Indirect Water Heater Efficiency

Inefficiencies

Heres one for ya- How about the stand by losses of putting a 3" or 4" steel pipe through the center of your stored water and allowing air to vent through the roof 24 hrs. a day. Seems a little inefficient to me. Kinda like insulating your wall and leaving your window open. A high efficient low mass boiler comes up to temp very fast. With insulated connecting pipe I have to conclude that even the worst indirect is hands down better than the best standard direct fired water heater. Is there really any question?

instantaneous hot water

this could be the answer if you need a lil hot water. i will stay with my indirect thankyou

Ummmm...

It's hard to separate all your points, but I'll try.

Re Heat exchangers: The water-water heat exchanger does not have to be 100% efficient. It simply has to be much more efficient than any other means of heat-loss. Then you simply run the water through it a couple of times and most, if not all, the heat transfer that could have taken place, will have. BTW, I once held a 99.98% HX in my hands... built for a NASA/DoD space application, NEAT!

Next, you ought to consider your analogies a bit more carefully. For example, your kettle and tube example could hold true, but not necessarily. A condensing boiler will have its water up to temperature with 90-98% thermal efficiency. Assuming a 90%+ HX transfer, and you have about 90% thermal efficiency at the tap (assuming 120°F water). Now, how many kettles do you know that run cold enough to cause all of the gasses impinging on them to condense continuously?

Granted, at very low water quantities, using any kind of tank appliance makes no sense. This is where tankless units make the most sense... but try comparing the amount of gas you'd have to go through to fill a bathtub with warm water from a kettle vs. the consumption of your water heater. I can virtually guarantee that the water heater will come out ahead due to its much larger HX and the fact that the bathtub won't go cold by the time your next shot of hot water from the kettle is ready.

This brings us to the next critical juncture, AFUE. Your standard gas water heater will have an AFUE of about 50-65%. The minimum allowable standard for boilers is 80%. Thus, you'd have to hook up a very inefficient IDWH to not come out ahead. Highly efficienct gas water heaters may make it into the 90%+ AFUE range as well, but why have two very efficient heating appliances when one can do the job of two perfectly well?

So how much energy do you lose by hooking up a IDWH to a boiler in a given year? Like many things in life, it all depends.

• A low-mass, condensing boiler won't care one bit to briefly come on to keep the IDWH at temperature, then quickly cool down again. This minimizes stack losses and maximizes the efficiency of turning potential chemical energy into useable heat.
• On the other hand, a old-school, high-mass system will burn through a lot of fuel before the boiler, IDWH are satisified, then continue to experience stack and standby losses for a long time afterwards. Some manufacturers like Viessmann seem to have developed systems with very low stack and standby losses despite falling into the high-mass category.
• Don't get me started on electric water heaters. Yes, they are 100% efficient at turning electricity into heat. But first, some resource is 30-60% wasted before being turned into electricity. Otherwise, you wouldn't see that the operational costs of electric units are much higher than even low-efficiency gas units.

So can I verify the claims of others? No, but utilizing the very laws of thermodynamics you cited earlier, I have come to the conclusion that my IDWH ought to be very efficient once it has been installed. That efficiency will be ratcheted up even further by the VitoSol solar water heater on the roof.

sounds like trouble

What would you do? How would you make hot water? Any flame under a tank of water will not last as long as indirects in what I have seen. The extreeeeeeeem heat from the flame burns out the bottom of SOME tanks. soooooo...... how would you do it.......??????efficency ..........I have to look at the big picture, I do not want to replace this in ten years. I want to clean it and get another 20yrs. If I replace it in 5 or 10 years, no savings in eff. will pay for the unit. The indirects I sell only lose 1/4 degree every hour and can stay warm for about 10 hours. Every system is differant and you can NOT say how much you WILL save(with out all the info and alot of time).....but we try to estimate.... EFFICENCY is only important if the unit runs long enough to get the savings....... Most indirects are built to last and made to store any energy generated by the boiler, not waste it.....Heaters with flames under the tanks are good for places you need really fast recovery and can work well and save if done right........

my take on this

RS,

Here the thinking that led me to where I have ended up on this, which is a boiler and well insulated indirect. (Mine happens to be a dual coil with solar, but the decision would have been the same without that.)

The first is your regular old water heater. It has a 80% efficiency burner, and as Troy pointed out, it has horrid standby loss because of the flue running right through the tank. Hands down loser, the one everyone likes to compare against. No need to discuss it further.

Second you have a boiler and an indirect. The HX efficeincy is quite good, as you loop it through the boiler rather than using a one pass system. The standby loss is better than the standard, but it still exists. You have pipe connections slowly pulling heat out and I doubt that accounted for in the manufacturers numbers, so I don't have any way to give exact numbers. My Stiebel-Eltron tank has 3" of polyurethane, so it's quite well insulated and a larger tank increses the volume to surface ratio (a good thing.) More on condensing boilers and indirects below.

Third, you have the instant gas heater. This unit has no standby loss, but none are available with condensing HX (they have them in Japan...) So the question is do you gain more from having no standby loss or from having a more efficient burner/HX. You have to estimate the tank temperature and water use to see where the crossover is. At low use, the demand will win, as use goes up the boiler will win. If you could get a condensing demand gas unit, it would be the hands down efficiency winner. You might even get the exhaust air out cooler than the combustion mix went in.

The problem with condensing boilers is that many of the boilers are set up for maximum recovery rate rather than maximum efficiency. You would really want to have the supply water be just hot enough to get the water to the temperature you want and get the return from the inderect below 130F. I don't know how all of them do it, but at least some don't give you this choice.

The rub for me and why I didn't go the demand route was the finicky function on the demand units. Everyone talks about the fact that there's a max flow rate, and that you can often go below the listed flow rate in the winter. Also, if you go below some lower flow rate, the unit won't heat at all. The burner has a limited modulation range, and when you go below the bottom it has to turn off. This is often in the 1 GPM range. So if I'm washing things in the sink and I turn on the water at under half of the full flow pure hot water rate (facuets are 2GPM max) you get no hot water at all. With the tank, you get hot water from a drip to a flood (till the tank runs out.)

jerry

Bump

For Steve Rockwell, a customer who likes to be able to make an informed decision.

Enjoy

And Ask Too Many Obscure Questions

Thanks Steve.