New Englander 25 PDVC????

'D' mode is the default mode for that stove.

'C' mode allows the stove to burn hotter, it adds more fuel to the burnpot on all heat ranges. It also changes the speed of the exhaust blower to pull more air into the burnpot to burn the extra fuel. Some owners change to mode 'C' when the temps outside drop below a certain temperature so that they can keep their home warm.

Never run the stove in 'A' or 'B' mode, the stove was not designed to work in these modes.

I switched my pdv from d to c cause the stove was having trouble keeping the house warm.. It made a huge difference in how the stove runs.. But burns a lot more pellets.. Basically with mine after the mode change when it was in d mode high is how it ran on low in c mode, well with heat output..

Does it change the ash in the burn pot and the glass getting covered???? running in c mode

The ash build up will not change... A huge part of that is the pellets.. Englanders love softwood top quality pellets that helps a lot with the ash build up... The glass will get cleaner bigger flames that air wash the glass.. When mine was on d the glass would get dirty too it's kinda normal for these stoves

I will have to look into finding soft wood pellets

Loving C mode when its really cold out !!

so far down to 10 heating setting on 4 been doing a good job mod d

If ur having no issues in mode d I would leave it be...