> Absolutely - but the important thing is that you can actually get a 20kW gas boiler, but you can't get a consumer 20kW heat pump.
If even you could, you may not want to. Instead one external heat pump handle heads on the top floor, which is generally bedrooms, and not occupied during the day; a second external unit to handle heads on the main floor, which are generally not occupied overnight.
Each individual smaller unit runs less because the load is more focused in 'zones'.
> Why do you say heat pumps only work in well insulated houses with large radiators?
Because heat pumps operate at a lower flow temperature than boilers. Smaller radiators designed to operate at hotter temperatures won't bring your house up to the target heat temperature operating at lower temperatures. They often need replacing. [1]
Trials of heat pumps in existing UK housing have found it difficult giving away heat pumps for free, because of all the extra work required for them to operate properly in the house. [2]
> Heat pumps are ~3x as efficient as the best gas furnaces so why would you need a better insulated house?
Heat pumps in comparison to conventional boilers are meant to be on all the time at a lower temperature, as they take a long time to bring a house up to target temperature.
They depend on good insulation, otherwise the heat never accumulates, similar to if you leave a freezer door ajar, it will never freeze.
> So why would you waste all that energy converting from renewable electricity to hydrogen then burning hydrogen (which is not super efficient anyway)?
At times, renewable energy drives electricity prices negative. We haven't got many ways to store this when it happens, converting water to hydrogen would be a good candidate, which can be delivered using existing gas infrastructure.
> Plus, heat pumps already work at scale
Not in existing UK housing, which would require major changes (insulation, underfloor heating, larger radiators), before they work.
> Are all properties suitable for a heat pump though? That's the question.
Generally, yes, but of course it always depends on specific circumstances.
Read up on heat pumps! They're really cool. And yes they're like air conditioning units that can cool and also heat. In addition to heat pump space heating there are also heat pump hot water heaters and heat pump clothes driers and it seems both are more efficient and non-polluting.
It feels magical that they can pull heat from outside in the winter, but that's what they do! The more recent units can even do it in very cold temperatures.
> Be that as it may, heat pumps at this time are cost prohibitive.
They range from sub-$1K single zone air-to-air wall/window units, to multizone split units, to ground source hydronic boiler/chillers that integrate with baseboard or radiant floors, which can get quite expensive to install, but that's at the high end. Generally speaking they're not really much more expensive than normal air conditioning. I've seen units that come in cooling-only and heating+cooling versions, and the cost difference is marginal (<10%) in those cases.
What's the limiting parameter here? Obviously heat pumps can be scaled up to arbitrary sizes - I've used unimaginably large ones in the chemical industry. Do you mean power vs cost compared to other options? Power vs volume? Power vs. weight?
Stockholm has a couple 40 megawatt heat pumps as part of a set of just seven heat pumps which provide building heat to an entire district of the city, pumping heat through 3000 kilometers of pipes.
The only limiting factor I'm aware of for most commercially-available home heat pumps is that they tend not to work well below a certain temperature. This minimum temperature can be lowered by increasing the size of the outdoor radiator and adjusting the pump and coolant pressures to match.
>> all in all fewer points of failure than a cutting edge heat pump that can actually produce heat when it's below 20f (-7c, i.e. the only kind of heatpump that might stand a chance at heating throughout the cold winters most of the northern US gets).
My family has one in Poland, it was -20C this winter already and it worked absolutely fine. It's some cheap unit, wouldn't call it "cutting edge". I think there's a simple resistive heater that de-ices the fins at low temps, but it kept the interior of the house at a (very toasty) 24C pretty much non stop even in those low temperatures. I have no idea why people keep saying heat pumps don't work in low temps. I had a basic split unit fitted to my home in UK, literally a basic £600 midea unit and apparently it should work down to -25C without any problem.
>>Gas boilers are far simpler units. Less moving parts, simpler electronics
Have you ever looked inside a modern gas boiler??? I hard disagree that it has less parts than a modern heat pump. A heat pump is like your fridge - there's an inverter, compressor, and a whole bunch of fins, that's about it. A gas boiler has multiple tanks, burn chamber, exhaust recirculation, at least 5-6 probes to measure every part of the process(and they all can fail in surprising ways that renders your boiler dead).
> There is no need to 'rip it out and replace the whole system' to use a heat pump; an air to air heat pump as used in many Scandinavian homes can be fitted at much lower cost without disturbing the existing central heating at all.
I understood that heat pumps work more efficiently in homes that are a) well insulated, and b) where the heating system is designed to work at lower temperatures (bigger radiators/underfloor heating and appropriate pipes). Whether or not a & b are must-haves or nice to have I'm not 100% sure about - I guess it depends.
> You can still use a heat pump that takes the heat from the exhaust air to warm the fresh air
Unless I'm misunderstanding part of this proposed setup, I'm pretty sure this doesn't work. The higher efficiency of heat pumps comes from the fact that the outdoors is an effectively infinite (for the purposes of a house) source of temperature differential. You can only move as much heat as exists, so you can't use a heat pump to multiply a finite heat source.
Gas-fired heat pumps are neither economical nor efficient for single-apartment or single-house systems. Internal combustion engines with shaft power output of 1-2 kW are inefficient, loud and maintenance intensive.
They can start to make sense from around 50 kW heating/cooling capacity and upwards, so the smallest units are suitable for 8-15 apartments depending on size.
> Then there's the cost. A gas heater is 3 times as cheap, and for that price, 3 times as expensive.
Only really a thing if there's a local gas network and subsised gas.
> Lastly, there's the issue of when it's really cold and the heat pump cannot extract heat from the outside air (this is much less a problem when you have an water to water heat pump, but they are significantly more expensive). Then the thing just runs on electricity. And that can be VERY expensive in certain parts of the worlds.
They work fine in -25c. Few people in the world live in places that are regularly colder than that for significant chunks of time.
Yes, and that's very nice if you have a heat pump. I suspect people with heat pumps use them for heat. It'd be weird if they didn't.
But, to restate my previous comment: if you have a natural gas furnace and a GPU, but no heat pump, and are deciding which to use for heat, it doesn't matter how efficient the heat pump you don't have is.
If you own your own home and have the capital to get a heat pump, great, do it. But a lot of people rent. They can't just swap out their furnace for a heat pump.
> Other than higher cost for the heat pump unit, are there any other issues with having one that's oversized for heating?
- Short cycling leading to lower equipment life (also true for gas furnaces, although heat pumps have more moving parts)
- Greater discomfort as the house heats up rapidly then cools down rapidly (especially if it has a leaky building envelope).
- Higher peak electric loads, possibly during hours of high electricity prices, leading to higher electricity costs.
> My understanding is that for cooling, having one that is oversized can be a problem because of humidity issues,
Yes, because moisture will build up when it isn't running.
> makes me wonder what can be done if a space has an imbalance between the size needed for cooling versus what it needs for heating.
Many heat pumps have different output ratings for heating and cooling modes to deal with this. Often however, this has as much to do with the distribution of the heated/cooled air and placement of supply registers, which is often an afterthought when the system is purposely oversized (which is presumed to make up for lack of air distribution design).
Yes, and a heat pump tends to run at lower output for longer periods of time versus traditional fuel fired boilers so the peak power demand would be even lower still.
> Heat pumps are the greatest thing because the home doesn't go through this massive hot/cold cycle. They might stay running at 10% once they get to temps to keep it at the temps.
That means you have inverter drive. Also nice because they don’t have a capacitor that eventually fails and won’t dim your lights when it turns on. Also great at reducing demand charges if your electric utility bills that way.
But heat pumps can run as an on or off only system. Depends on what you install.
> a heat pump can put even 400W of heat in a room for every 100W of electricity
You mean it will transfer heat from the outside of the house to the inside? Over here we have the more traditional halogen space heaters (or central heating, with oil).
> Also just a guess: is the story with heating that your heat pump is sized based on your max heating load, and so losing up to 25% of its capacity on the coldest days means it would never be able to catch up?
The unit I have has a HSPF2 rating of 10.5. It's very efficient at the transfer of the heat available. Heat pumps can extract more energy than the consume. New heat pumps can operate over 300% efficiency. The best furnaces are generally in the mid-90 percentages.
I have a dual-fuel system and can set where the heat pump locks out. The system is rated to a COP of 1.75 at 5F. Meaning it's still over 100% efficient at that temperature. The unit will operate down to -22F but would be at, likely, less than 50% capacity of output. To save on wear and tear of the unit, however I have my setpoint lockout at 15.
It would have to be significantly below 0 for days on end to have a problem. But even then the furnace would be providing primary heat. There are people in my area that heat their entire home with ductless heat pumps [0]. My unit is rated at 48k BTU. The furnace at 100k BTU. I only really "need" the furnace for emergency. But we do have sub-zero highs rather often and during those days the heat pump would be running continuously.
I also have an 80 gallon hybrid heat pump water heater which has saved us thousands over the last few years. 98% of the year it only uses the heat pump.
If even you could, you may not want to. Instead one external heat pump handle heads on the top floor, which is generally bedrooms, and not occupied during the day; a second external unit to handle heads on the main floor, which are generally not occupied overnight.
Each individual smaller unit runs less because the load is more focused in 'zones'.
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