Munchkin Diagnosis and Replacement Size Check

in a cast iron boiler, any heat you gained by condensing water out of the products of combustion you hopefully lose later when you reevaporate the water as the system comes yup to temp. you will lose it unless your system never gets up to a temp that is safe for the hx.

the latent heat of vaporization is significant, if you design the system to give you low enough return water temps to condense most of the water produced from burning the hydrogen in the fuel it is at least another 5% of heat. where you don't get the efficiency is if you designed the system wrong. steam systems work almost entirely off of the energy in that phase transition.

What I am trying to get at, is. Mod/Con boilers are category 4 boilers. That is, they draw their combustion gases from outside the building and expel their combusted gases outside the building.

We can agree that condensation occurs when the temp drops below the dew point, and also, heat energy travels in a straight line to what ever is coldest, relativity speaking. Mother Nature seeking equilibrium. The effects of Heat Energy as measured in BTUs.

On a blistering cold day in the depths of winter, -5 deg day is not uncommon in my area. No, I don't live in Minnesota where the people. when the temperature get up to 45 degs, they put on their shorts and go out for a picnic.

That -5 deg air is coming into the combustion chamber where some of the BTUs are going to the HX and the bulk going to heat the incoming air (how is that economical)? Reminds me when I was a kid, at dinner, the older brothers got first choice and I was left with the scraps. Better if it all goes to the HX.

So, I think, it isn't condensation providing the economic benefits of a condensing Mod/Con, but the turn down ratio (ramping) and ODR. Of course, the modest water capacity of a Mod/Con as compared to CI helps.

Weather a boiler takes inside air or outside air it makes no difference. The air leaving through the flue is replace by outside air either leaking in the house or intentionally brought in. That goes for any boiler.

A mod con runs at a lower flue temp than a CI boiler so it is putting less heat into the flue gas than a CI boiler so it has to be more efficient.

mattmia2, says, "…air is free." "The air, the air, is everywhere…", from the play, "The Dawning of Aquarius". The ultimate lesson on Supply and Demand. The demand is unlimited, but the supply is unlimited which is why it's free. If the air comes from outside or from in the house, regardless, it's going to be heated. If your supply temp is set at 120 degs, would it take more heat energy to bring that water to 120 degs if the combustion air is -5 degs or more heat energy if the combustion air was at household temp of 70 degs or the same?

EBEBRATT-Ed, where the combustion air comes from does make a difference in economy of operation which is what I'm saying. Why does a Mod/Con run at a lower flue temp than a CI given the same rating?

Alan (California Radiant) Forbes, why do you do everything possible to make them condense? There is a limit to heat emitters before you run out of heat energy. It's like the train analogy, a train pulls into a station and a bunch of BTUs get on board. The train get to the next station and a bunch of BTUs get off. It get to the next heat emitter and a bunch more get off. It gets near the end and no BTUs get off. There are still BTUs on the train that want to get off, but they don't. Why not? Maybe because the loading dock is so crowded with BTUs that there isn't any room on the dock for the ones left on the train to exit. It's called stasis. The room is the same temperature as the train. Temperature is the motor that moves BTUs from one point to another. But you know all that. I have fin tube baseboards with an indirect.

@HomerJSmith

The same boiler heats the combustion air weather it is through the combustion process taking in cold outside air or taking the air from inside the building using indoor air.

Mod cons have a heat exchanger designed to lower the flue gas temp so moisture in the flue gas will condense. its designed for that purpose.

The concept of a condensing boiler is it has a secondary heat exchanger that the flue gases pass through extracting the additional heat so your flue gas runs around 10° or so of the boiler operating condition.

A conventional boiler 400° or so flue gas temperature.

I too have seen a fair share of direct connected Munchkins, other brands also. Properly pumped, assuring proper flow and temperature ∆ across the HX they will work fine.

I recall Dave Davis of HTP had a direct piped Munchkin heating his swimming pool, back in the early wild west days of condensing boilers.

i mean i did, i just said latent heat of vaporization.

When condensing or evaporation is said, that's another way of saying phase change and phase change is all about latent heat.

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