Shower or bath?

Mike2

Forum Supporter
Went for water metering recently as the girls have left home, almost. We have gas CH but an electric shower. Common wisdom is that it is cheaper to run a shower than a bath. I am sure that is the case for me, as a shower is less than a minute, but when the girls visit they commonly shower for 30 minutes! :eek:

So, where is the sweet spot? I believe the shower is a 12kw one. It is also considerably cheaper to heat water with gas than with electricity, but unfortunately electric showers take a cold input.

There is some really out of date discussion here:

http://forums.moneysavingexpert.com/showthread.php?p=19253499

I loved the story about the son's girlfriend taking a 45 minute shower and the parent watching the fuel consumption in dismay.
 

Brian

Administrator
Staff member
You could have a shower plumbed into your cold and gas heated hot water system.:)
 
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Mike2

Mike2

Forum Supporter
You could have a shower plumbed into your cold and gas heated hot water system.:)

Too late. You mean a mixer? The head of water is not sufficient, so a power shower would need to be fitted, and that then uses even more water, so I understand. I think one person on that thread is describing one of those, where her shower fills a bath in very few minutes.
 

dandnsmith

Forum Supporter
The head of water is not sufficient, so a power shower would need to be fitted
That must be a very old system - even my 15 year old system runs at mains pressure for delivery of both hot and cold water.
 

af123

Administrator
Staff member
Too late. You mean a mixer? The head of water is not sufficient, so a power shower would need to be fitted, and that then uses even more water, so I understand. I think one person on that thread is describing one of those, where her shower fills a bath in very few minutes.
I've just had a new shower fitted on gravity fed system. Only 2m head (so approx 0.2 bar of pressure) so I fitted a pump too. The pump is 1.5bar and the shower head aerates the water and has a 9l/m flow limiter built in. I calculated I could shower for 25 minutes before draining the hot water tank assuming a 75% hot water input. I like a long shower when I have the time but still wouldn't take longer than 10 minutes!
 
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Mike2

Mike2

Forum Supporter
That must be a very old system - even my 15 year old system runs at mains pressure for delivery of both hot and cold water.


Two years old actually. Hot water is never at mains pressure as far as I know. No instantaneous heater could deliver it that fast, and the other sort relies on a header tank. If you have a good head of water, eg, a block of flats, you are OK.

We decided against a combi boiler as I had heard of so many reliability issues. So we went with a condenser boiler.
 
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Mike2

Mike2

Forum Supporter
I've just had a new shower fitted on gravity fed system. Only 2m head (so approx 0.2 bar of pressure) so I fitted a pump too. The pump is 1.5bar and the shower head aerates the water and has a 9l/m flow limiter built in. I calculated I could shower for 25 minutes before draining the hot water tank assuming a 75% hot water input. I like a long shower when I have the time but still wouldn't take longer than 10 minutes!

So a 10 minute shower would be about 90l, or roughly 31p worth of water? The other cost would be in heating 90l of water to a reasonable temperature.

As far as I can see, I pay 3.78p per kWh for gas and 12p for electricity. So my 12kW electric shower would cost about 24p for a 10 minute shower. Using gas would cost maybe 8p. My regulator is at 7.6l/min so is slightly slower than yours. That would add up to 55p for a 10 minute electric shower and 39p for a gas heated one.

Looks like my daughter's shower costs me £1.50 a shot, though! (Not counting multiple showers and a bath in one day!) 7.5*30*0.35=78.75p for water and 12*0.5*12=72p for electricity. (I have no idea whether that electric cost is accurate, as the control reduces temperature but increases flow!)

Edit: I would guess that from surface of water tank to shower head there is no more than 1m.

Edit2: Water consumption for us is about 0.3cu m/day, and considerably more if our daughters visit.

Edit3: Dunno where that guess came from for water costs. It was totally wrong! I have changed the figures. Our estimated 110l costs £384 a year, so the cost per litre is 0.35p
 

Ezra Pound

Well-Known Member
Hot water is never at mains pressure as far as I know. No instantaneous heater could deliver it that fast, and the other sort relies on a header tank.
It is possible to pressurise the water held in a hot water tank from the (cold) mains water pressure, this is obviously not an instantaneous hot water system
 
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Mike2

Mike2

Forum Supporter
It is possible to pressurise the water held in a hot water tank from the (cold) mains water pressure, this is obviously not an instantaneous hot water system

I haven't heard of that being done. There would need to be a non-return valve on the cold supply to stop pollution, but slime mould in the header would no longer be a problem!

Edit:

http://www.plumbase.com/article/11/differences-between-vented-and-unvented-water-cylinders.html

Looks like the US has them. You would need to take expansion in the hot tank into account too.
 

Black Hole

May contain traces of nut
My hot water system is a condensing combi gas boiler. There is no hot storage tank, the hot tap water is heated on demand and is at mains pressure. An automatic valve and heat exchanger system either heats the central heating circulation or the tap water according to demand (water has priority), and a small heat reservoir provides smoothing (the burners only fire up when the reservoir needs topping up).

Because the hot feed is at the same pressure as the cold feed, I can run a thermostatic mixer shower straight from the hot and cold feeds. The fitting instructions are supposed to ensure the shower head can never dangle in water, and thereby suck stale water into the supply if the pressure went negative (what are the chances?).

There is one big problem (not counting the requirement that nobody flushes the loo or runs the hot tap - that goes for most passive showers): if the hot water demand isn't great enough, the boiler alternates between heating the water and heating the radiators, so the damn shower alternates hot and cold! This is more likely in the summer than the winter. I get around this using a "waste gate" - I turn the sink hot tap half on, to ensure a sufficient demand.
 

Ezra Pound

Well-Known Member
Although a combi boiler is fed by mains water pressure, the flow is restricted on it's path through the heat exchanger in order to raise the temperature by some 40 Deg. C, if the Domestic Hot water flowed out of the boiler at the same rate as the cold water flows from the mains, the combi boiler would never get it hot enough, it only works because the flow is restricted
 

MikeSh

Well-Known Member
They have an expansion vessel similar to the ones used in central heating systems where there is no header tank. They also have an hot water Cylinder that can take 3 bar of pressure

Yes. We have this.
Originally the house had the usual - cold water tank in loft, hot tank in airing cupboard. We had a power shower for a while, then got rid of the airing cupboard for space to rearrange the bathroom and went combi. That was just rubbish, so then I heard about mains pressure hot water and changed it to that. Brilliant.
 

MikeSh

Well-Known Member
Went for water metering recently as the girls have left home, almost. We have gas CH but an electric shower. Common wisdom is that it is cheaper to run a shower than a bath. I am sure that is the case for me, as a shower is less than a minute, but when the girls visit they commonly shower for 30 minutes! :eek:


Common wisdom was largely based on word of mouth and 'surveys' where people estimated how long they showered for, etc. I think the surveyed average shower was 4 mins.
About a year ago I saw an item which blew a serious hole in that wisdom. Technology has made it possible to accurately meter and datalog individual bathrooms without breaking someone's bank, so (and working from memory now) someone metered up 1000 homes and recorded their shower use. Result: average use is actually about 7 mins and water use is not much less than baths.

In our house with a high pressure shower we did the leave the plug in while showering test (my wife showers, I bath) and it filled the bath higher than I fill it - and this is a big bath with a wide end for the shower. So no saving there :(
 

Black Hole

May contain traces of nut
Although a combi boiler is fed by mains water pressure, the flow is restricted on it's path through the heat exchanger in order to raise the temperature by some 40 Deg. C, if the Domestic Hot water flowed out of the boiler at the same rate as the cold water flows from the mains, the combi boiler would never get it hot enough, it only works because the flow is restricted
So what? It works, even when filling the bath.
 
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Mike2

Mike2

Forum Supporter
So what? It works, even when filling the bath.


I think the point was that it isn't really mains pressure like the cold tap is. It's just mains fed, like an electric shower.

Your description of your shower on a hot day is interesting. I would not want to do that on a water meter, and one thing our electric shower does well is maintain a constant flow and temperature, even though it costs a fortune doing it! How it does that I have no idea. Perhaps it doesn't run at 12kW all the time. You set a temperature on it and the flow changes, but after that both remain constant. There is also a lower power setting for the summer (reduced flow per temperature setting), and I notice that one daughter always changes that back to the winter setting with higher flow. Grrrrh.
 

Black Hole

May contain traces of nut
I think the point was that it isn't really mains pressure like the cold tap is. It's just mains fed, like an electric shower.
Actually, there is not a great deal of difference in the flow rate between my hot and cold taps. The reason is that I don't have the main stop cock full on, it is set to strict the overall flow rate to what is reasonable from a bath tap.

The same discussion applies to any water installation - pressure is mains pressure until a flow starts, and then the pressure (at the tap) depends on mains pressure at source minus the flow rate x pipe resistance. This distinction is largely irrelevant, because it's a common factor.
 
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Mike2

Mike2

Forum Supporter
So "mains pressure" means "stopcock pressure" now. :D

I am sure with a high enough header tank you could greatly exceed stopcock throttled pressure, for a while, until the water ran out!
 
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