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Open Therm

What is Open Therm? 

Open Therm is a form of communication between the room stat and the boiler enabling the boiler to deliver modulating flow temperatures depending of the required heat output. Boiler manufacturers have started to add open therm into their new range of boilers, some require the manufacturers own controls for it to work, others are compatible with smart thermostats such as NEST.

Older on/off style of controls were a very primative form of communication between the boiler and the room stat. Lets look at an example where the room is currently 15 degrees, and you want to make it 20 degrees. You would turn your programmer/timer on for heating, turn your thermostat to 20 degrees, the boiler would fire up and reach a flow temperature of 75-80 degrees and would keep this going until your room stat reached 20 degrees and then turned your boiler off. At this point there is still 75-80 degrees in your radiators and even though your boiler is off, your radiators will continue giving off heat as they are cooling down, as a result this might make your room temperature go up to 21, 22 or even 23 degrees!! Completely over shooting the desired 20 degrees you had set and now you’re too hot! In the mean time your radiators cool right down to around 30 degrees and your room temperature starts to drop as well. It continues dropping and drops below the 20 degrees you set on your room stat and so turns your boiler on again. Your boiler now has to heat the system water up again to 75-80 degrees by which point your room temperature has kept dropping down and might have dropped down to 19, 18 or even 17 degrees and now you start to feel cold. So there was a continuous cycle of over shooting and under shooting the desired room temperature you set, so you’re either too hot or too cold.

With a smart control that has open therm such as NEST, it constantly monitors both the room temperature and system temperature and delivers the right amount of heat to achieve the desired temperature. For example if we used the same room that was 15 degrees and you wanted to be 20 degrees, the smart thermostat would learn how long it takes your house to heat up. It might start with a flow temperature of 70 degrees and then lower the flow temperature the closer it gets to the desired room temperature. So when the room reached 19 degrees, it might know that a flow temperature of 50 degrees is all that is required to get the room to 20 degrees. Once the room has reached 20 degrees, the smart thermostat will keep monitoring the temperature and sense if it starts dropping. It will then tell the boiler to send out a steady flow temperature of say 40 degrees which will be enough to keep the room at 20 degrees (each house’s flow temperature will vary but the principle is the same). The result of this is not only a much more constant and accurate room temperature meaning your comfort levels will be much much better, but also the boiler will stay in condensing mode far longer and so help reduce your heating bills.

Open therm can take getting used to, as there will be times where your radiators are just luke  warm to touch, but the room is at the desired temperature and you are warm. Yet some customers will think their boiler is not working properly because their radiators aren’t as hot. Please get in contact with MD Plumbing and Heating if you would like to know more about open therm and if you would like to have it installed in your home.