Pellet Boiler Not Moving Water Fast Enough

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New Member
Nov 10, 2023
Macon, France
I recently installed an Alfa Plam Commo 21 pellet boiler on a five radiator system that also has a mixer from a defunct heat pump. With the power set to the lowest setting (1 out of 5) and requesting water heated to 60C, the pellet boiler fires up, heats the water to 70C and shuts down, waiting for the water to cool. This takes about 30 minutes.

The first two radiators heat pretty much as expected. The last two do not heat at all. I have tried many things, such as restricting exit flow from the first radiators. A few times, the hot water pipe from the radiator stayed cold and the return line filled with hot water.

The basic problem seems that the pellet boiler pump cannot push water through the system fast enough. I have tried putting the pump from the old mixer online to assist, but the pump just gets hot and does not seem to move water.

I have flushed and bled all five radiators. What is the best way to get water moving? Replace the pellet boiler's pump with one with more power? Install an external pump? All ideas are appreciated.


  • Pellet Stove Flow B Radiators.pdf
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Looking at your schematic.
I'm pretty sure the water just takes the easiest path, that is one or 2 radiators getting water.
you need something to balance your flow; example, Caleffi Quicksetter valve.

Can you close off the 2 radiators that receive hot water, and see what happens then?
I suspect that there is some kind of blockage in Radiators 1 and 2 because when 4 and 5 have their outflow restricted the entire system flow decreases dramatically. Radiator 3 seems pretty happy regardless of what I do. I am running some tests today. I figured out how to turn the old system's pump on and off, too.
I tried many things yesterday. I am impatient and naive, bad combo. The good news is that my underfloor system, which I thought irreparably trashed, works. The bad news is that I still cannot get the basic flow I want through my five radiators, so I am keeping the underfloor shut off for now.

I tried to force water to the most distant radiators by closing the lockshields on the closest radiators. It seemed to overwhelm the pellet stoves pump. In any case, I got to the point that heated water was exiting the stove through the return line.

My budget is very tight. The stove's pump is a Wilo Type RS25/6-3 P on the highest, 93 W setting. Wilo's online documentation truly sucks, more than their pumps it seems (ha, ha). I think is does:
at 0 meters max 5.8 cubic meters/hr;
at 4m height, 3.4 cubic meters/hr

Is this too small to use with 5 radiators three meters above the stove (Alfa Plam Commo 21)?

If I need to install an external pump, does anyone have recommendations? Thanks.
I discovered the internal Wilco pump was not turning, probably because my central heating pipes were not as clean as I thought. I did carry 5 heavy radiators down steps, flushed them, and back up. I have figured out quite a bit about Alfa Plam hydro pellet stoves—not easy when you only have a bad translation of a bad, incomplete manual. And, OMG, you should see the Google translation of defunct Slovenian forums.

The Access Code that lets you get to all the settings is A9.
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Good you worked your way through it.
If you have a dirty system...
A side stream filter or a Y trap would be a good addition to your system.
When i built mine i installed a side stream filter,had to change it after a week, then it went 3 months and haven't changed it in 10 years now.
A hot water spin down filter is an option also. Put one on my system and I couldn't believe the amount of stuff it caught in the first week or so. IMHO it's cheap insurance if a pump impeller decides to give up the ghost. I've read a few stories where folks Water-Air HX gets plugged up by an impeller and they have to buy a new Water-Air HX.
Thanks for the suggestions. I am going to look into the filters. I now have the 21 kW Alfa plam sending water to the other side of the house and the mixer from the old system, with its own pump, sends water to the downstairs underfloor system.
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Here's a tip, although I doubt many others have a set-up like mine. I put a pellet boiler on my defunct heat pump/electric heat system that had a mixer for radiators and underfloor. Things were going pretty well but yesterday the pellet boiler never got its water temperature high enough to turn on its pump. This is unusual because normally it heats up so fast I have trouble with it going into cooling cycles too often.

It seems the pump on the mixer was pushing cold water from the entire system through the pellet boiler before it had a chance to warm up. Not good, I know. If I leave the mixer's pump off until the pellet boiler heats up, it seems to work.