What to do with Hot Water?

[horseman]

As a licensed plumber who specialises in gas appliances I have been asked this before. I feel I am not best to comment regarding an electric mains pressure storage water heater, however my guess would be any more than a week then turn it off. For a gas mains pressure storage unit even on older cheaper lower efficiency units I have proven that it is best to do nothing with them at all.
A tank of hot water, when up to temp will be maintained at that temp with just the pilot light burning and the insulation around the tank, as long as no one is home and drawing off hot water it will use virtually no gas while you gone.
The gas the pilot light uses is absolutely minuscule.

As far as continual flow units or solar systems are concerned the efficiency is not much better than the 'better' storage systems. The continual flows are so full of electronics that they are very troublesome and prohibitively expensive to repair, the solar systems generally don't pay for themselves in their lifetime.

 

[Austman]

The continual flows are so full of electronics that they are very troublesome and prohibitively expensive to repair

We don't know yet. We have 12 x STIEBEL ELTRON units in our building all operating for the past 8.5 years with zero problems so far and no repairs ... So we don't actually know how much it costs to fix one. But when you don't use hot water - there is no cost. And when you want it - it's there. Perhaps pick the right brand?

 

[amaroo]

As a licensed plumber who specialises in gas appliances I have been asked this before. I feel I am not best to comment regarding an electric mains pressure storage water heater, however my guess would be any more than a week then turn it off. For a gas mains pressure storage unit even on older cheaper lower efficiency units I have proven that it is best to do nothing with them at all.
A tank of hot water, when up to temp will be maintained at that temp with just the pilot light burning and the insulation around the tank, as long as no one is home and drawing off hot water it will use virtually no gas while you gone.
The gas the pilot light uses is absolutely minuscule.

As far as continual flow units or solar systems are concerned the efficiency is not much better than the 'better' storage systems. The continual flows are so full of electronics that they are very troublesome and prohibitively expensive to repair, the solar systems generally don't pay for themselves in their lifetime.

Was told something similar. Solar with a gas (bottle) heat pump, apparently the same set up with electric heat pump continues to boil the billy all day, every day.

 

[j.espoir]

Make yourself a nice cup of tea with it and it will help your thinking process.

Isn't coffee better?!

 

[Sprucegoose]

I'm mostly interested in cost (and its an interesting theoretical exercise).

The "climate change" impact per year on a global scale is negligible. I don't want this to be a political thread - let's leave it at that.

Strange post given i just responded to a post of yours concerning F class travel and here you are postulating about hot water costs !!

what gives?

 

[horseman]

We don't know yet. We have 12 x STIEBEL ELTRON units in our building all operating for the past 8.5 years with zero problems so far and no repairs ... So we don't actually know how much it costs to fix one. But when you don't use hot water - there is no cost. And when you want it - it's there. Perhaps pick the right brand?

Sorry, Gas continual flows are troublesome and prohibitively expensive to fix. There are very very few three phase systems out there, i knew they were out there but yours is the first actual installation I've heard of.

 

[munitalP]

Ok, rough calcs not knowing insulation (heat loss), consumer rate, tank volume and design, if you have your temp set at 60 degrees...

200L tank, Will cost (on my off peak Sydney extortion costs) approx $23.20 to heat from cold (10 degC). This will take approx 42 minutes. To maintain 60 deg C with no water draw off and less than 5% heat loss through insulation per day, the element will need to operate at full power consumption (resistive load, costs more) for around 3 minutes per day $1.65 approx, $11.60 per week to maintain temp. This assumes your thermostat is working correctly and the water is allowed to cool 5 degrees before the thermostat operates the relay rather than a poorly calibrated thermostat which has very low differential and effectively hunts between 1-2 degrees, a common fault my sparky friend told me.

So leaving it on for 3 weeks not used works our at $34.20, reheating $23.20

My initial calcs were a bit wrong, 2 weeks is the break even rather than 3

Please note a few assumptions made regarding costs and efficiency! Doc James, as yours is a dual element, it does cost more than mine as a single element.

 

[Pushka]

@ Pushka

No - it has a vacation setting!

Phew.

If it was me, I'd turn the heater off if going for longer than a week. But luckily we have gas and instantaneous which only wastes water!

 

[docjames]

Ok, rough calcs not knowing insulation (heat loss), consumer rate, tank volume and design, if you have your temp set at 60 degrees...

200L tank, Will cost (on my off peak Sydney extortion costs) approx $23.20 to heat from cold (10 degC). This will take approx 42 minutes. To maintain 60 deg C with no water draw off and less than 5% heat loss through insulation per day, the element will need to operate at full power consumption (resistive load, costs more) for around 3 minutes per day $1.65 approx, $11.60 per week to maintain temp. This assumes your thermostat is working correctly and the water is allowed to cool 5 degrees before the thermostat operates the relay rather than a poorly calibrated thermostat which has very low differential and effectively hunts between 1-2 degrees, a common fault my sparky friend told me.

So leaving it on for 3 weeks not used works our at $34.20, reheating $23.20

My initial calcs were a bit wrong, 2 weeks is the break even rather than 3

Please note a few assumptions made regarding costs and efficiency! Doc James, as yours is a dual element, it does cost more than mine as a single element.

Thanks Mr! - Looks like i'll just use a two week "line in the sand" until it dies then i can either
- get solar
- plumb a gas main to my house
- move to Queensland

 

[SFSC]

A few more details to follow on from my post #35. As mentioned, the heating is done using a heat pump only. The tank is rated to take a booster element but it isn't fitted. The household consists of Mrs SFSC and myself so we find it convenient to turn off the power to the hot water the day before we go. The 325 litres in the tank is always enough to cover our showers and any washing up on the day of our departure. The overall amount of energy remaining in the tank is now less because we have used some of it and thus the tank will reach ambient temperature sooner into our absence, halting energy loss across the insulation a bit sooner.

(OT) If the weather conditions allow (i.e. not too hot or cold) I usually power down the supplies to the air conditioners at this time. This prevents them drawing a continuous 5 watts or so in the control circuits while we are away.

When we get back, I turn on the HWS. The off-peak extended hours tariff usually means we don't have to wait too long before the unit kicks in. If the weather is mild I may leave the air conditioners turned off at the board until they are again required.

 

[horseman]

For what it's worth, crooks will often check out meter boards looking for circuits turned off indicating occupants are away. Your power company can supply you with a lock to keep the crooks out.

 

[JessicaTam]

Ok, rough calcs not knowing insulation (heat loss), consumer rate, tank volume and design, if you have your temp set at 60 degrees...

200L tank, Will cost (on my off peak Sydney extortion costs) approx $23.20 to heat from cold (10 degC). This will take approx 42 minutes. To maintain 60 deg C with no water draw off and less than 5% heat loss through insulation per day, the element will need to operate at full power consumption (resistive load, costs more) for around 3 minutes per day $1.65 approx, $11.60 per week to maintain temp. This assumes your thermostat is working correctly and the water is allowed to cool 5 degrees before the thermostat operates the relay rather than a poorly calibrated thermostat which has very low differential and effectively hunts between 1-2 degrees, a common fault my sparky friend told me.

So leaving it on for 3 weeks not used works our at $34.20, reheating $23.20

My initial calcs were a bit wrong, 2 weeks is the break even rather than 3

Please note a few assumptions made regarding costs and efficiency! Doc James, as yours is a dual element, it does cost more than mine as a single element.

Based on the photo it is a 400 litre tank, if that changes the time frame to break even.