Basically, how fast a water heater or boiler can heat water. Recovery rates are generally measured by how many gallons of water can be raised 90-degrees in one hour. If a water heater, for example, stores 50 gallons of water, a recovery rate suitable for radiant heating would be the ability to heat 1 1/2 times its storage amount within one hour. In other words, it could heat 75 gallons each hour. The best water heaters on the market can heat almost three times their capacity. Recovery rate is important in radiant systems because the water in the radiant tubing cools down to room temperature between heat cycles. Depending on the size of the zone, this could be as much as 20 or 30 gallons of water. So, when the system kicks on, this 70-degree water enters the water heater and “dilutes” the temperature. A water heater with a good recovery rate will heat the water back up quickly and return the radiant system to its desired operating temperature (125 degrees). Gas and oil fired water heaters offer the quickest recovery rates, electric water heaters the slowest.