Efficiency of an indirect water heater

I think you eventually get good transfer but that is because you start out with poor transfer and the tank temp drops so the approach gets much wider and the transfer gets better. I've thought about either pre-firing the boiler on flow to the shower or adding a brazed plate hx before the tank at the inlet and in the return to the boiler.

AI does have it wrong. The indirect efficiency is directly related to the efficiency of the heat source. Subtract a factor based on the efficiency of the transfer process including the surface area of the heat exchanging surfaces and any loss thru the insulated jacket to the ambient air. But to compare a tankless coil to an indirect is like comparing apples to pineapples. They both are apples but they are completely different fruits. And there are no pine cones involved.

From experience I can tell you that an oil fired boiler with a tankless coil that is maintaining 160° in the off season will operate the oil burner for a few minutes every 4 hours as a result of the vent connection to the chimney. That same oil fired boiler will fire every 20 hours for a few minutes to recover the loss of temperature in the tank. That is assuming no one uses any hot water for that 20 hour period. (like when you are on vacation). I had a timer on my oil burner just to test this theory. It is not a theory anymore. it really happened with my Buderus indirect and my Buderus boiler with Riello burner over 30 years ago.

That is why I recommend Indirect water tanks over tankless coils and tankless wall hung water heaters. You need to live like a European when you use a tankless water heater. Us Americans like to have as much hot water as we can get. 

Tankless coils are ok for two people but in this day and age it doesn't make sense to me to keep a hot boiler 24/7.

As far as indirect go I have an electric HWH. I have been on vacation for up to two weeks and come home to still have luke warm water after 2 week and mildly hot water if gone for 1 week and I shut it down when I leave. I assume indirects are insulated as good as a EWH.

So I think indirects are a good choice.

Tankless water heater seem nothing but trouble from liming up the HX due to high temp heat transfer and flow issues I know a lot of people who put them in and have ripped them out.

HPWH are expensive to purchase and who can fix one if they bite the dust? A homeowner certainly isn't going to invest in the refrigerant tools needed for repairs and neither are the plumbers who install them. Many have given up on them as they just turn into an EWH. They are an expensive throw away appliance. Even if you get a rebate you wont get a second rebate

nice graph @hot_rod

I’d advise everyone to watch that acceleration curve in the years ahead

@hot_rod Could you please comment on this efficiency comparison chart from Energy Kinetics? It's for different boiler types that also heat DHW. I don't doubt its correctness, but I wonder what assumptions are made for the conditions under which the non-EK boilers are running. Are these truly typical efficiency numbers for cast iron boilers with indirect or tankless coils?

We have a natural gas fired water heater with a typical draft hood and standby losses up the flue, so we're probably 70% or less efficient there. I would love eventually to go to an Energy Kinetics System 2000 with an indirect tank that would (a) eliminate our existing 2 large cast iron boilers and (b) improve our hot water efficiency. I know Roger from EK has commented here before that the EK efficiency comes from (1) low thermal mass, (2) low water volume in boiler, and (3) post purge.

Our combined system efficiency for DHW and cast iron boilers is probably around 70-75% now, and EK advertises 85% combined system efficiency for the System 2000 with DHW, so in theory we could probably improve our efficiency 10-15% by going to a System 2000 at 85% system efficiency.

HPWH is 2% of the market. Not popular and not perfected yet. When they are done experimenting and start to sell more the price may come down

How the heck did this turn into sexist comments??? Hopefully somebody is no longer a member here if that's how they behave on a public forum.

The actual efficiency of a system, can be determined by measuring the actual heat losses of its components.

Attached is a chart showing the efficiency of an actual 80 gallon Direct Fired DHW Tank with the total heat losses broken down into three major categories. In this system, only 47% of the Total Heat goes into heating DHW for use. The rest of the Heat goes into Standby Heat Losses, Recirculation Heat Losses and Flue Heat Losses.

Timers were used to collect the data on energy use for Standby and Recirculation Heat Losses during a fixed period at night when there was no actual DHW use.

This is the actual data on an old Direct DHW Tank system, just before it was converted to a stainless-steel Smart Indirect Tank fired by a ModCon boiler with much higher thermal -efficiency.

Indirects also have much lower Standby Heat losses.

You say the measurements were taken during night when no DHW was being used. Does that mean your system had hot water recirculation for instant hot water at each tap? For systems like mine with no recirculation, does that mean our recirculation loss would be zero? I assume there's some heat loss anyway in hot water flowing to each tap even without recirculation, but maybe that's hard to quantify.

Hi jesmed1 -

Yes - Your heat loss without recirculation would be zero. The supply piping cools down and the heat losses drop to zero.

In large systems with recirculation, the heat losses due to recirculation can be significant, because it keeps both the Supply and Recirculation piping hot 24 / 7.

hot_rod - when it was converted to indirect was the piping, including the recirc line done then also?

No - The original system was an 80 gallon Direct Fired Tank with Recirculation - that is what you are looking at.

The 80 gallon Direct Fired Tank was replaced by a 120 gallon Smart Indirect which greatly reduced Flue Losses and Standby Losses.

The Recirculation system was converted from flow-control to temperature-control using CircuitSolvers from ThermOmegaTech primarily to prevent pin-hole water leaks. The recirculation pump was reduced from 10 GPM to 2 GPM at this time. Better control reduced recirculation heat losses.

hot_rod - On a fired tank the loss up the flue depends on the draft I suppose., In addition to loss through the tank insulation.

On a direct fired tank, unless you have a flue damper, which my example does not have, most of the standby heat loss is up the flue, rather than through the tank insulation. When the tank is not firing, heat losses are still drafting up the flue at a great rate.

I'll chime in here - much of the questions can be answered with the BNL study as it published "summer domestic hot water efficiency" results for indirect tank systems with cast iron (38.3% with 84 AFUE to 68.3% with 89 AFUE) and condensing boilers (58.7% with 95 AFUE), tankless coil boilers (31.0% with 84 AFUE to 40.6% with 83.9 AFUE), direct fired water heaters used for heating and hot water (57.1% with EF 68 and 57.5% with EF 59), and of course a non-condensing low mass boiler with thermal purge and external plate heat exchanger (74.9% with 87.5 AFUE). There was also an imported oil combi boiler tested (47.9% with 82.8 AFUE); this should not be compared to low mass condensing gas combis.

A couple of comments:

1. Low mass with thermal purge and a plate heat exchanger has high efficiency when the burner is running, and virtually eliminates standby losses with a very effective thermal purge. The low mass with thermal purge to a storage tank is even more efficient for condensing boilers as it pulls cold makeup water from the bottom of the tank and can condense through nearly the entire hot water tank heating cycle. With the oversized plate heat exchangers Energy Kinetics uses, the boiler can maintain return temperatures as low as 80°F, but typically they are set at 110°F or lower for exceptional condensing and performance. As AFUE is tested near 120°F return temperatures, that means the boiler efficiency in this mode is even higher than shown with AFUE. Thermal purge means that very little energy is left wasted at the end of the hot water cycle (it can purge against a reservoir of cold water at the bottom of the tank to heat the water from energy left in the boiler after the burner turns off).
2. Indirect tanks have coils that heat up as the tank water heats, so the return water temperature to the boiler increases. This really hurts the efficiency of condensing boilers, and moderately hurts the efficiency of non-condensing boilers. Higher mass means more heat wasted in the boiler when the burner turns off; heavy insulation can help.
3. The best efficiency of a storage tank (no open flue) and accounting for standby losses is around 95%. This is probably well represented by an electric tank UEF; these are between 0.92 and 0.95, so 5% to 8% standby losses.
4. Tankless coil boilers are very inefficient, and have poor hot water delivery, we can do better and always should.
5. Chimney and vent losses occur in the off cycle because boilers without thermal purge finish hot. With thermal purge, these losses are also nearly eliminated. The hot water purge cycle only needs about 5 minutes to capture the energy left in the boiler, then there's no more energy to waste up the chimney; no need for draft dampers, etc.

Best,

Roger

Thanks, Roger. I always learn from your informative posts. Someday I hope to be fortunate enough to own an Energy Kinetics boiler!