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Comment by __MatrixMan__ | Hacker News Reader

Have you ever tried contacting an HVAC company and asking them to convert your AC to a heat pump so that you can use it to heat your house in the winter? I did, they told me that it was a nice idea in theory, but that it was the kind of work that nobody did unless they were teaching a class or something. The way to go is to just tear out my AC and have a heat pump installed. Nevermind that an AC is technically a heat pump.

So in practice, they're quite different.

> heat pumps provide no advantage

If you're worried about carbon emissions, heat pumps provide an advantage over AC / fuel-burning furnace combo's. I assume this is what the article was talking about.

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In theory such a thing is possible. In fact, I've even sketched out what I'd need to do to my one to convert it. I may still do it as a science project -- it's an old unit (~2007), so if I destroy it it's not the end of the world.

I can completely understand why no HVAC company would want to touch such a project. It's entirely experimental, would take a skilled technician, and has a high chance of not working perfectly. They would much rather slap together 2-3 installs in the time it would take to plan and assemble your science project. Remember that most of their techs are literally just installers, and there's a huge incentive to "rip out and replace" rather than "diagnose and repair".

If you could find a retired HVAC tech, you might be able to convince them to help.

> Have you ever tried contacting an HVAC company and asking them to convert your AC to a heat pump so that you can use it to heat your house in the winter?

Of course not, that's like asking your car mechanic to convert your ICE car to an EV.

While the principle of operation in a heat pump and AC unit is exactly the same, an AC-only unit is missing hardware to be able to be used to provide heat, and it's not just a matter of replacing a part or adding an optional feature. It would require major surgery.

It's basically two 3-way valves to flip which coil has high pressure and which coil has low pressure. If you get freezing temperatures, you may also need to do something to manage the icing that would happen outside, which may be via a suitable sensor to detect that state and running it in reverse for a calculated duration or such to melt the ice off.

It's really not much that needs to change. Efficiency may not be too good in heating mode, though, but in spring and fall it should work much better than a furnace.

Is there not a difference of capacity?

It takes more energy to bring a house up to 20°C in -15°C weather in a reasonable amount of time than it does to bring down the indoor temp of 20° from an outdoor temp of 35°C.

Their point still stands, which is that a heat pump provides no advantage in the summer, so the article was wrong in indicating that it did. But yes, not all ACs can be run as a heat pump.

I don't know where you get your ACs, but any I've seen installed this side of the turn of the century has a heating setting.

I use the AC unit (air to air heatpumps) in my attic mostly for heating in Winter and it works fine. The big downside is noise, (air to water) heatpumps let you move the noisy bit to another space which increases comfort.

You can build a silent-enough forced convection condenser to put in the room and put the noisy part away, without water, and cheaper than the passive convection water setup.

For cooling you can't use the typical passive convection radiators anyways, so might as well invest their cost into making the cooling and heating refrigerant coil silent...

Also c.f. a normal domestic fridge: no noise beyond the piston compressor.

Forced air is however going to cost you quite a bit of efficiency. And in mid-winter just the airflow is noisy enough, not just parts of the unit.

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