Flue condensation after boiler replacement

Kaos

Helping someone with an issue with a one year old boiler replacement. There is significant condensation dripping down from the boiler. Installer is blaming flue liner and wants to do a direct wall vent to fix. Of course for $$$.

I'm pretty sure that isn't the issue and want to get some opinions on it.

The new boiler is a Weil McLain CGi series 4 mid efficiency unit. The place has what looks like cooper fin convectors. Running the boiler for about 20min the flue gas temp right by the boiler never got above 120F.

There is no bypass or any primary/secondary piping on the unit. I'm pretty sure what the installer proposes won't fix the issue without modifying the piping.

GroundUp

Sounds like it's over-radiated and needs either a bypass or p/s because the thermostat is being satisfied before the boiler can reach temp

Jamie Hall

This^^^. The flue has nothing to do with it. This does, however, need to be addressed, as that is a very good boiler — but it won't last long under those conditions.

hot_rod

the heat emitters always dictate the operating condition of the boiler. Can you add up all the fin convectors to get a btu requirement?

Copper tube convectors generally heat quickly and boilers rarely need return protection with fin tube?

Bob Harper

Per the manual, the return temp should not get below 130F. I'm guessing you need numerous changes here. Consider installing a bypass using a globe valve to throttle it so you can temper the return water. I'd put the circulator on the supply so you're pumping away. You need to provide the delta T, combustion analysis including stack draft, circulator rating in GPM, and your aquastat settings. Your stack temp is way too low to generate natural draft. This unit comes with a 15 second post-purge, which is way too short. You could install a delay on break timer to get at least 2-4 minutes pp. If you have an ODR, you'll need to contact the mfr. for advice per the manual.

The chimney should be lined and properly sized. They claim this unit is CAT I and CAT III but with the oversized flue, long vent connector with offsets (unsupported) and needs a draft inducer. They rate it 84% AFUE even though most sources consider 83% the max to still be considered CAT I. Yours is effectively a CAT II.

Combustion analysis will show if it is being underfired, which would contribute to the problem.

A barometric damper would add dilution air, stabilize the draft and reduce condensation.

jesmed1

@Bob Harper How does adding dilution air reduce condensation? Wouldn't dilution air further lower the flue gas temp? I'm just a homeowner trying to understand how this works.

hot_rod

if you do decide to repipe, I would add a return protection valve. You need a valve or device that can respond to the return temperature, automatically.

These are standard equipment in most gasification wood boilers. They work for any boiler that needs absolute protection.

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Kaos

Would the manufacturer recommend install work well enough? Might have a hard time convincing the installer that is already blaming something else to install a thermostatic bypass.

hot_rod

it really depends on what the load is connected to the boiler.

Use a 2000 sq ft radiant slab, 4” thick

That’s about 25 yards of concrete

X 4000 per yard, 100,000 lbs of mass

Specific heat of concrete is 0.21 so 0.21 btu/ lb of concrete to raise it 1 degree. 21,000 btu to raise that slab 1 degree

It a. 25 btu/ sq ft design load, a 50,000 btu/ hr cast boiler

Start up with a 60 degree slab temperature 5 flowing

Return to the boiler is 60 or so degrees, maybe, maybe the boiler can lift that 10 degrees, probably more like 3-5 degrees

So the boiler SWT is 70- 75 degrees. Remember the load ALWAYS dictates the operating condition of the boiler. Unless you get in the middle with some sort of control.

Where would I set the bypass valve or pump to mix the temperature up to 130? What pump or bypass can add 80 degrees to the bypass? Where would it get that temperature?

If I set the bypass To 5 gpm, the slab never gets heat🤨But the boiler would be protected

Ain’t gonna happen until the 100,000 lb mass warms considerably.

Although a condensing boiler would be running 97% or more under that cold return. Peeing out gallons of condensate😁

You could bang, bang the boiler pump off with a 140 degree aquastat on the return, a poor man’s boiler protection. I suspect it would be a few seconds of run time, a few seconds off time.

With a thermal bypass valve, it slowly releases the supply at the rate the boiler can keep up. Think of slipping the clutch in your truck on a hill with a heavy load.

Gravity conversions are another high mass system. Between the piping volume and the cast mass. Although generally not nearly the mass of a concrete slab. It’s easier to come up with the numbers for a slab as opposed to hidden piping

Especially if you paid for the concrete

Bob Harper

https://forum.heatinghelp.com/discussion/comment/1826185#Comment_1826185

It does lower the stack temp when open, which is due to high draft conditions. It reduces condensation just like a draft hood, which is the same as opening a window or door when showering. You reduce the Rh%. A fan assisted furnace does not have dilution air.

I have a thermostatic bypass valve on my boiler as Hot Rod suggested and it works great to maintain return temps and prevent thermal shock. I put a globe valve in the bypass so I can throttle it as needed. Ball valves are isolation valves and horrible for throttling. Gate valves are a little better, but their Cv is still worse (higher) than a cheap globe. This means you could get erosion from cavitation and require a larger valve for a given flow/ pressure profile.

hot_rod

A globe valve with a tapered plug, ideally. Flat disc type are not so flow friendly, or accurate.

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Steamhead

Before we repipe anything, we need to see if the boiler is under-fired.

Kaos

Thanks for the comments folks.

One more question. What's the chance of an aluminum liner rotting away after a season under this type of operation?

Bob Harper

At those temps? Very easy. Get a 316Ti ss liner with a lifetime warranty. Try to get a smoothwall one.