Most efficient water heater?

[Bubba]

AO Smith has the Vertex water heater that gets 90% Weil-McLain has a boiler that does 98% at low temp Bubba

[ransley]

92-93-94 even 98% water heaters are common even 96% boilers, even a 94% tankless. AO Smith Cyclone tank, Takagi tankless and a Canadian firm makes a 98% commercial hw boiler, 5 years ago I installed at my apt a 92% 1900000 btu AO Smith Cyclone. these are all condensing units,

[hallerb]

on a tankless feeding a regular tank, it should cost no more to operate than a regular hot water tank.

the tankless initially heats the water to whatever it can, then sends the water to a regular tank that does its normal job.

endless hot water regular tank conveniences and the only extra cost is the line between the tankless and regular tank, ideally it should be short and well insulated.

true the tank will have normal tank losses.

today i have to stop at home depot and while i am there price some hot water tanks. just to verify some of these issues:)

[trader4]

Maybe not, but is sure costs a lot more in terms of buying and installing 2 water heaters, one of which is tankless and more expensive. With this approach, you incur the higher cost of tankless and by having the second regular tank, you still have the standby losses, which defeat most of the advantage of the tankless that justify it's expense. I fail to see the point. Plenty of folks have a gas tankless for their whole house needs and are happy with it.

[ransley]

There is no point to this approach, its backwards and will loose you all the savings you just paid for. If the tankless and tank are 82% efficent you are heating with one 82% burner and keeping it warm with another 82% burner. You are heating with the tankless and allowing it to cool in the tank, at about a 20% reduction in efficency rating. Most of what you just paid for in increased efficency goes up the center of the tank and out the chimney. The tank if hooked up should before the tankless and only hold water unheated to allow it to warm up by the surounding air to temper it, it works for me. Even better is to strip of the insulation on the tank, your basement will always be warmer then the incomming water main.

[ransley]

For pricing a tank look at a cheap uninsulated well tank.

[Blattus Slafaly]

If you're gonna spend lots of money on a water tank you might as well get an oil fired demand water heater. Gives you unlimited hot water and no cost to maintain a tank of hot water. If you want to put a tempering tank in your hot attic save even more.

[Wayne Whitney]

A much better way to do this is to get an electric tank water heater, remove the heating elements, and wire the thermostat to run a pump on a loop to the tankless heater. Incoming cold and outgoing hot are from the tank iteslf.

This way, the standby losses are that of an electric tank, which is less than a gas tank due to the lack of a flue down the middle. An advantage over tankless only is that the delivered hot water pressure is higher, because the pressure drop from a tank is noticeably less than from a tankless.

When I get around to installing solar hot water, this is probably the way I'm going to go; the solar can be on another loop from the tank.

Cheers, Wayne

[Don Wiss]

Uh, where am I going to get oil?

Don (e-mail link at home page bottom).

[ransley]

No point to having a tank circulate or change anything as that since you are still talking same burner efficencies, this is dumb.

[hallerb]

my tankless add on was only for use when familiy is visiting, the remainder of the time my 50 gallon high btu tank is fine.

now 7 people pile in here, and it can become a problem espically when incoming water temperature is 40 degrees

[ransley]

Its alot of money to put in a tankless and not get the savings year around, first you need to get the supply tested with all other gas apliances running to be sure no upgrade is neded. Do 2 people shower now at the same time, I dont think you will benefit having a tankless before a tank and it will actualy cost more to run since both units burners are probably near in efficency, I put my tankless after my tank with bypass valves incase my old tank leaks, but i havnt used it since installing the tankless, the cheap Bosch.

[hallerb]

its more of a idle thought, the minor standby losses of a regular tank dont bother me, and our tank is plenty big enough, except when family visits. with washing clothes, doing laundry and showering its a busy hot water using place. and our showers have the flow restrictors removed.....

but a new kitchen dining room gut job is a lot more likely and probably better of use of money:) kinda nervous the economy may hurt our income:( and gasoline is killing my service business

[ransley]

With shower restrictors removed you have to be real carefull, and measure shower output since tankless are real specific on Gpm and the amount of temp rise, [ on the coldest day, when gas pressure is low and everything gas is on, water is 38f incomming] you still need a hot shower. For many a cheap unit would not work, but I guess that really depends on your incomming mains gpm.

[Donna Ohl]

I think the reason for this is somewhat misleading.

If I understand this correctly, almost all the heat energy put into an electric heater gets put into the water. Basically, the water cools the heater coils down by taking the heat off the heater coil.

In the case of a gas water heater, the water cools down the flame by taking heat off the flame (figuratively speaking) but a LOT of heat goes up the flue.

They baffle the flue to slow down the rising air but they have to let the hot air out. If they cooled the hot air to room temperature, it wouldn't rise and get out of the house and that would be a bad thing from the standpoint of carbon monoxide poisoning.

So, I think the fact that all none of the heat energy that went into the electric coils goes up any flue - it's all absorbed by the water - is what makes the electric water heater 98% efficiency.

But, as someone stated, I suspect the power generation is about 70% efficiency, so, the true efficiency of electric water heating must be vastly lower than 98% taking distribution into account.

But, how can we account for that true efficiency?

Donna

[Donna Ohl]

I should have mentioned that I searched for that PDF during my GAS water heater replacement.

That PDF only contained residential GAS water heater specifications (very many hundreds or even a thousand or more).

It did not have any residential ELECTRIC water heater efficiency ratings (some of which approach 98% due to the fact no heat goes up the flue; it's all absorbed by the water).

Donna

[ransley]

Electric are all 100% efficent, all energy consumed is used to heat water, and energy factor should be near 100 as well with great insulation. Almost all gas water heaters burners are about 80-83% efficent, but an additional 17-20% goes up the chimney 24 hrs a day, Energy Factor ratings account for loss up the center uninsulated flue part of the tank and reflect overall efficency, which for most gas tank is 50-60 with one I saw of 70. Condensing gas water heaters, Boilers, furnaces, are different, have a second exchanger that lowers flue temp to near room temp and are forced out the flue by a fan. A condensing 93% water heater wont loose 20% in flue loss since the fan stopped some of the heat loss, but even the best condensing tank water heater of 93% may only be 83% Energy Factor [I guess]. Condensing tank water heaters are really commercial units costing thousands. AO Smith has them, I own one a 175000 btu unit, a Cyclone. For most, electrics are and always will be more expensive to run unless you have a cheaper Hydro Dam nearby, since for most oil- gas products generate electricity. Someone stated 70% for electric, that is not true to you for what you consume and pay, he was talking about transmission line loss, for you electric tank is 100% efficient, but here electricity is still 30% more than NG. If nobody in your neighboorhood has an electric furnace then you can bet Ng is still cheaper per Btu. Now in the last 6 months all petroleum products are going up fast, but electric will follow in the long run.

[CWatters]

Correct.

so, the true efficiency of electric water heating must be

Average power generation isn't anything like 70% efficient. Typical efficiency for a coal fired station is 30-40% with only the latest generation achieving 60%+. Gas fired around 47% and nuclear around 38%. Then another 5-6% is lost in transmission. The average depends on what mix your country has but I can't see it being much above 40-50% overall by the time it reaches your house.

This web site compares the cost of different fuel sources in the UK. It's the only site I've seen that takes into account boiler efficiency. The key figure is the middle one "Pence per kWh after boiler efficiency". The actual boiler efficiency is in brackets...

Note that electric heating is indeed 100% efficient but the cost of that electricity makes it expensive to run.

Heat Pumps have efficiencies of over 100% and in the case of a ground source heat pump (GSHP) around 350%. This more than compensates for the loss of efficiency producing the electricity needed to power. Overall a GSHP is the cheapest system to run (ignoring capital costs). It would be interesting to know if anyone makes a small scale gas or oil powered GSHP and how the efficiency of those compare.

In theory it would _just_ be possible to use the heat from a GSHP to power a Stirling engine to power the GSHP. This would not violate COE because there is a heat source (the sun) providing power into the system. However most of the heat produced by the GSHP would go into the stirling engine with very little left over to heat your house. Stirling engines that big would also be rather big physically. Overall such a system would be too big and expensive to be practical - but it would be free to run.

[CWatters]

Actually it's 100% efficient. I mean your reasoning is correct.

[hallerb]

you can get too would up over efficency ratings, nothing is 100% even electric loses a little to the room.

and one must be aware that cost to buy can exceed savings on whatever your trying to be more efficent with