Electric shower v using combi boiler hot water - most efficient way to set it up?

BooJewels

Like everyone else, I've been looking at my energy use and have identified a couple of areas where I can make savings.  I'd already changed lighting to low wattage LEDs as they came onto the market and even upgraded some as newer versions were even lower wattage.  My energy use is quite high, but that's largely through choice or work commitments, comfort  etc.- like heating and dehumidifying the cellar to keep damp at bay, keeping a second large chest freezer as this saves me shopping as often, or buying at good prices to save some pennies, having lighting on all day as the house is dark.

One area that I have identified for modification though is showering.  I use an instant electric shower which, as I spend longer in there than most would find decent, is going to become even more expensive.  I want to look at using my combi boiler to supply the hot water.  When I had my previous combi boiler installed, we looked at it then, but due to water pressure or something, they said it wasn't suitable.  I've now had a newer Vaillant combi boiler and wonder if that's perhaps no longer the case.

What I have done, just to give it a cheap practice run before making a plumbing commitment, is order a shower hose that fits over the bath taps, to see if it does indeed generate enough pressure to shower with and to see what sort of flow and temperature I get.  I already have a suction bracket that I can use to position the shower head in the short term and the hose should be long enough to give it a try.  Unfortunately, the taps are at the wrong end of the bath to simply swap the taps to a shower mixer.

What I don't fully understand, is the difference between an instant on shower providing hot water direct from cold and the hot water coming from the boiler and the impact it has on energy use to heat the water / flow / pressure / time getting warm / water used etc.

I understand that a kWh of electricity costs me around 28p and the same with gas is a little over 7p.  If my 9kW x 8 minute electric shower costs me 34p (1.2kWh)  - how can I compare it to the gas supplied equivalent  - simple maths would suggest it should be around 9p - but I doubt it's that simple.  I'm not on a water meter - so that's not a cost factor at the moment - I suspect the hot tap method will use more water though.

For example, my shower water temperature is a little over 39°C - but the water out of my tap in just over 55°C - too hot to shower under.  So what would work better - lowering the boiler temp to 40°C for showering and just using the hot tap, or keeping it at 55°C and using some cold water mixed with the hot to taste.  If I run cold water at the same time as the hot, does this reduce the amount of pressure for the boiler/hot water supply?  Which would be most efficient - not just in energy used, but flow pressure etc?

I realise that it's going to need a bit of trial and error and meter reading (will turn heating off before I shower and it does 3 decimal places, so should be pretty accurate), but the hose isn't coming until tomorrow and I like examining these ideas and would like some pointers before I start getting the bathroom wet!

northernsoul

I have done some tests. I have a Vaillant 837 28kw combination boiler.
At the new rate of 7.33/kWh these are the costs I found for various lenghts of  showers ( from gas meter readings)
I use a large rainforest shower head and the shower temperature is too high for some people so plenty hot enough (they turn it down from maximum temperature)

3 min shower  9.7p (one reading)
4 min shower 13.8p (average of 9 readings)
5 min shower  15.7p (average of 6 readings)
6 min shower 17.5p (one reading)
Do not take showers longer than 6 minutes.
Note that after the 4 minute shower costs go up per 1 minute extra about 1.9p (so approximately 1.9p for every extra minute in shower - I guess)
So maybe 21.3p for a 8 minute shower.
I think my hot water temperature is set at 50C (away at the moment)
The temperature is also hot enough for a very hot full bath (to overflow) which now costs 42.8p
Hope this helps

BooJewels

Many thanks @northernsoul - much appreciated.  That's coming out at more than my simple calculation - which I thought was probably too simple to be accurate - if only because the higher temperature of the water from my boiler would require more energy to reach than the 39°C-ish I'm getting out of the shower. 

However, if you're using a large shower head, your water flow rate might be higher and going through more water - therefore more heating - so might be a bit more generous than I might experience.  The shower hose I'm getting for my experiment only has a basic shower head, so will perhaps have a slower flow rate - if indeed that's how it works - this is the stuff I'm not sure about - does it need more water for a larger head, or does a smaller one use the same volume of water, but it comes out under greater pressure?

I think it's going to need some experimentation and rushing up and down the stairs to read the meter before and after.  But I feel comforted by the fact that I'm not the only one daft enough to bother doing that.

northernsoul

I believe a rainforest shower head uses more water. Not 100% sure though because might be more flow through a normal shower head. But still think more water is needed to be heated up through a rainforest shower head.
I can only guess because I have not measured the amount of water used.

theoretica

You can get special shower heads which add air to the mix so it feels like you are getting a good shower, while using less water.  Heating less water - by whatever means - will reduce costs.

BooJewels

northernsoul said:

I believe a rainforest shower head uses more water. Not 100% sure though because might be more flow through a normal shower head. But still think more water is needed to be heated up through a rainforest shower head.
I can only guess because I have not measured the amount of water used.

I don't know either, hence asking here.  What I have been doing lately is putting the bath plug in (shower is over bath) to do some of my in-shower tasks using the standing water, after I've switched it off - saves some electricity.  I know, from this, approximately how much water I use in my 8 minutes - so if I repeat the process with a boiler water shower, I should get a comparison of how much water that uses.

Thanks @theoretica - I was looking at one of those when choosing my shower hose.  For the purpose of this experiment, it's just a basic shower head without even any adjustment features.  Should it work as I hope, either I'll be happy to continue with my make-do arrangement, or I'll install something properly - in which case, if I'm bothering with the expense, I'll be able to choose just what I want.  Then the major issue will be in trying to find a willing plumber able to do the work in anything less than about a year.  If my efforts to find a roofer are any guide.

Does anyone have any thoughts of what would work better - be more efficient - re-setting the boiler water temp to 40°C and just using the hot tap, or keeping the boiler temp at 55°C and adding cold water at the tap to taste?

theoretica

BooJewels said:

Does anyone have any thoughts of what would work better - be more efficient - re-setting the boiler water temp to 40°C and just using the hot tap, or keeping the boiler temp at 55°C and adding cold water at the tap to taste?

Do you elsewhere in the house like the water at 55?  Switching back and forth would annoy me. You could always test by lowering the temperature to see what you think.  Generally only heating as hot as you want, and not hotter and then cooling, is likely to be slightly more efficient.  Less heat will be lost in the pipes.

BooJewels

theoretica said:

BooJewels said:

Does anyone have any thoughts of what would work better - be more efficient - re-setting the boiler water temp to 40°C and just using the hot tap, or keeping the boiler temp at 55°C and adding cold water at the tap to taste?

Do you elsewhere in the house like the water at 55?  Switching back and forth would annoy me. You could always test by lowering the temperature to see what you think.  Generally only heating as hot as you want, and not hotter and then cooling, is likely to be slightly more efficient.  Less heat will be lost in the pipes.

Yes, I like to wash up at that temperature - I wear rubber gloves and like the water hot.  It would need to be cooler if I washed the pots bare handed probably. 

As I asked in my OP - I don't know which of those scenarios would give the best temperature to flow ratio - I don't understand well enough the methodology of the water heating process and the impact of running hot and cold together.  I don't really want to be adjusting the boiler ideally - but neither do I want to be futzing with the taps too much trying to get to an acceptable showering temp either.  If I spend 2 minutes doing that each shower, it might negate any saving.  Obviously, if I do decide to do it properly, I'd get the right sort of mixer tap so that I only need to switch it on at a pre-determined mix.  But for now, my test hose will have an end each going to the hot and cold taps, hence asking if adjusting the boiler to my end temperature might be the least wasteful solution.

We used to stay in a caravan with a water heater fed shower and there was quite a knack to getting the right temperature to shower under without stopping the heater by drawing too much cold water and reducing the flow through the boiler and tripping it - and you had to start the hot tap first, then add the cold.  etc etc.  I'd like to avoid that sort of faffing about if I can.

northernsoul

Takes about 40 seconds for my shower to become warm. Less when the central heating is on

northernsoul

My water temperature is set at 45C not 50C as said previously. Any lower and the shower temperature was not high enough. Personally I  think heating to a higher temperatures uses more energy , even though it might be a shorter time 

markin

You could all do a quick l/m test measure 10sec.

The cold water will try and push up the hot tap pipe, When you fit a mixer tap you need none return valves fitted, but then maybe a combi has better matching pressure than an open vented tank.