Nasty Condensation with New Pellet Boiler

[AlfaFrance]

I installed a new 21 kW pellet boiler a few weeks ago, an Alfa Plam Commo 21, which is rebranded under many names in Europe. After many false starts, I have it functional, feeding both radiators and an underfloor system of pipes.

I am surprised it seems to take about 100 lbs. of pellets a day to heat a 1900 square foot house. Maybe I can get that down.

My biggest problem now is that I get about a cup of black water coming out of the bottom of the pellet stove each day. At first, I thought it was rain coming down the flue, but nope, it continues after the rain stops so it is almost assuredly condensation. I haven't done a close examination to see exactly where it is coming from.

Any suggestions how to get this under control would be greatly appreciated!

 

[EbS-P]

I installed a new 21 kW pellet boiler a few weeks ago, an Alfa Plam Commo 21, which is rebranded under many names in Europe. After many false starts, I have it functional, feeding both radiators and an underfloor system of pipes.

I am surprised it seems to take about 100 lbs. of pellets a day to heat a 1900 square foot house. Maybe I can get that down.

My biggest problem now is that I get about a cup of black water coming out of the bottom of the pellet stove each day. At first, I thought it was rain coming down the flue, but nope, it continues after the rain stops so it is almost assuredly condensation. I haven't done a close examination to see exactly where it is coming from.

Any suggestions how to get this under control would be greatly appreciated!

Can you describe how it’s vented? Pictures are welcome.

 

[ABMax24]

What's the water temperature in and out of the boiler when this happens?

Boilers require a minimum inlet temperature to avoid condensation within the boiler, depends on the model, but lots of them want water no cooler than 60c entering them. If the water is too cold it causes the moisture in the flue gases to condense.

Second option is there is a leak internally in the boiler somewhere.

 

[AlfaFrance]

I have seen much strange behavior I cannot explain. There was a minimal manual that was badly translated. I found a crude forum where everyone wrote in Serbian. I keep hoping that I can find some documentation. I also had a defunct heat pump/emergency heat unit to deal with (Buderus/Geminox) that talked to a mixer that handled the underfloor pipes. I put that mixer in emergency mode and use its pump to send water through the underfloor pipes.

The pellet boiler uses a Micronova controller from Italy it seems. I can't swear to it but I think when the boiler temperature gets too far above the requested temperature it goes into a cooling cycle and if cooler water returning from the radiators hits the heat exchange with the burn chamber, that is when the water on the floor is worst.

I have seen installation instructions for other pellet burners where they say to install a three-way mixer that loops water back to the pellet boiler until it gets up to temperature.

The controller has 5 levels of "power". I am not sure what that means. The boiler seems most stable when I request 150F water at power level 2.

I am going to collect more data. Does anyone know what the "power" setting means? There is a blinking number below the "power" setting, too.

 

[maple1]

I would bet your return water temp is way too low.

 

[sloeffle]

I have seen installation instructions for other pellet burners where they say to install a three-way mixer that loops water back to the pellet boiler until it gets up to temperature.

(broken link removed to https://store.tarmusa.com/collections/lk-armatur-featured-products/products/lk-820-thermovar-1)

 

[AlfaFrance]

I would bet your return water temp is way too low.

I think you are correct. I am trying to figure this out again today, taking the back cover off the unit to see what is going on. I am 90% certain these things that look like flow restrictors came with the boiler. I unpacked a lot of boxes that day. The in and out water pipes are both nominal 1-1/2 fittings and these disks are a loose fit. Nothing in installation instruction about them.

If they are flow restrictors, why is the hole off center? Do you think I should use them?

 

[AlfaFrance]

Moderator? Please delete my last message. I am pretty sure those disks came with a faucet set.

 

[Mr._Graybeard]

I would set water temperature at a minimum of 180f, not 150. You don't want the water jacket below 140 at any time if you can help it.

 

[AlfaFrance]

The manufacturer recommended a max. water temp setting of 170F, so I put it there to see how it goes.

Taking the back off the pellet boiler let me see where the black water comes from—right out the flue pipe junction at the back of the stove. I am always surprised how far leaking water can travel. At least now I can get a pan under it.

So, it looks like I will be installing a thermostatic mixing valve.

Being new to this and provided with poor instructions makes it hard. I have a controller with three settings: 1. Desired room temp. based on a sensor out the back of the stove. 2. desired water temp. and 3. "Power" you can set from 1-5.

I think "Power" sets the pellet feed rate and blower intensity, right?

 

[Mr._Graybeard]

If 170f is the best the boiler can do, you may need a buffer tank to hold heated water. Power dial may set the pellet feed rate.

 

[AlfaFrance]

What do you think to fix condensation? They say the 55C mixing valve fully opens at 60-62C. Or should I install a 60C valve that fully opens at 65-67C? Looking at the FAR products. Less cost and easy to obtain in France.

 

[Mr._Graybeard]

I'd go for the 60c valve. Do you have a double-walled flue pipe? The inner surface of the flue may be cool enough to allow moisture in the exhaust to condense. A double-wall pipe may reduce that. Any ash residue that deposits in my boiler flue is dry as dust.

Here's a brand that's popular in the States, DuraVent.
(broken link removed)

 

[AlfaFrance]

Thanks. I have 8 cm coming out of the stove that transitions to the old wood stove flue of 15 cm. It goes straight up and becomes double-wall 18 feet above the stove . 6 feet above that it it goes through the roof.

 

[Mr._Graybeard]

A lot of your condensation may be occurring in the flue, not in the stove. Maybe you can do something to keep the exhaust gases hot, such as wrapping the flue pipe in insulation. Or going to double-wall right out of the stove.

Another thought -- the 15-cm pipe may be too big to force exhaust gases to make a quick exit into the atmosphere, so the gases can linger a bit and condense. I've had pellets with relatively high moisture content -- burning them would produce a vapor cloud when they hit the outside air but the flue remained dry. My 4-inch vent pipe (10cm, maybe) rises about 2 meters before going horizontal -- the entire flue is less than 5 meters long. People have used longer flue pipes with pellet burners, but the fact that you're getting that moisture at the stove's flue exit makes me wonder if it's running down into the stove.

I'm just guessing at your situation, obviously. I'm totally unfamiliar with the boiler you're using.