Why Some Rooms Are Colder (Or Hotter) Than Others
I Am A Building Scientist.
This means I study and understand how heat and moisture flow through buildings. It’s been a pretty cool thing to learn because we can explain the root cause of problems in homes to people – and often these problems have mystified everyone else for years. We get to be the heroes – and being the hero is fun.
I started a part of our company called Dr. Energy Saver in 2009. Since then we have saved millions and millions of dollars in annual fuel and electric cost for heating and cooling for our homeowners in CT and NY, and for people all across America through the contractor dealers that we recruited and trained.
So think about the one room that is always cold in the winter in your home. (Or you might think of the room that is always hot in the summer. The problem is the same just in reverse.) Here is what I know already. That room is NOT the one with the thermostat – right?
The thermostat is the brain of the heating system. The furnace (or boiler) is a slave to it. When the temperature drops below the temperature you have it set for, it calls down to the furnace – “Hey we need some heat – turn on”.
When the temperature gets to the set point, the thermostat calls down to the furnace again – “Hey we are warm enough, turn off.”
What this means is that no matter what is going on in the room with the thermostat, it’s always comfortable there. So, what about the rooms without the thermostat?
Well, for those rooms, it is a simple matter of BTU’s in, and BTU’s out. A BTU (British Thermal Unit) is about the same amount of heat in a wooden kitchen match. Your furnace’s size is expressed in BTU’s – such as “80,000 BTUs per hour”. In short, BTU is a unit of heat.
In a hot air heating system those heat units are delivered via the distribution of warm air from the furnace into the room. In any given room you need to balance the units of heat it needs to stay comfortable with the units of heat you are delivering.
If you do not have enough warm air going into a room necessary to keep it comfortable, then the thermostat will tell the furnace to shut off when the area with the thermostat gets up to the thermostat set temperature, but before the problem room does. That’s pretty easy to understand, and we can fix that.
In some cases, it may be very simple. For example, let’s say someone closed the registers one day and no one ever opened them. So – open them. Or if you have floor registers and someone put a throw rug over the register or a box or a piece of furniture. So – move it and let the warm air into the room.
In other cases, it may be that there is not a return duct into the room. Most bedrooms do not return ducts. There is a big return duct in the ceiling in the hallway, but not in the bedroom. This is ok when the bedroom door is open because the doorway IS the return air path. And for every cubic foot of air that gets sucked out of a room via the return, a new cubic foot of heated air from the ductwork is allowed to come into the room, bringing more heat.
The problem is when you close the bedroom door. There’s an old joke amongst building scientists. Your daughter Jenny’s room was comfortable until she got to be a teenager and started closing her door at night – then her room was cold.
To solve the problem jumper ducts or transfer grills can be installed to get the cooler air that’s in the room out, so more heat can go in from the supply ducts.
When it comes to colder rooms, there is another thing we look at too. What is causing the rooms to be cold? Is it a corner bedroom with two exterior walls instead of one like other bedrooms? Is there a lot of glass? Is the duct that gets the warm air there leaky or crushed or disconnected in the attic? Are there big air leaks in that room like from can lights in the ceiling or a knee wall space like in a Cape Cod home? Something else?
By stopping the heat from leaking out if we can, we can change the balance of heat units between the room with the thermostat and the problem room.
We need to lower the heating load in the problem room and/or steal heat units from the room with the thermostat so that both areas get up to a comfortable temperature at the same time.
It’s the same for rooms that are too hot in the summer, just in reverse. We need to get more heat units OUT of the problem room so it cools the same as the room with the thermostat. It’s amazing that almost no HVAC contractors consider all these factors when hearing a complaint of a room or rooms that are too hot or too cold. They don’t think about where the air is going or where it is leaking out of the building envelope, but are only focused on their equipment instead. If you have some rooms that aren’t comfortable, you tend to stay out of them for the summer or winter. We can fix it so you can use your whole house all the time.
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