Instant water heating

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rayonline

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#116651 5-May-2013 23:02

We have it for gas cooking and water heating. I have been curious about this for some time when I started looking at my bills more indepth. On the Smarter Homes website it says instant gas is more efficient than electric cyliner water heating.

I have been reading the gas meter, it doesn't move when gas isn't being consumed. For a 12 minute shower, water temp set at 43 degrees (Rinnai). It consumed 4.11kW (after applying the multiplyer conversion as on my bill) which is 28c. Today's lunch and dinner preparation was just over $1.00 (12:00 - 18:30). This is ex GST and ex the daily charge.

When water was being consumed the gas meter was moving smoothly, like there wasn't a delay in the spinning of the numbers.

I read on another webpage where people said they found instant gas more costly than electric cylinder too, so who's right?

March bill we used 1200kW of gas.

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#811821 6-May-2013 01:01

43 degrees seems a tad warm for a shower (at least for me - I'm set at 39 degrees)
Or are you also mixing in cold water?

rayonline

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#811847 6-May-2013 08:30

43 is set for all the hot water eg - washing dishes or hot tap. We don't use a dishwasher :D Yes mix cold water yeah ... thru the showe head.

NonprayingMantis

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#811876 6-May-2013 09:25

Having a boiler means you are heating up a load of water, then keeping it warm until such time you use it.

Having the instant water heating means you only heat the water you use, so there is no energy wasted through lost heat

Stan

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#811898 6-May-2013 10:06

Natural Gas is MUCH cheaper per unit than electricity your rinnai is somewhere around 90% efficient. So yes should be around half the price.

farcus

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#812070 6-May-2013 14:38

rayonline: 43 is set for all the hot water eg - washing dishes or hot tap. We don't use a dishwasher :D Yes mix cold water yeah ... thru the showe head.

you could probably save a little if you dropped the temp down a couple of degrees.
At the moment you are paying to heat the water to 43 and then adding in cold water to cool it down.
Usually a temp between 39 and 42 degrees should be hot enough to do everything you have noted above without the need to add in cold water.

timmmay

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#812085 6-May-2013 14:53

farcus:
rayonline: 43 is set for all the hot water eg - washing dishes or hot tap. We don't use a dishwasher :D Yes mix cold water yeah ... thru the showe head.

you could probably save a little if you dropped the temp down a couple of degrees.
At the moment you are paying to heat the water to 43 and then adding in cold water to cool it down.
Usually a temp between 39 and 42 degrees should be hot enough to do everything you have noted above without the need to add in cold water.

Mixing in cold water increases the volume through the shower head, reducing the amount of hot water used. Heating to say 80 degrees you would use half the hot water, but it would take more energy to heat.

Not sure turning the temperature down would have any effect on the bill.

CYaBro

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#812102 6-May-2013 15:34

I have hot water set to 41 for my showers, kids get set to 40 and the wife has hers at 42.
None of us use the cold water at all in the shower, just have mixer set to full hot.
When washing dishes I set it to 55.

Opinions are my own and not the views of my employer.

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John2010

532 posts

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#812103 6-May-2013 15:36

farcus:
rayonline: 43 is set for all the hot water eg - washing dishes or hot tap. We don't use a dishwasher :D Yes mix cold water yeah ... thru the showe head.

you could probably save a little if you dropped the temp down a couple of degrees.
At the moment you are paying to heat the water to 43 and then adding in cold water to cool it down.
Usually a temp between 39 and 42 degrees should be hot enough to do everything you have noted above without the need to add in cold water.

Your physics is a definitely a bit pooh

. As I think Timmmay is alluding to, if one uses higher temperature water then you use less of it; when mixing it is the quantity of heat in the hot water one is interested in not the quantity of hot water used.

Apart from some miniscule extra pipe loss during flow and from the stagnant volume when no flow in the pipes the result is the same (that miniscule extra loss is only that attributable to the small volume in the pipe and heater exchanger, and to the temp delta from the few degrees lower temperature setting one might have used).

So unless one goes mad heating to (dangerous) temperatures way above the desired end temperature at the consuming point there is no difference worth considering.

farcus

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#812111 6-May-2013 15:44

John2010:
farcus:
rayonline: 43 is set for all the hot water eg - washing dishes or hot tap. We don't use a dishwasher :D Yes mix cold water yeah ... thru the showe head.

you could probably save a little if you dropped the temp down a couple of degrees.
At the moment you are paying to heat the water to 43 and then adding in cold water to cool it down.
Usually a temp between 39 and 42 degrees should be hot enough to do everything you have noted above without the need to add in cold water.

Your physics is a definitely a bit pooh . As I think Timmmay is alluding to, if one uses higher temperature water then you use less of it; when mixing it is the quantity of heat in the hot water one is interested in not the quantity of hot water used.

Apart from some miniscule extra pipe loss during flow and from the stagnant volume when no flow in the pipes the result is the same (that miniscule extra loss is only that attributable to the small volume in the pipe and heater exchanger, and to the temp delta from the few degrees lower temperature setting one might have used).

So unless one goes mad heating to (dangerous) temperatures way above the desired end temperature at the consuming point there is no difference worth considering.

except that it has been my experience that most people begin by turning their hot water tap up full and then gradually increase the cold to attain the desired temperature.

timmmay

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#812112 6-May-2013 15:48

farcus: except that it has been my experience that most people begin by turning their hot water tap up full and then gradually increase the cold to attain the desired temperature.

Only people who like to be burned. Anyone sensible turns on cold then adds warm until it's the right temperature. If you went full hot in my shower, with a 55 degree output (65 internally but mixed on leaving the cylinder), you'd scald yourself. I had extra cold mixed in to stop people (ie me) burning themselves.

John2010

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#812162 6-May-2013 17:28

farcus:
except that it has been my experience that most people begin by turning their hot water tap up full and then gradually increase the cold to attain the desired temperature.

Specific heat of water is 4190J/kg.K
Assume a generous shower flow running 15l/min (15kg/min) (as in Networkn's showers now

) with hot full on while someone (but not most of us) plays with the cold water.

Assume you play with it for a wasteful 1 minute but using your advice have, say, reduced the in line heater temperature by 4C.

Then the heat content of water saved per shower event = 4190J x 15kg x 4C = 251.4 x 10^3 Joule approx. = 0.07kWh approx.

Assume gas is used at 10c/kWh (which is a little more than the OP is paying) then cost per event will be 0.07kWh x 10c = 0.7c approx.

Assume 4 showers/day, everyone wasting water, and a month of 31 days then cost of heating the wasted water will be 0.7c x 4 x 31 = 87c/month saved if you lower the in line heater temperature by 4C and you choose to waste water.

So even if one chooses to waste a lot of water in the way you say and for the long time I have allowed, and for the 4 person household each showering once per day and wasting water I have used, the cost saving from lowering the temperature of the water is minimal.

And if you don't waste water you save nothing by lowering the in line heater temperature. Not wasting water seems the sensible way to go.

rayonline

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#812169 6-May-2013 17:49

Ok, we turn on the cold water side first and then set the desired temp. I have done that too quickly still starting at the cold side and it was a bit too hot.

We get a bill of near $200 for electricity which is fine, we got the new meter put in not tooo long ago. But for gas which we just use for water and cooking stove, that itself is $150 incl GST and all the daily charges. 1200kW use.

If we had electric water could our power bill be $300 for the month? $100 for cooking/water - due to $150 discounted to $100 when the daily charge isn't needed.

Nety

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#812196 6-May-2013 18:34

Depends on what kind of electric heater. You have traditional hot water heaters, solar boosted and heat pump hot water. The first would likely be more. The second two likely less.

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Aredwood

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#812337 6-May-2013 22:02

Gas is definitely cheaper. Assuming you pay 20c per unit of electricity 1200 units (the amount of gas you said you used) would have cost you an extra $240 per month on top of your current electricity bill. Or $300 if you pay 25c / unit. Since you said your gas bill was $150 this means that switching to electric hot water and cooking will cost you an extra $90 @20c / unit or $150 @ 25c / unit.

Take any stories of gas hot water being more expensive than electric with a grain of salt. Did they replace a low pressure electric cylinder with mains pressure gas. Therefore going from about 6L/min shower flow to 20L/min. (Easily achievable if you connect the old low pressure shower mixer and rose to mains pressure). Were they connected to piped natural gas or LPG? Natural gas has a higher daily charge than LPG but a lower unit charge than LPG. Also did they have night rate electricity for their hot water? Night rate electricity is similar to LPG cost wise on a per unit basis.

Although for very low users electricity for hot water is cheaper since the extra daily charges out weigh the cheaper unit cost. But you are definitely not one of those users.

In otherwords the answer to the "Is gas or electricity cheaper?" Question depends on how much you use, where you live. And whether you will be using Natural gas or LPG. Edit - this last bit is a general reply rather than a reply to the OP /Edit

Linuxluver

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#812385 6-May-2013 22:38

This is an interesting thread as I own two flats that both have instant hot water via Bosche gas water heaters. The water pressure is mains.

In one of the flats the tenant asked me to check the shower head as it was behaving "oddly".

The head had lots of flexible wee 'rubber' spray points set in a chrome fascia. The 'problem' was that after a few minutes these wee spray points would begin to point in all directions and the water coming out of them was almost painful as the stream was so small and strong.

They showed me how they used the shower: pull the tap out full and go for it.

Aaah......Yes. Try just pulling the tap out half way (less volume) and let us know how you get on.

The wee spray heads were warming and becoming more flexible...and (partially) 'inflating' due to the sustained maximum pressure. At this point the spray could be pointing in any direction....

Reducing the flow sorted it out.

I wish I has those kinds of pressure issues at home....where I have a gravity-pressure tank of electrically heated hot water.

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