Making hot water without a heat exchanger by routing combustion gas directly through water?

cowdog

Boiler heat exchanger is a frequent point of failure of hot water hydronic heating systems.

Can we avoid using a combustion-water heat exchanger by routing combustion gas through water, making direct contact with water?

Exhaust could be a little acidic from sulfur in fuel, we should add baking soda into the recirculating water to avoid corrosion to metal.

For efficiency we can also add a small heat pump (about the same cost with a portable AC unit) to condense vapor in the humid combustion gas after passing through water.

retiredguy

Before I retired there was a company that did just that but the heated water was not for domestic use but was mostly used to heat water for use in a concrete plant. Could this be done for household domestic usage, I am not sure, it would take someone smarter than me to make that determination.

ratio

I'm sure you'd transfer heat to the water. I'm also sure you'd transfer all kinds of yummy chemical compounds to it as well. I wouldn't want to drink that water. Or wash with it. Or touch it. No matter how much filtering you do to it.

pecmsg

cowdog said:

Boiler heat exchanger is a frequent point of failure of hot water hydronic heating systems. Can we avoid using a combustion-water heat exchanger by routing combustion gas through water, making direct contact with water? Exhaust could be a little acidic from sulfur in fuel, we should add baking soda into the recirculating water to avoid corrosion to metal. For efficiency we can also add a small heat pump (about the same cost with a portable AC unit) to condense vapor in the humid combustion gas after passing through water.

Condensed Vapor is extremely acadic!

Jamie Hall

Yes you would warm the water. And turn it into dilute nitric acid with a trace of suphuric -- pH probably around 2 or so, and capable of dissolving gold. It would also have any trace metals which were in the fuel (there always are some) or in the combustion air, and probably an interesting assortment of low level concentrations of various interesting organic compounds with scary names, some of which, like dioxins (a normal combustion product) scary properties as well.

But would it me more efficient?

Not if you were using a well-designed condensing water heater from the beginning...

Mad Dog_2

Joke?  I don't like the idea.  Too easy for Cross Contamination..Mad Dog 🐕 

HVACNUT

I can't even picture it in my head. A vertical or horizontal flame(s) going directly through (interacting with) the path of the water to a flue or exhaust? 
Is the water side under pressure? Actually, I guess there is no water side.
How would you direct the combustion gases to the exhaust? I imagine a high point, but not directly. That's a lot of mixing. 
And the combustion itself would need to be like welding under water, and hot enough to heat a vessel to 140° minimum. 
Even if it could be done, are there any benefits?

Mad Dog_2

Those fishes are gonna sleep 💤 with the fishes...in this environment.   Mad Dog

Larry Weingarten

Hi @cowdog , The dead guys did this many years ago. I have a water heater from 1895 that does this. Here's a cutaway of a similar one.
These were called contact heaters, where water and combustion products were mixed directly. They claimed to be 92% efficient. I assumed the acidic water cleaned better. You may note that venting wasn't a concern either

Yours, Larry

cowdog

Jamie Hall said:

Yes you would warm the water. And turn it into dilute nitric acid with a trace of suphuric -- pH probably around 2 or so.

Saturate the water with baking soda to absorb the acids. Use a device to change water once the pH is lower than 7.
We don't need clean water for room heating.

Jamie Hall said:

Not if you were using a well-designed condensing water heater from the beginning...

We need equipment affordable by the poorest Americans.

cowdog

HVACNUT said:

I can't even picture it in my head. A vertical or horizontal flame(s) going directly through (interacting with) the path of the water to a flue or exhaust? 
Is the water side under pressure? Actually, I guess there is no water side.
How would you direct the combustion gases to the exhaust? I imagine a high point, but not directly. That's a lot of mixing. 
And the combustion itself would need to be like welding under water, and hot enough to heat a vessel to 140° minimum. 
Even if it could be done, are there any benefits?

1. Water side cannot be under pressure. To heating radiators by circulation pump.
2. Simplicity is the benefit. Simplicity = affordability + low maintenance. High efficiency water heaters are expensive.

Steamhead

Larry Weingarten said:

Hi @cowdog , The dead guys did this many years ago. I have a water heater from 1895 that does this. Here's a cutaway of a similar one.
These were called contact heaters, where water and combustion products were mixed directly. They claimed to be 92% efficient. I assumed the acidic water cleaned better. You may note that venting wasn't a concern either

Yours, Larry

IIRC, Burnham tried this with their "Opus" gas boiler some years ago. Obviously we never see them now.

hot_rod

There are direct fired water heaters out in the world😉

https://www.maximizersystems.com/direct-fired-water-heaters

cowdog

hot_rod said:

There are direct fired water heaters out in the world😉

https://www.maximizersystems.com/direct-fired-water-heaters

Nice! I hope direct-fired oil and wood fuel heaters could reduce cost of these units.
Natural gas price has increased too much. Residents cannot afford the warmth. Wood chips are very cheap.

Jamie Hall

"We need equipment affordable by the poorest Americans."

That is a political question, not an engineering or scientific one. Go talk to the EPA or the Consumer Products Safety people.

cowdog

IIRC, Burnham tried this with their "Opus" gas boiler some years ago. Obviously we never see them now.

In this forum thread, looks like the "opus" cannot tolerate hard water. Don't know exactly why. Maybe there needs to be some filtration and flushing system.

cowdog

Jamie Hall said:

"We need equipment affordable by the poorest Americans."

That is a political question, not an engineering or scientific one. Go talk to the EPA or the Consumer Products Safety people.

Solution 1: subsidize them (we already do with LIHEAP and some EPA credits/rebates, but apparently not sufficient, many people still cannot afford)
Solution 2: make simpler and cheaper equipment that need fewer repairs. Personal computers went a long way on this one.

STEAM DOCTOR

@Erin Holohan Haskell @cowdog. Whether or not the government should subsidize anything, is a discussion for a different forum. Whether or not this particular idea is mechanically or otherwise feasible, is very much a discussion for this forum. 

Erin Holohan Haskell

STEAM DOCTOR said:

@Erin Holohan Haskell @cowdog. Whether or not the government should subsidize anything, is a discussion for a different forum. Whether or not this particular idea is mechanically or otherwise feasible, is very much a discussion for this forum. 

Yes. Thank you @STEAM DOCTOR.

Steamhead

cowdog said:

Steamhead said: "IIRC, Burnham tried this with their "Opus" gas boiler some years ago. Obviously we never see them now. "

In your link, looks like the "opus" cannot tolerate hard water. Don't know exactly why. Maybe there needs to be some filtration and flushing system.

I didn't include a link. The ones I had are long dead.

Larry Weingarten

Hi, I seem to remember that PVI made modern day contact heaters with a 99% efficiency. It was not clear by looking on their site if they still make these contact heaters. They seem to make bigger, commercial equipment.
Yours, Larry

jumper

hot_rod said:

There are direct fired water heaters out in the world😉

https://www.maximizersystems.com/direct-fired-water-heaters

One I saw had flame going down from water induced air flow. The was a heat exchanger down in the pan and directly heated water was recirculated. I forget how often recirculated water was changed. It was supposed to be super efficient at producing relatively low temperature (~100°) water.

Jamie Hall

Have to admit that that the beauty of direct contact approaches lies in having no heat transfer problems in a heat exchanger, plus they are inherently condensing systems, so the efficiency can and indeed sould be very high. The two downsides is making sure you are getting good complete combustion --which has to be done before contact with the water -- and then the water quality which results which is likely to need tweaking.

Matt_67

One of the concrete batch plants by us has a direct fired aggregate heater. www.vortomech.com It works well but I don’t think it would be scalable for residential use.

ethicalpaul

If a dead person once did it, it is obviously a good idea. Case closed!

EdTheHeaterMan

ethicalpaul said:

If a dead person once did it, it is obviously a good idea. Case closed!

Unless of course, the death was caused by the contaminated water!

This sounds like the beginning of a Marvel Comic story about AquaHulk

This ordinary man that one day inadvertently bathed in the thermally heated water from an experimental Maximizer water heating system, at the brink of death, suddenly transformed into the words first amphibious super-hero

cowdog

Jamie Hall said:

Have to admit that that the beauty of direct contact approaches lies in having no heat transfer problems in a heat exchanger, plus they are inherently condensing systems, so the efficiency can and indeed sould be very high. The two downsides is making sure you are getting good complete combustion --which has to be done before contact with the water -- and then the water quality which results which is likely to need tweaking.

It also lies in water, being a natural cleaner, is able to wash out air pollutants from flue.
Wood, coal and oil flue have much less pollutants released to air after sufficient contact with water. The resulting dirty water can be periodically discharged as sewage, which is then centrally processed in the city's wastewater treatment plant.

Jamie Hall

Oh boy. Well, you do have a point. However, I have to mention two things. First, of an overall basis, I'm not at all convinced that moving contamination from one place or medium to another is a very good idea, although humans have been doing it for millenia. Second, a municipal wastewater treatment facility is not set up for, nor does it do a very good job at, managing very low levels of what are basically industrial contaminants (I've designed a number of the years, and run a few myself).

So, not to put too fine a point on it, that's not, in my view, a very good justification for direct contact...

cowdog

Jamie Hall said:

Oh boy. Well, you do have a point. However, I have to mention two things. First, of an overall basis, I'm not at all convinced that moving contamination from one place or medium to another is a very good idea, although humans have been doing it for millenia. Second, a municipal wastewater treatment facility is not set up for, nor does it do a very good job at, managing very low levels of what are basically industrial contaminants (I've designed a number of the years, and run a few myself).

So, not to put too fine a point on it, that's not, in my view, a very good justification for direct contact...

You mean fly ash, soot and creosote cannot be neutralized by anaerobic and aerobic digestion of sludge?

I think

• fly ash from wood burning are mostly minerals naturally contained in soil, since wood grows from soil.
  
• soot is basically carbon, once it attach to soil to become the "peat" part of soil.
  
• Creosote is naturally produced by wildfires, since you don't see a natural forest full of creosote, it can be degraded in natural environment.
  

WMno57

https://en.wikipedia.org/wiki/Polycyclic_aromatic_hydrocarbon#Human_health

WMno57

Here are some Polycyclic Aromatic Hydrocarbons that have not broken down or degraded in 65 million years.
https://en.wikipedia.org/wiki/Polycyclic_aromatic_hydrocarbon#Natural_fires
"High levels of such PAHs have been detected in the Cretaceous-Tertiary (K-T) boundary, more than 100 times the level in adjacent layers. The spike was attributed to massive fires that consumed about 20% of the terrestrial above-ground biomass in a very short time."

Jamie Hall

My comment stands.

Solid_Fuel_Man

There was someone who made a condensing woodstove with this idea. They sprayed the water down a pipe which induced the draught. The naturally heated water was run through a small fan coil to extract latent heat. I'd expect the water to get nasty fast. I think there was a bypass to get the fire going and reach secondary burn temperatures first. Then the condensing mode was started. 

I'll go through some archives and see if I can find it. I'd like to experiment with that someday in my shop. 

Big Ed_4

Combustion is acidic , it would kill the fish....

jumper

Big Ed_4 said:

Combustion is acidic , it would kill the fish....

bah! who tells fish to drink it? Seriously, pollutants have to go somewhere. Down the drain; into the air; or get buried.