Now is the time ! Every year we hear that some grower had problems with toxic fumes or ethylene because of heater malfunction. Summer to early fall is the ideal time to check for cracks in the heat exchangers. Even a very small crack can cause serious problems during a cold day because the greenhouse environment is tight and gasses accumulate over time. Just a few days before you think the heaters will be used, check all air inlets and fume exhaust pipes. It is not uncommon for some animals like squirrels and birds to make their nests in the pipes blocking air movement. An ounce of prevention is worth... well, you know how it goes!