to tap.  But you could have been heating it 10 or more degrees warmer.

That wasn't true for me.  My combi had replaced a cylinder at some time in the past and all the hot water pipes went from the boiler to where the cylinder had been in the centre of the house (6 metres or so of large bore pipe) then back out to the hot taps

Yes in my parents house - 5 apt semi - they were same  place -and sensible - in kitchen and kitchen immediately below bathroom - so runs similar and short.

In my small 2 bed all electric terrace it's much sillier - airing cupboard at back of house - kitchen and bathroom at front - so run several litres off to get hot at tap in kitchen especially - leaving same litres of full hot to cool in pipe.

good_thyme

Thank you for the interesting and helpful replies, we are going to digest them all and see how it goes

victor2

Bit more reading for you...

We changed our 9KW electric shower for one running off a combi boiler.

I (discreetly!) timed the OH in the combi fed shower. She spent 5min 42sec.

That time under the electric shower would have used 0.86kWh of electricity. In the gas shower it used 2.29kWh of gas. However, the combi fed shower takes a good 30 seconds to warm up, as the pipe run is quite long, where the electric shower was instant - but the water flow in the combi fed shower is much better.

So you could say the electric shower would be for 30 seconds less and only use 0.78kWh.

Price 0.78kWh of electricity against 2.29kWh of gas and the gas is still cheaper, although not by as much as you might think.

Gas bill increased as a result, but electricity bill fell by more.