Hot Air Doesn’t Always Rise

When we were designing the house I wanted a recirculating duct from the living room on the parlor floor to the bulkhead 4 flights up. I figured in the winter we could pull the hot air from the top of the stairs down to the parlor, and in the summer we could push cool air up where it was hot.

Problem was, the architect wasn’t too enthusiastic about the idea, and his mechanical engineer told me air in buildings didn’t work that way – that the hot air wouldn’t rise. That contradicted everything I had been taught in grade school. He said hot air only rises when there’s moving air. The air in a well-sealed house with the windows shut doesn’t move.

I didn’t believe him and when we got to the end of the project the recirculating duct was one of a short list of things I felt like I would have done differently if I could do it all over again.

Well, who’d have guessed – but the mechanical engineer was right. It’s winter and the top floor is at least 5 degrees cooler than the lower floors. It’s not because of poor insulation – we’ve got an R62 roof and R22 walls and great windows. At first I thought it might be just be that the room where I was when it struck me had had the door closed. But then I moved into the stairwell and it was the same there.

In the summer the area up in the bulkhead would get incredibly hot, but apparently that was because of solar heat gain from the large bulkhead windows (and glass door). But in the winter there’s less solar heat gain even – though the bulkhead is designed to capture winter light and discourage solar heat gain in the summer.

Now I’m perfectly happy the recirculating duct wasn’t put it. The air is cold up in the bulkhead – there’s no point of pulling cold air down into the living space in the winter. And likewise with the summer – we don’t need the bulkhead air conditioned.

The moral of the story is engineers know what they’re talking about. Who’ve guessed? lol

Our Heating & Cooling Choices

Going into the renovation process we had never chosen a heating and cooling system before. Our tilt-n-turn windows meant we couldn’t have window air conditioners. And when I initially thought through the HVAC choices I wasn’t really in favor of forced air since it tended to be expensive, dusty, and take up a lot of space.

We wound up going with with hot water radiators plus mini-splits. Some of the money we saved going with rads, we spent on expensive European radiators (Runtal). We also didn’t go overboard on a zoned system with lots of thermostats. Instead we opted for pretty simple Danfoss valves. The end result looks like this…

runtal radiator with danfoss valve

The exposed pipes will be painted white when it’s all done. Most of the radiators in the house are two units high – that one is in the rental bedroom wall and shorter than the rest – so to get the same BTUs we have everywhere else we had to go with the model that’s 3 units high.

That’s inherently a pretty low-tech solution – a valve and a radiator. It’s just the radiator looks better than most and the valve is thermostatic – so it’s based on temperature works better than regular radiator valves.

[Incidentally – avoid steam radiators – there’s no such thing as a high efficiency steam boiler. To get high efficiency you need hot water radiators, not steam radiators.]

Because there’s a valve on every radiator we sorta get a zoned system where every room is a zone. It won’t work quite as well as a real zoned system with thermostats for every zone, but it also cost a lot less. And if we stay on top of it, it will probably perform about as well as a more sophisticated system.

There will be one thermostat for the heating system. A friend’s post on Facebook turned me on to the Nest thermostat. If you haven’t checked it out, take some time and watch the promo videos on their site – it’s incredibly cool. I can’t wait to get one.

The one “problem” that we’ve realized is the issue of curtains. With radiators under all the windows – if we hang curtains all the way to the floor they’ll cover the radiators…

runtal radiators below windows

That’s a bit of a problem because the tilt-n-turn windows limit our window covering choices. They swing inwards, so putting blinds at the top of the window framing is sorta impossible. We were thinking of doing a lot of curtains, but now worry the radiators won’t work very well when the curtains are pulled.

One area where a forced air system might be better is air exchange. Since the house is well insulated, we may not have enough fresh air entering the house. That is easier to solve with a forced air system – you just incorporate a fresh air intake. But honestly I don’t think we’ll have much of a problem – 2 people in 3,000 sq. ft. shouldn’t be a problem.

We are happy that we have mini-splits and that they’re both heating and cooling. The house is warm enough down to about freezing. It maintains 50+ degrees when it’s in the high 30s outside. That means we probably won’t turn on the boiler until it gets down around freezing. If we just need to take the chill out of the air in a single room (or if our tenant wants it to be warmer than we do) we can use the mini-split.

The mini splits are a bit of an eyesore – we tried to hide them the best we could, but they’re still there hanging on the wall. I don’t think that will bother us too much. To us the visual noise of the mini split unit isn’t any worse than the soffits you often have with forced air systems. If you’re the type who doesn’t want to see a mini-split hanging on your wall there’s always the concealed models you can tuck inside a closet. Those have the advantage of being able to cool/heat two rooms (like a bedroom and an en-suite bathroom) – but the concealed models aren’t quite as efficient due to the ductwork.

One other thing to mention is that we have radiators at each end of the house, and since the bathrooms are in the center of the house we were worried they could get a little cold. We were worried they could get a bit cold, so we put under floor heating pads in each bathroom with programmable thermostats. It’s not very efficient heat, but it won’t be used all that often – so it should be fine.


Some of the comments asked how the radiator piping was configured. Here’s a diagram to explain how it’s been done…

radiator heating diagram

The model numbers are Runtal unless except where they say Slant/Fin. Since we have exposed brick on one side, both pipes are actually on the same side of the building – right next to each other. There are also valves to bleed air from the system in key locations (such as at the top of each riser). Notice that each radiator / loop has a Danfoss valve on it, so we can (nearly) turn off the radiator in rooms and the other radiators will get more flow.