Actually, Australia had exactly this scheme and it worked for many decades until we got a government who has tried to move us in the direction of the US system. I know many people who have come here from the US and commented on how simple and effective the heatlh system is (I believe canada has the same system?).
Steam = water vapour. You might be thinking of fog, i.e. lots of little droplets of liquid water suspended in air. I like to remember the latent heat of vapourisation of water as equivalent to heating the water from 40 to 540C - steam by weight burns like 540C water.
I've got a good book called 'Heat Transfer' in the Schaum outline series that covers lots of tidbits like this. The ASHRAE handbook is also full of goodies (I've heard).
This boundary layer thing is quite interesting - for many applications the boundary layer is more significant than the material in question. For example, in air-air heat exchangers there is almost no benefit to using copper foil over plastic film (the main benefit would be the higher melting point!). Another example is the good old space blanket. It could be made from just aluminium foil, the boundary layer and the high reflectivity and low emissitivity mean that despite the huge conductivity of Al, the blanket is a remarkably good insulator! (They make them from aluminised mylar for strength, incidently)
At -40 the water holding capacity of air is negligible. Even at 0F it is pretty much irrelevant. Furthermore, a higher humidity reduces evaporation from the skin, a far more effective way of cooling bodies than the specific heat. The specific heat of water vapour is 1.8J/gK, the specific heat of air is 1 and the partial pressure even at 0C is less than 1%. On the other hand, evaporating a gram of water sucks 2.2kJ of heat out of your body.
Wind chill is very dangerous, on the other hand, as not only is more cold air moving past you, the thermal conductivity of the boundary layer keeping you warm increases proportionally with air speed.
If a tankless water heater saves 30% over a tank one, then for it to make sense the solar system needs to last as long, be no more expensive than the tankless premium and add a 30% efficiency gain (or some combination that adds up to the same thing). I'm dubious that it can achieve this.
I can buy a solar retrofit kit right now for $1kau (and I've heard that similar things exist in the US for $600) that has a 10 year guarantee. It would cost less than 20% of the gas of a tankless system, maybe even none of the cost. If the solar part is good enough you don't need to spend much on the water heater - a 20% efficient water heater in conjunction with a 90% available solar system uses half the gas of the best fully gas systems. With modern evacuated tube collectors 90% seems like aiming low.
Evacuated tube collectors are such good collectors that the heat output end gets warm to the touch when they are sitting in my lounge room under only incandescent light. And they are cheap. I predict that within 10 years most people in melbourne will have some kind of tube collector system.
I'm dubious about the method used to measure though. His claim that tanks last a lot longer than people expect runs true. My dad has had his storage hot water unit for at least 15 years now, having had two anodes installed.
The less you use, the better deal the tankless units are.
The less you use the easier it is to provide all the energy with solar.
But in the limit, the water doesn't get to the target temperature with a tank unit. A tank unit has all day long to get the tank back up to temperature after my shower is over. That's why a tank water heater has 34 kBTU input and a tankless one has 175 kBTU.
They're all fast recovery types these days, which means that they can keep up with most demand rates at higher efficiency. The main advantage to a large tank is the thermal mass, which means that you can use a higher efficiency burn, and the large size, which gives you more room to do your heat exchange.
Btw, BTU is a measure of energy, not power.
The Bosch 250SX tankless heater has an energy factor of 0.82, which I believe means that 82% of the energy is used to heat the water, and 18% goes up the vent.
Sure, and I can buy an aquamax storage gas heater than costs half the price and gives me an expected efficiency of 94%, including tank losses - I just rung the company, who also sell a rebadged bosch (rebodged basch?).
Yeah, ok. I'll look more closely. Extraordinary claims and all that. Perhaps 'standard' storage heaters don't have much insulation. Lets say we use 100L of hot water per day. That's 14MJ of heat. What insulation would we need to make the waste heat 34% of the total? 14*0.34/0.66 = 7MJ of heat waste each day through the insulation. The means 87W heat lost, over a 1.5m^2 area, or an R-value of 1.6. That's not an unreasonable insulation level (corresponding to say an inch of fibreglass), so perhaps it is plausible after all. That's also assuming a 100% efficient storage heater (reasonable - condensing units get something like 95%) and a 100% efficient tankless heater (less reasonable). A smaller tank, more water usage or more insulation would reduce this, in the limit you get a continuous flow system.
We also haven't considered the benefit of efficiently heating the house whilst make hot water (cogeneration). If a unit is 94% efficient at heating the water, you can interpret that as 94% efficient at heating the house with the waste heat. I bet you spend more on heating than on hot water.
When our existing sytem dies we'll probably get a condensing continuous flow system (if they make them then) with solar preheat (hey, for $300 I think you'd be mad not to). Until then I'm going to stick with our 1 star gas unit with solar preheat, perhaps wrapping a spare roof bat around the outside.
The DOE site claims that 94% of the people who installed a solar system thought it a good investment, which runs contrary to your claim that they cause problems.
Has some interesting observations. It seems that a standard US hot water service is woefully inefficient, which is perhaps where the DOE gets their 'can be as much as' from.
Thanks for this interesting discussion - you made me check my assumptions and they were off a little.
Sorry, was there a mistake in my maths? Appeal to authority seems rather weak when I've given you numbers. Much better to point out where I went wrong. Of course the tankless manufacturers say their systems are more efficient. I just don't see the advantage in the numbers. The same sites seem to talk about the pilot light a lot, which is amusing because the pilot light only really wastes energy in tankless designs.
I can't say the wikipedia entry is very compelling. It says 'can', which is a weasel word. Their point about losing lots of heat in the pipe of course applies to both technologies (but not to point of use systems), but there are two obvious points. firstly, where only hot water is useful (shower?), or you want to save water, you can use something like the envirosave: http://www.abc.net.au/newinventors/txt/s1389495.ht m
secondly, if the pipes are inside the insulation, the heat is not really wasted as it heats the house.
I have a 130L tank. It is heated to 50C. The water comes in at 15C and needs 19MJ to fill.
It is wrapped in R3 insulation with a surface area of 1.5m so it loses 0.5W per delta deg C. Thus it loses 17W of heat when hot. That's 13 days worth of storage equivalent to 1 heating cycle.
So no, you're wrong, most of the energy is needed to heat the water. If I can preheat the water to just 30C (piece of cake, doesn't require any thing fancy) I save 40% of my gas bill straight away. My prototype, costing $100, heats 50L of water on a cloudy day to 80C quite comfortably which is about 70% of the energy used by the heater.
Where do you get your $50 less figure from? 95% or more of the cost of DHW is heating the water from ambient. I can't see how removing the tank can change this.
They bother because tankless heaters take up less space and never run out of hot water. Until recently tankless heaters were woefully inefficient. Now they're nearly as good as a condensing storage heater (storage is the standard term here for tanked DHW services).
"but that's assuming natural gas prices remain the same as they are now"
And that's the big question nobody knows the answer too. I suspect they will stay pretty much the same (inflation adjusted), as there is apparently plenty of gas. I'm certianly tempted by the tankless designs, as far as getting a quote (about $1100 for a decent one).
Where do you get your $50/pa less on natural gas figure from? Storage and instant heat systems are about the same efficiency, with storage coming out slightly ahead. Or are you comparing to your existing system?
Ok, wiring in the walls is nice. Did you look at the wiretrax idea?
An old (say 50years) used house is probably a better deal, at least in australia, where houses built in the 80s are falling apart.
Do you live in a hot climate or a cold climate? pumping the waste heat outside really only makes sense if you spend more each year on cooling than heating. clothes dryer vents are too small to be efficient for moving heat. A better solution would be to just return the air to the room in winter (using a big fan) and to the attic in summer. An AC seems way over kill. I live in a climate where it regularly reaches 45C (110F?), and my computer works fine under those conditions with just a $10 Lowes type box fan parked in the side:)
For sound insulation it is hard to beat mass. Use those hollow concrete bricks (cinder blocks?), perhaps filled with dry sand.
Of those issues the installation/capital cost is the killer (and the reason we don't have solar hot water heating). You don't seem to have done the sums? An interesting question is 'how cheap would a complete package have to be before it makes sense?'.
I'm working on a $300AU ($200US) retrofit system using evacuated tube collectors and off the shelf parts (and keeping the existing gas storage heater tank). So far it seems quite promising, producing about 90% of the total water heating energy we need. $300 is the differential cost between a 1 star and a 6 star (some arbitrary efficiency units provided for all domestic appliances), and my system (which isn't very much my, and very much cheap chinese evacuated tubes) saves about 8 times as much energy as the difference.
Would you consider that? At $300, with our water heating bill, you could replace the unit every year and still save money. How much do you spend on water heating each year?
Most of the commercial systems sold today are complete rip-offs, but a friend noted a complete 20 tube system, including a stainless tank, for $695 US. My design is only 5 tubes and thus wouldn't have enough bandwidth for a family.
Most of my space is the result of a money/space tradeoff and an over-developed sense of self-reliance. I have tons of tools so I can do things myself instead of paying someone to do them for me. The other big problem is that I'm involved in so many different things, each of which take up some space (e.g. piano + drum kit = 120 sq. ft.).
Fair enough! What sorts of problems are these though? I know a number of people with solar DHW and they are all very pleased with it (one recently replaced his 25 year old system when the panel finally corroded through. I'm interested to know what problems you've seen.
Slashdot Mirror
Actually, Australia had exactly this scheme and it worked for many decades until we got a government who has tried to move us in the direction of the US system. I know many people who have come here from the US and commented on how simple and effective the heatlh system is (I believe canada has the same system?).
Steam = water vapour. You might be thinking of fog, i.e. lots of little droplets of liquid water suspended in air. I like to remember the latent heat of vapourisation of water as equivalent to heating the water from 40 to 540C - steam by weight burns like 540C water.
I've got a good book called 'Heat Transfer' in the Schaum outline series that covers lots of tidbits like this. The ASHRAE handbook is also full of goodies (I've heard).
This boundary layer thing is quite interesting - for many applications the boundary layer is more significant than the material in question. For example, in air-air heat exchangers there is almost no benefit to using copper foil over plastic film (the main benefit would be the higher melting point!). Another example is the good old space blanket. It could be made from just aluminium foil, the boundary layer and the high reflectivity and low emissitivity mean that despite the huge conductivity of Al, the blanket is a remarkably good insulator! (They make them from aluminised mylar for strength, incidently)
Unfortunately, yes: http://en.wikipedia.org/wiki/Mosquito_fish
They can bet on solar, which is even worse in terms of taking up space, and is expensive, and only works for half the day.
If you're after heat or light, solar is very cheap. And those are the two biggest energy uses outside transportation.
At -40 the water holding capacity of air is negligible. Even at 0F it is pretty much irrelevant. Furthermore, a higher humidity reduces evaporation from the skin, a far more effective way of cooling bodies than the specific heat. The specific heat of water vapour is 1.8J/gK, the specific heat of air is 1 and the partial pressure even at 0C is less than 1%.
On the other hand, evaporating a gram of water sucks 2.2kJ of heat out of your body.
Wind chill is very dangerous, on the other hand, as not only is more cold air moving past you, the thermal conductivity of the boundary layer keeping you warm increases proportionally with air speed.
It worked fine in totem without quicktime. I want to know why he had a CD blank before they were released?
Doing the former would require that he attach his name to his complaints
And you're Don Giovanni. And I'm the Magic Flute.
They had 707 jet passenger lines (1950s) when the book was published (1962).
It's so the angels can stay on their clouds without flapping their wings, right?
Amazing, he was born in 1970 and made props for a movie release in 1977! (WP says he updated the R2D2s for the new movies)
If a tankless water heater saves 30% over a tank one, then for it to make sense the solar system needs to last as long, be no more expensive than the tankless premium and add a 30% efficiency gain (or some combination that adds up to the same thing). I'm dubious that it can achieve this.
e aters_-_Some_Surprising_Facts.shtml
I can buy a solar retrofit kit right now for $1kau (and I've heard that similar things exist in the US for $600) that has a 10 year guarantee. It would cost less than 20% of the gas of a tankless system, maybe even none of the cost. If the solar part is good enough you don't need to spend much on the water heater - a 20% efficient water heater in conjunction with a 90% available solar system uses half the gas of the best fully gas systems. With modern evacuated tube collectors 90% seems like aiming low.
Evacuated tube collectors are such good collectors that the heat output end gets warm to the touch when they are sitting in my lounge room under only incandescent light. And they are cheap. I predict that within 10 years most people in melbourne will have some kind of tube collector system.
An interesting (though maybe old) article:
http://www.askthebuilder.com/451_Tankless_Water_H
I'm dubious about the method used to measure though. His claim that tanks last a lot longer than people expect runs true. My dad has had his storage hot water unit for at least 15 years now, having had two anodes installed.
The less you use, the better deal the tankless units are.
The less you use the easier it is to provide all the energy with solar.
But in the limit, the water doesn't get to the target temperature with a tank unit. A tank unit has all day long to get the tank back up to temperature after my shower is over. That's why a tank water heater has 34 kBTU input and a tankless one has 175 kBTU.
They're all fast recovery types these days, which means that they can keep up with most demand rates at higher efficiency. The main advantage to a large tank is the thermal mass, which means that you can use a higher efficiency burn, and the large size, which gives you more room to do your heat exchange.
Btw, BTU is a measure of energy, not power.
The Bosch 250SX tankless heater has an energy factor of 0.82, which I believe means that 82% of the energy is used to heat the water, and 18% goes up the vent.
Sure, and I can buy an aquamax storage gas heater than costs half the price and gives me an expected efficiency of 94%, including tank losses - I just rung the company, who also sell a rebadged bosch (rebodged basch?).
What sort of house heating are you using?
Yeah, ok. I'll look more closely. Extraordinary claims and all that. Perhaps 'standard' storage heaters don't have much insulation. Lets say we use 100L of hot water per day. That's 14MJ of heat. What insulation would we need to make the waste heat 34% of the total? 14*0.34/0.66 = 7MJ of heat waste each day through the insulation. The means 87W heat lost, over a 1.5m^2 area, or an R-value of 1.6. That's not an unreasonable insulation level (corresponding to say an inch of fibreglass), so perhaps it is plausible after all. That's also assuming a 100% efficient storage heater (reasonable - condensing units get something like 95%) and a 100% efficient tankless heater (less reasonable). A smaller tank, more water usage or more insulation would reduce this, in the limit you get a continuous flow system.
t igate831.html
We also haven't considered the benefit of efficiently heating the house whilst make hot water (cogeneration). If a unit is 94% efficient at heating the water, you can interpret that as 94% efficient at heating the house with the waste heat. I bet you spend more on heating than on hot water.
When our existing sytem dies we'll probably get a condensing continuous flow system (if they make them then) with solar preheat (hey, for $300 I think you'd be mad not to). Until then I'm going to stick with our 1 star gas unit with solar preheat, perhaps wrapping a spare roof bat around the outside.
The DOE site claims that 94% of the people who installed a solar system thought it a good investment, which runs contrary to your claim that they cause problems.
http://www.bchydro.com/business/investigate/inves
Has some interesting observations. It seems that a standard US hot water service is woefully inefficient, which is perhaps where the DOE gets their 'can be as much as' from.
Thanks for this interesting discussion - you made me check my assumptions and they were off a little.
Tankless designs have got a lot better in the last 10 years. I do like airing cupboards, very useful in damp places (I lived in scotland for a year).
Sorry, was there a mistake in my maths? Appeal to authority seems rather weak when I've given you numbers. Much better to point out where I went wrong. Of course the tankless manufacturers say their systems are more efficient. I just don't see the advantage in the numbers. The same sites seem to talk about the pilot light a lot, which is amusing because the pilot light only really wastes energy in tankless designs.
t m
I can't say the wikipedia entry is very compelling. It says 'can', which is a weasel word. Their point about losing lots of heat in the pipe of course applies to both technologies (but not to point of use systems), but there are two obvious points. firstly, where only hot water is useful (shower?), or you want to save water, you can use something like the envirosave:
http://www.abc.net.au/newinventors/txt/s1389495.h
secondly, if the pipes are inside the insulation, the heat is not really wasted as it heats the house.
Find me some real numbers.
I have a 130L tank. It is heated to 50C. The water comes in at 15C and needs 19MJ to fill.
It is wrapped in R3 insulation with a surface area of 1.5m so it loses 0.5W per delta deg C. Thus it loses 17W of heat when hot. That's 13 days worth of storage equivalent to 1 heating cycle.
So no, you're wrong, most of the energy is needed to heat the water. If I can preheat the water to just 30C (piece of cake, doesn't require any thing fancy) I save 40% of my gas bill straight away. My prototype, costing $100, heats 50L of water on a cloudy day to 80C quite comfortably which is about 70% of the energy used by the heater.
I thought very small things like that were done with photo resists and electrodeposition?
Spot on. If only the rest of humanity would work this out...
Where do you get your $50 less figure from? 95% or more of the cost of DHW is heating the water from ambient. I can't see how removing the tank can change this.
They bother because tankless heaters take up less space and never run out of hot water. Until recently tankless heaters were woefully inefficient. Now they're nearly as good as a condensing storage heater (storage is the standard term here for tanked DHW services).
"but that's assuming natural gas prices remain the same as they are now"
And that's the big question nobody knows the answer too. I suspect they will stay pretty much the same (inflation adjusted), as there is apparently plenty of gas. I'm certianly tempted by the tankless designs, as far as getting a quote (about $1100 for a decent one).
Where do you get your $50/pa less on natural gas figure from? Storage and instant heat systems are about the same efficiency, with storage coming out slightly ahead. Or are you comparing to your existing system?
Ok, wiring in the walls is nice. Did you look at the wiretrax idea?
:)
An old (say 50years) used house is probably a better deal, at least in australia, where houses built in the 80s are falling apart.
Do you live in a hot climate or a cold climate? pumping the waste heat outside really only makes sense if you spend more each year on cooling than heating. clothes dryer vents are too small to be efficient for moving heat. A better solution would be to just return the air to the room in winter (using a big fan) and to the attic in summer. An AC seems way over kill. I live in a climate where it regularly reaches 45C (110F?), and my computer works fine under those conditions with just a $10 Lowes type box fan parked in the side
For sound insulation it is hard to beat mass. Use those hollow concrete bricks (cinder blocks?), perhaps filled with dry sand.
I wish I had a basement.
Of those issues the installation/capital cost is the killer (and the reason we don't have solar hot water heating). You don't seem to have done the sums? An interesting question is 'how cheap would a complete package have to be before it makes sense?'.
I'm working on a $300AU ($200US) retrofit system using evacuated tube collectors and off the shelf parts (and keeping the existing gas storage heater tank). So far it seems quite promising, producing about 90% of the total water heating energy we need. $300 is the differential cost between a 1 star and a 6 star (some arbitrary efficiency units provided for all domestic appliances), and my system (which isn't very much my, and very much cheap chinese evacuated tubes) saves about 8 times as much energy as the difference.
Would you consider that? At $300, with our water heating bill, you could replace the unit every year and still save money. How much do you spend on water heating each year?
Most of the commercial systems sold today are complete rip-offs, but a friend noted a complete 20 tube system, including a stainless tank, for $695 US. My design is only 5 tubes and thus wouldn't have enough bandwidth for a family.
Most of my space is the result of a money/space tradeoff and an over-developed sense of self-reliance. I have tons of tools so I can do things myself instead of paying someone to do them for me. The other big problem is that I'm involved in so many different things, each of which take up some space (e.g. piano + drum kit = 120 sq. ft.).
Know the problem well! (including the piano)
Fair enough! What sorts of problems are these though? I know a number of people with solar DHW and they are all very pleased with it (one recently replaced his 25 year old system when the panel finally corroded through. I'm interested to know what problems you've seen.
Wouldn't Polish come under the category of Localisation?