Dumb beginner question about RMHs

Sorry if this has been asked and answered a million times...it was on my mind a little while ago and I realized I hadn't seen it addressed in what little RMH info I've been exposed to.

The point of the "mass" is to absorb the heat going through the exhaust vent. Thus by the time the exhaust reaches the chimney it has cooled quite significantly, as I understand it.

What I'm wondering is what keeps those exhaust gasses from cooling so much before they exit the chimney that they condense, causing water vapor to drip back down?

(Asking because there are maximum heights specified for the chimneys of conventional combustion-powered furnaces, fireplaces, and other appliances, intended to prevent exactly this problem.)

Condensing exhaust wrings the maximum heat content from the fire, so is a good thing if you have the draft to support it and a system built to handle condensate. An internal chimney that doesn't lose heat to the exterior, and a structure layout and detailing that give good draft, are important here.

Most actual RMHs that I know of don't get such cool exhaust that condensation is a serious issue. If the mass cools the gases below say 100F, the mass at that point will barely be feeling warm on the surface and is not much use. This issue along with friction informs the standard recommendations for duct length in piped masses or bell internal surface area.

Hi Ned;
A properly running RMH will have a minimum exhaust stack temperature of 140F -200F
At these temps your condensation is insignificant.
The chimney ideally is located inside the home and uses an insulated section where it enters the outdoors.
A fully outdoor chimney must be insulated from top to bottom.

However, if your firewood is not properly dry then you will get condensation no matter what.

thomas rubino wrote:Hi Ned;
A properly running RMH will have a minimum exhaust stack temperature of 140F -200F
At these temps your condensation is insignificant.

Ah, okay. That solves the mystery then. I was mentally comparing a high efficiency class iv gas furnace, whose exhaust is so cool it can be piped with PVC.