Acidic discharge water isn't the word for it. How are you going to dispose of it? Most of these furnaces won't fully exhaust the combustion by products and you have to deal with essentially battery acid water that has to be pumped somewhere.
Oh, really?
Fine. Lets do this. Time for me to put on my recently conferred adjunct professor of HVAC hat, hop on my soap box and pontificate on.
When any hydrocarbon is combusted in atmospheric conditions, you get a fairly similar redox reaction. Since natural gas is what we're talking about here, and the hydrocarbon component of interest is methane, we'll start there:
CH
4 + 2 O
2 → CO
2 + 2 H
2O
This assumes complete combustion, a stochimetric mixture with no areas of flame front impingement to result in the formation of the thermodynamically less favorable product, carbon monoxide. Correctly operating furnaces use a mixture of primary and secondary combustion air to ensure this reaction fully takes place.
Now, on old furnaces like what Dave is soon to be rid of, some heat from these combustion products is relieved of in the heat exchanger. This heat ends up in the inside air. On a furnace like what Dave currently has, this is probably somewhere around 60-70%. The rest of it, still at 350F+ degrees, is exhausted to the vent stack. This hot exhaust is more buoyant then its surroundings, and this heat creates draft in the vent stack, carrying the flue gases up and away to atmosphere. And that, boys and girls, is how natural draft furnaces work.
The furnace Dave is getting does and interesting thing. It takes these hot gasses, and just like its predecessor, cools them in a heat exchanger. But instead of stopping there, it has a second heat exchanger (and incidentally ususally lower in the furnace for an upflow configuration so it receives the coolest return air, thus making a counter-current heat exchanger for added efficiency but necessitating a draft inducer fan to pull the combustion gasses "downward") cools them further. It cools them below 350F... down down down, and at 212F (ish), an interesting thing happens...
Remember that chemical equation? Well that 2 H
2O in there is steam! And when those gases cool below 212F, that steam condenses back into water. In the process that steam gives up tremendous heat, chemistry geeks will recognize it as the latent heat of vaporization.
And that is how we end up with furnaces that can recover 90+% of the heat in combustion gasses. And the gasses cool enough that we can vent with PVC.
Now, on to acidity...
Carbon dioxide in the presence of liquid and vapor water has a tendency to form carbonic acid. H
2CO
3. Not a lot of CO
2 ends up in this state, because carbonic acid, unlike - for instance – battery acid (sulfuric) - is a weak acid.
That's not to say that weak acids are 'weak' in a non-chemistry sense. For example, no one in their right mind would consider glacial acetic acid in the same safety category as household vinegar. But the denotation of weak acid does tell us that the acid doesn't have a tendency to fully dissociate into hydronium and hydroxide ions – the rate at which this happens can be found by looking at an acids K
a (or more conveient stated in a log scale) pK
a value.
It just so happens that I have a copy of the CRC in front of me. Lets see... Carbonic Acid pK
a...
DISSOCIATION CONSTANTS OF INORGAINC ACIDS AND BASES. Pg 5-92
Carbonic acid ---> pK
a 6.35
This is an equlibrium constant, and it relates to an equlibrium equation for the dissociation of acids in aqueous solutions. That equation for us is:
[H+][HCO
3-]
Ka = ------------------
[CO
2(aq)]
and pK
a = -log(K
a)
Okay, lets apply this with Henry's law and work through it.
In flue gasses, you can expect about 8.8% CO2 [
1]
CO
2 (aq) = K
CO2P
CO2But what's the K
CO2?
Page 5-152. SOLUBILITY OF CARBON DIOXIDE IN WATER AT VARIOUS TEMPERATURES AND PRESSURES.
Love the CRC.
At 100C, atmosperic (where condensation is taking place): K
CO2 = 1.96x10-4
P
CO2 = 8.8%/100%x1Atm = 0.088 Atm
CO
2(aq) = 1.96x10-4 * 0.088 Atm = 1.725x10^-5
Plug it in to the acid base equlibrium accounting for the second form of acid, bicarbonate; solve for a hydronium concentration:
6.35 = -log([H+]^2 / [1.725x10x-5])
Hydronium concentration = 1.73x10^4. Therefore, PH is 3.76.
Battery acid (that is, the acid used in lead acid batteries) is reported, depending on charge state, to have an acidity of about 0.8 PH.
PH is, however a logethrimic scale, so for a better comparison, lets convert back to actual hydronum ion concentration.
K
a = 0.44933
Lets compare this to the acidic concentration of condensing furnace effluant:
(1.73x10^4 / 0.44933 ) * 100% = 38,500%
In other words, no, condensing furnace condensate is actually nothing like “battery acid”. You are completely incorrect.
Edited to add: Please note, these are
worst case numbers, in which full mixing and dissociation to theoretical limits are allowed to happen.
