Condensing in flue for efficiency?

Brad_9

I’m curious if increasing the vent length on my Rinnai water heater will increase efficiency by condensing the exhaust so that it gives its latent heat to the incoming combustion air. They even suggest and supply a condensate drain on the vent if it’s over a certain length. (The fact that I’m using a Rinnai for heating is another story, wouldn’t do it again, though it’s ok so far)

I’m assuming that in a condensing boiler that the exhaust is brought down to condensing temp by the return fluid, if it’s cool enough, by long enough contact with a heat exchanger. Thus giving up its latent heat to the return fluid on its way to being heated again. I’m also assuming that in non-condensing heaters, like a tankless water heater, that the exhaust does not condense because of the short time the exhaust is in contact with the heat exchanger. Am I correct in these statements? If so then why is vent condensing not used as another mode of condensing heater, or is it?

Will condensing exhaust giving up its heat to the incoming combustion air increase efficiency as much as giving up that heat to the return heating fluid? Does the surface area of the vent need to be very large to be effective? It seems to me that incoming air would likely be colder than return fluid in all cases. Also that transferring that heat to either the return fluid or incoming combustion air would increase efficiency, but by different amounts?

Any clarity on this would be appreciated.
Thanks,
Brad

Mark_7

Rinnai

spec. max. height of 5' without condensate collector installed with a total of 21' max, vent height were condensat collector is required. condensate collector not required for horizontal venting right of unit, grade to out side. units are great for dom.hw hven't tyied any for hydronic heat yet.

Kevin_16

Tankless Water Heater

Hello,
You propose a good question about the condensation however the fact that Rinnia has a concentric vent where the intake air is on the outer diameter and the exhaust is on the inner diameter only hinders their product. This is because the exhaust is going to cool down much faster because it will be releasing it's heat to the intake air and you run the chance of destroying the heat exchanger. If you are looking for a really good fresh air intake tankless heater you should really try the Takagi T-KD20. This unit works great and costs less then the Rinnia.

prm_5

Heat exchanger damage

If the Rinnia has is designed to condense, why would condensation in the exhaust duct matter? I would think that heating the incoming air would boost efficiency a bit.

Wardner

Cooler air when mixed with fuel produces more heat.


Why do some internal combustion engines have intercoolers?

Why is intake air on vehicles drawn from front of vehicle?

What is the function of hood scoops?

Why do dragster crews pack ice around the induction systems? (not sure about current practice, but they used to.)

The aim here is to produce denser air which has more oxegen molecules per cubic foot than lighter (read warmer) air. It seems to me that preheated air on a closed combustion boiler would hurt efficiency. Trinity is another mfg that does not use coaxial intake/exhaust although I believe they sell an optional adaptor.

scrook_2

but...

the auto engine is an INTERNAL combustion engine, so you want the incoming air/fuel mix as dense as possible aso you can get as much mass in each cylinder as possible to get as much power (energy) not as much efficiency, as possible. If you wanted to increase th eefficiency you would (among other things) cool the exhaust gas as much as possible and use that energy to propel the car -- though that isn't practical, the exhaust isnt hot enough to recover energy from easily, and you'd have to confvert the head energy to mechanical energy.

The boiler is an EXTERNAL combustion engine and the dersired output is heat not mechanical or other forms of energy, so to maximise efficiency you want the exhaust gasses as cool as possible, and you want to return as much of that heat as possible to the boiler where you want to transsfer as much as possible to the water that is circulated to the radiation. If you wanted to get as much heat out it as possible without making it bigger, then you'd want to increase the density of the air/fuil going in by cooling it and (in the interest of efficiency) sending that head into the living space (or to whatever was to be heated). Think of the flue pipe as becoming a small hot air furnace to supplement the boiler's hot water.

Kal Row

there diff on as far as power - when it comes to emissions

they are a lot alike - you want as cool and as slow a flame front at the stocimetric point as possible, so that all the fuel (hydrocarbons hc’s) are converted into H2O and CO2,
unfortunately, you need to run with a drop excess air to guarantee, that no CO (carbon monoxide) is formed, do to lack of O2, – you always get h2o, but what connects with the carbon atoms of the hydrocarbon, depends on air to fuel ratio, if he mixture is too rich ie: if your gas pressure is to high, or the jets are bad, or there simply is not enough air due to flue restriction or no fresh air supply, then you will get deadly CO instead of CO2 and if there even less air then the carbon just falls out (not having any o2’s to connect with), and coats everything

so you set the output for 1-2% O2 left over to guarantee that no Co is formed – and if you lose a little heat – tough

I waiting for them to put in 02 sensors in the flue and set the mixture on the fly – the auto world has brought them down to 30-50 retail for a heated typo (which a boiler would need) and they tell you instantly how much free o2 you have in the exhaust – trouble is, an 02 sensor is a zirconium/platinum battery, that uses 600f plus air as it’s electrolyte, and releases 200-900 millivolts depending on the O2 content, and when the zirconium is used up, it’s dead just like any other battery, so it’s a continuing maintenance item – (like every three years) something people hate

jim sokolovic

It's about time...

someone separated fuel burning appliances from cars. Everyone seems to want to make analogies between the two, only because cars are exiting, glamorous, and we all interact with them closely. In terms of delivering power for motion out of the energy available in the fuel, what is the average car's efficiency, 40 %? The auto industry could only wish they could compare their product's performance to high efficiency heating products when they want to boast about performance!

Lurker_2

what...

you thought turbos run on wind? tis the heat dat makes da power

Kal Row

it's only because

1/3rd energy goes out the exhaust, 1/3rd goes into the water, and 6% goes into mechanical losses, and the combustion dynamics are much faster, severe and variable -
yet they are doing remarkably well under the circumstances

we could learn and utilize a lot of the technology to bring us up much higher than we are today –

fully modulating with active mixture control and condensing boilers is a step in that direction

it always boils down to money – the rising gas prices are limiting a boilers real economic life today to ten years instead of 30

we haven’t even touched combination energy extractors –
like viessmann radiant dome grid, large thermocouple grids in the flue outlet that would allow us to take the lower heat from the flue and concentrate it into one higher temp heating element on a colder day when return temps are hotter than a fully condensing flue, or use air heat extraction from flue on colder days

Jack_23

Rinnai/venting/condensation

The EF on the Rinnai Continuum is .82 for NG and .87 for lp. If you run a longer vent you will certianly condense as the incoming air will continue to cool the flue gases. I represent Rinnai and I always recommend a short vent, if possible. As with Cat III appliances which require special vent material the DV pipe for the Rinnai adds up $ in a hurry. I have to think that the very small increase in efficiency is just not worth the added cost of the vent. IMHO. Another thing on the Rinnai's is that you absolutely want to grade the vent to the outside on the horiz installs. If there is any question of condensate getting back to the unit you definitely want to protect the equipment with the condensate collector. As a general rule of thumb in the business happiness side of things, if you want a condensing piece of equipment, go ahead and buy one. Do not try to create one. Why push it? It is and excellent piece of equipment when properly applied. Happy Heating!

jerry scharf

they are different animals

You betcha. Burning fuel where heat is the main goal and weight is not a significant issue is one thing. Burning fuel to create mechanical force and maintain low weight is a whole different thing.

There are certainly components that are built for the automotive world that can be applied to boiler operation, but that doesn't make them comparable in any way.

If there was even a few percent of the money that is invested in automotive engine R&D available to boiler systems, my bet is that the changes would be breathtaking. As one data point, Toyota sunk over a billion dollars of R&D into hybrid engines alone. I bet any boiler manufacturer's R&D department would kill to have that kind of resources.

Or imagine if the auto industry started using 1/4hp ECM motors for something. Things change when the minimum order is 100,000 units.

jerry

Jack_23

Actually...

a great turbo reference, as long as we are talking about cars is Corky Bell's "Maximum Boost". I'm rebuilding a 75 BMW 2002 Turbo now. Should be on the road in early May. it is actually the velocity/vol. of exhaust gases which powers the turbo (gets the turbo spinning). the heat is an unavoidable byproduct of the combustion process, but is not the power source. Mr Bells book is outstanding in that it tells the turbo story in a way which chowderheads like me can understand!