Tankless DHW over 100% Efficiency?

AJinCT

So the other day I was thinking about a new Navien tankless unit my friend got, and I was wondering... In the dog days of August, could a gas condensing tankless DHW heater go over 100% efficiency? If the intake air is 95F, and the water it's heating is coming in at 55F, could the exhaust drop below 95F on a 199k unit running at 20k or a 150k unit running at 15k? Maybe the heat exchanger isn't actually big enough to do that, but isn't it theoretically possible that a unit with a large enough heat exchanger could go a fraction of a percent over 100% efficiency? Or am I missing something?

unclejohn

Maybe by the AFUE but not in real life. We all have to give up something to get some thing.

Jamie Hall

Yup. Second law of thermodynamics. The energy or work out is always less than the energy in...

What you are missing is that efficiency is -- or should be --- properly measured by taking the energy content of the fuel used and dividing that into the energy content of the useful output. Efficiency on things such as boilers and water heaters is almost always actually determined indirectly, rather than by direct comparison of the energy input and useful heat output. This can give rise to some very strange numbers indeed...

In the example you cite, if you calculated the efficiency and neglected the heat input from the warm intake air, you could indeed get apparent efficiency numbers over 100%. In fact, if you didn't really need your output water temperature to be greater than the air temperature, you could happily get an infinite efficiency!

njtommy

I believe it's only that efficient on low fire. So if you preheat your hot water with solar you could be up to those numbers.

I only use about 5 therms a month for hot water in the summer time and some light cooking. My service charges are more then my NG charges.
That's with a Navien combi boiler

Zman

AJinCT said:

I know AFUE is completely hosed up as an efficiency measure, due to losses in the boiler to the unconditioned space and all of that.

And yes, the Second Law determine that you can't have efficiency over 100% from a combustion efficiency perspective alone.

But in the case of a tankless water heater, it would effectively be sucking heat out of the outside air in the process of heating it, which I'm thinking could get the efficiency based on fuel in and heat out slightly over 100% in exactly the right conditions...

Yes, true, that would just be a giant radiator in the air at that point. I'm cooking the books the same way that you say a heat pump might be 380% efficient, or an HPWH is 285% efficient by not counting heat from the air, since it is not purchased fuel like electricity, NG, or LPG.

You are correct that a heat pump water heater that gets it's energy from the indoor air is deceiving.
A heat pump that get's it's energy from any source outside the heating envelope is a real number.
I always feel like the true energy used needs to reflect what it takes to get it there. Electricity is one of the worse with approx 70% generation and transmission losses.
All fuels have some cost associated with processing and transportation.

bmwpowere36m3

You cannot get 100% efficiency... nor 100%+

Simple physics.