Heat pumps = simply running your AC in reverse
It escapes me such a simple concept could take so long to be considered for homes instead of radiators
this post was submitted on 01 Aug 2023
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It's a simple idea, but it's not quite that simple.
When it gets to be around freezing outside, you have to deal with frost buildup on the outdoor unit.
And as temperatures fall, output and efficiency generally falls. So you need an oversized unit to heat your house on the coldest days, but an oversized unit isn't great the rest of the year.
Historically, heat pumps were only good if it never got down below freezing. Now, modern cold- climate heat pumps are efficient well below freezing and Mitsubishi's models advertise that they deliver 100% of their output down to -23F/-30C. Between adding variable inverters, better defrosting, etc they've come a really long way in the past decade.
It gets below zero in the north east in the winter. Heat pumps stop working at 20-30F and the system has to switch to classic/emergency heat. They are great for fall/spring (or summer as an AC), but useless for winter.
The bigger issue is that it is extremely expensive to install ductwork, wiring for 1 or more thermostats, and a shiny new heating/cooling system in many existing homes that use classic radiator heat. Depending on where the oil tank is located, it may require removal as well (example: if it is underground, depending on state/municipal laws).
That's not necessarily true now the newer systems can go to as low as -15F which in the north only happens for a few hours a year so still a reduction in heating gas/oils needed
...which in the north only happens for a few hours a year...
I know the guy in the TC video used Chicago as an example but the "few hours a year" thing simply isn't true for many of us.
Where I live we had many days below -15f including a week where itpretty much stayed between -20 and -30 for nearly a week straight.
A Heat Pump will still work, even here, but you need to be careful about which one you purchase and how it handles cold weather.
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I got a quote last year as my furnace is original to the house (33 years old) and we don’t have AC.
It was going to be $25,000, $5k of which was installing new ductwork because the existing ducts aren’t insulated enough.
Easy pass. I’ll wait for prices to (hopefully) come down.
It’s probably not gonna come down.
I’m in HVAC, the industry is insane right now with the cost of equipment alone, let alone the hourly rate for the installers.
Honestly, 25k for a complete overhaul is probably the lowest you’re gonna get.
I’d look into mini split heat pumps. Ditch the ducts all together, and you’ll probably save a few grand.
Same in the UK. I'm looking at 20k and that's after the 5k grant. It has to come down a lot to be viable.. I just don't have that kind of cash.
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First off, as the other poster replied, that isn’t true about modern heat pumps. They continue to work below freezing, and many support an “eheat” resistive heating mode, obviously only good if you still have electricity, but that’s true of all heat pumps. Generators or solar+batteries become much more important.
But the beauty of heat pumps is that you don’t need to install ductwork. Look at mini splits. You can do zoned or single room installs. No ductwork required. One of the huge upsides of mini splits are you do get “instant” zoning. You can stop heating and cooling unused rooms to a human comfortable temperature.
You can also get systems that retrofit into existing forced air ductwork.
I wouldn’t be surprised if someone made or will make a heat pump water heater for hydronic radiators.
You can also run the element that is typically outdoors inside if you have enough space in a basement, for example, which stay a pretty consistent temperature all year long.
How does that work then.. I get it for the portable ones, you stick the pipe out of the window, but last time I priced up a mini split it was more than twice the cost of a single room due to the installation work involved. There has to be ducting, they're not magic..
You drill a small hole and run some hoses outside from each room.
It’s definitely more expensive than a window air conditioner, but a lot quieter and more efficient, and provides heat.
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That is refrigerant dependent. For example R744 (plain old CO2) works well efficiently down to -4F, -20C and down to -40C/-40F just with some efficiency drop.
Main issue is CO2 needs a constant high pressure heat pump system, since it needs to be highly pressurised to be fluid at all. In ambient it sublimates (goes straight between gas and solid aka dry ice).
However that is a solved issue. Working CO2 heatpumps are off the self commercially available these days. Just still little more expensive as I understand. However prices should come down with production economies of scale, upon CO2 taking over due to pollution, toxicity, flammability, green house considerations. He nastier chemicals weren't used for being all the ways superior, but due to it being easier to make the heat pump units (be they running in heating or in cooling) due to lower pressure requirements.
Since CO2 and ammonia were the original refrigerants. Used in large ice production facilities early on, where their specific needs weren't issue even for earlier technology. Large, purpose built, stationary industrial facility had no problem accommodating the needed massive pressures by just really massive and heavy pipework.
However these days the propeller head people developed micro channel tubing and heat exchangers to keep the high pressure CO2 in control.
Heat pumps stop working at 20-30F
Yeah crappy ones. This does not apply to the ones designed for cold climates. Mine still beats electric radiators in efficiency at -22f
In Scandinavia we run ours down to -4F or colder. Efficiency goes down but they are still better than 1:1 until then.
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I've spent a quarter of my life in Australia and never seen heat in a house (which is nuts, because Melbourne and Hobart winters are pretty close to Vancouver winters omit the one week of snow we get here in Canada).
The benefits of having hot water and heating from the same source I'd guess
I’ve never seen anyone using their furnace to run hot water. Radiator water loops are closed loops and I’m sure you wouldn’t want that water to be used for anything you’d use hot water for.
I think it’s mostly a factor of ACs weren’t historically efficient. They were all on or all off. In the more recent designs that are ultra efficient they use variable speed compressors.
They were also historically less reliable than a furnace and certainly more difficult and complicated to service.
It's kind of amazing that with a radiator you turn 1kW of electricity into 1kW of heat energy but with a heat pump depending on the efficiency you might get 2kW of heat or even more with the same amount of electricity.
Not really. Heat pump is not creating heat unlike resistive heater. It's just transporting it. One must remember even though freezing temperatures are cold for humans, for physics and universe those are still relatively high temperature environment. After all 0 Celsius is 273 positive Kelvin degrees. So that is 273 Kelvin forth of heat to pump around. Well say in -30C, still 243 K worth of heat to pump around. So the issue isn't is there heat, the issue is the practical mechanical and thermodynamic realities of he pumping. Which in practice comes down to can you find suitable refrigerants with suitable phase change characteristics to pump around.
Which for normal ambient temperatures on Earth is "yes". Just usually matter of how high pressures one has to use and other nasty features of the materials. For example recently even just CO2 has been started to use more again, issue with it mostly being it has to operate at higher pressures than more traditionally used refrigerants. They just don't call it CO2 in the bizz, it's refrigerant fluid R744. That works down to -56.6 Celcius given its triple point temperature. So it won't heat one in antarctic -80C winter, but for most of Earth even in cold climates -56.6 C is plenty. Problem just is it has to always work under high pressure, since in ambient pressures CO2 just sublimated from solid to gas. Pumping around solid blocks of dry ice isn't very convient for continuous heat exchange process machine.
Which adds some cost to the pump components. On the other hand... CO2 is pretty darn harmless. As long as concentration locally isn't too high, humans, animals and plants are perfectly used to handling the gas they exhale. It is non flammable, it is green house gas, but it is green house gas we naturally exhale. Some unit leaking it doesn't change much, since usually CO2 for industrial use is extracted from waste product gas, that would end up in air anyway.
Plus on need be it can be distilled from air, it's just energy intensive. Which is why "carbon capture" isn't a bigger thing. We know how to do carbon capture. It's just energy intensive and thus on climate impacting massive scale energy prohibitive.
I watched the technology connections video. So I should know. But I don't really know. What's the difference between a heat pump and a split air conditioner?
A split air conditioner is will generally have an indoor component (evaporator) and an outdoor component (condenser).
When the compressor runs, the evaporator will get cold and the condenser will get hot.
A heat pump also has a indoor and outdoor components, along with a new item called a reversing valve. The reversing valve allows the system to switch which component is the evaporator and which is the condenser.
When the compressor runs and the reversing valve is enabled, the evaporator is inside and you get cold air.
When the compressor runs and the reversing valve is disabled, the condenser is inside and you get hot air.
Thank you that was very clear!
Isn't that what most split AC units are currently capable of? At least I've seen them almost all with the ability to produce heat when I was browsing for one.
All of the split units I’ve seen are heat pumps.
Heat pump means that it’s reversible, split package is just the physical arrangement of the pieces.
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Mini split you mean? Those are also heat pumps but used in homes that don't have duct systems installed. So theoretically you get large mini splits on each floor to heat or cool all the spaces. Downside being if you close a door and there's no mini split in that room, it will be more affected by the outdoor air.
I may be way off here. But I believe a mini split and heat pump are the same thing? At least where I live the term is interchangeable.
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"the" video? He has many on heat pumps. Get watching!
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You can get one that can go both directions. Heat in winter and cool in summer.
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Is this the reason why they're so expensive?
Just had a 48btu unit installed. 2,000 sqft, 3 - 9k wall units, 1 - 18k ducted unit. Operational down to -22 F.
$20,000 and then a $4,300 rebate and later a $2,000 tax credit. Down right affordable. Would have been even less if new ductwork didn't have to be retrofitted.
There are many rebates for their installation. Check your city, state, federal, and utility for kick backs. That's having a huge effect on demand.
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