Why is one bedroom colder than the rest of the house?

Question

I had a home built last year and we have one bedroom that is 6-10 degrees colder than the rest of the rooms in the house when its cold outside.

The heat is radiant and the system has temperature gauges on the manifolds which tell me what the temperature of the water is going out and coming back. I have reflective bubble insulation between the floor joists and the water is coming back only 2-3 degrees colder than when it leaves. I have three zones and this zone is our main living area, two spare bedrooms and the bathroom. The bath is between the two spare bedrooms and the same radiant tubes heat all three. The bath is warm, one of the bedrooms is warm, but the other is cold. I have had additional insulation put in over the bedroom and have installed aluminum plates around the perimeter to increase the heat output into the floor. This room is on a corner, as is the other spare bedroom, however this room had an additional 8 feet of exterior wall over the other spare room. I have been told that is why it is cold. It also has northwesterly exposure which I am also told is adding to the problem.

So my question is, will an additional exterior wall, even if it is only 8' long, as well as northwesterly exposure, make a bedroom 6 - 10 degrees colder? The heat is getting there, it just isn't keeping that room as warm as it should be.

Answer 1

In my experience as a Home performance contractor, situations like you describe are almost always a problem of air sealing. There is likely something about the way the room was constructed that allows more air infiltration. Possible causes would be an attic stairs located in the room, one of the exterior walls having an overhang, the room is connected to a cantilevered porch or maybe it's over a garage. It's highly unlikely that 8 feet of properly insulated wall would cause the temperature differences you're describing.

Answer 2

The additional wall area is not why the room is colder. The room is colder because the heating system was either specified, designed or installed without properly accounting for the additional wall area.

Answer 3

Are the radiators the same size? If baseboard, compare the length that has fins on it. If in-floor, is the tubing on the same spacing.

You may be able to tell this with a thermal imaging camera, or alternately a IR remote thermometer, and a whole bunch of spot readings.

In the crawl space is there a difference in how the floors are insulated?

If there is outside air circulation is that opening under your room.

How sure are you that the zones are actually what you think they are? Turn the zone off, and see if all parts of it cool off together.

Thought experiment:

For a given room heat is coming in from the radiant system and leaving through the outside wall. An overly simple vision is that heat from floor to room = k*Af(Tw - Tr) where Tw is the temp of the water and Tr is temp of the room Af is the area of the floor.

Heat leaves the room by r*Aw(Tr -To) where To is temp outside. k is constant depending on having x linear feet of heating pipe per square foot of floor, and somewhat dependent on the floor covering. Aw is the area of the wall. r depends on wall construction.

A standard bedroom is 12x12. If it is not on a corner it has 12 feet of wall to heat. If it's on a corner it has 24 feet of wall to heat. You have 8 extra feet for this room, and it's a corner so you have 32 feet of wall to heat.

If you are running the same temperature water, then I would expect that the room with twice as much wall will be cooler. As the room temp drops Tw-Tr increases, increasing the heat flow in, and Tr-To decreases decreasing the heat flow out.

If you keep one room constant, I would expect the temp of the cold room to remain a standard fraction of the difference between the right room and the outside. E.g. if you maintain the right room at 70 F, and when it's 30 F the cold room is at 65, then the cold room is running 1/8 (5/40) of the too cold.

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Solutions:

• Put more radiators in the cold room.

• Increase the insulation in the walls of the cold room so that it has about the same total U value as the right room.

• Put that room on it's own zone.

• if the pipes can be divided for the two rooms, making in effect subloops, then put a flow restrictor on the right room so that you can balance the heat load.

• Leave the cold room door open during the day.

• Replace the door with a louvered door so the room exchanges air with the rest of the house even with the door closed.

• Use a fan.