We've been seeing a lot of stoves with people complaining they aren't heating like they should. Your heat exchangers can get covered in a coating of carbon and ash very quickly. This actually insulates the exchangers from getting hot and cuts down drastically on the heat blown into your room. If you have a plate protecting your exchangers from the flame it needs to be removed often as fly ash will pack in there completely blocking the heat from them. It's best to wire brush or carefully scrape them back down to the metal.
