A heat pump just takes longer to heat then a furnace? Why?

I’ve seen this stated multiple times, here is one such statement:

Yes, a heat pump can replace both a furnace and an air conditioner. The only drawback to both is that a heat pump will take longer to provide heating and cooling to your home.

from Heat Pump vs Furnace - The Main Differences in 2022 | 2023 Fix-it 24/7

That seems to me just saying that the heat pump is undersized, and the solution is to get a ‘larger’ unit. But is there something going on here that heat pumps actually need to take longer but somehow can be equal powered to a furnace in BTU’s/h.

I guess if the output temperature of a heat pump is more dependent on room temperature input into the system than a furnace that sort of makes sense maybe. Like an extreme example, if a furnace output temperature is 150F no matter what, but the heat pump temperature is 20F over input temperature it would take longer for the heat pump to get up to it’s max. But that seems like a idealized example that only works with unobtainium.

Heat pumps decrease in efficiency as the ambient air temperature drops. They are extracting heat from the air outside so as the temperature falls there’s less heat to extract. At around 20F conventional heat pump efficiency drops rapidly, better compressors maintain more reasonable efficiency at colder temperatures. But no matter what, it will take more time to extract heat from colder air than warmer, unlike fuel based heat where the heat is produced by burning the fuel.

In normal operation heat pumps are incredibly efficient, producing more heat than could be obtained from the electricity used to power them.

This I know, but that’s not what I’m saying or seeing. Yes what you say is claimed in addition to this.

More like this situation, if you come home from vacation and set your thermostat from a vacation setback temperature to regular living temperature you have to wait longer for a heat pump to warm up the place. This statement seems to be in addition to what you state about lower temp’s = lower efficiency and capacity.

A heat pump is providing air that is warmer than your house, but not hot like a furnace. So, maybe it’s blowing 90 degrees and a furnace would be blowing 110 degrees. Both will warm up your house to 68, but the heat pump will take longer.

Would not this just make the heat pump less powerful than the furnace? I know a less powerful furnace would likewise take longer but should still achieve 68F. So is not the heat pump equivalent to a less powerful furnace? And why not just get a more powerful heat pump?

In theory a larger heat pump can heat just as fast but you’ll have to move a lot more air requiring additional or larger air ducts or more ductless heads. The solution is to use a remotely programmable thermostat so you can start warming your house before you arrive home from your vacation. For daily use just set your thermostat to start heating earlier. BTW: Hot air systems provide heat faster than other heating systems, if you switched to hydronic heat through baseboards and radiators, or conventional electric heaters, it would take much longer to get a room up to comfortable temperatures than with hot air.

As I understand it, it’s just the different ways they provide heat. The most powerful heat pump is the world will still put out heat at 90 degrees (using my example–I don’t really know the exact temp). They’re not designed (or capable) of putting out 110 degree heat. Burning heating oil or running an electric furnace can generate much hotter air. However, it will use much more energy to do it.

Furnaces put out 120+ air. All my registers measure 120F at least. As for the speed-to-heat of the heat pump, they provide nearly instant heat, just like an air conditioner provides cool air immediately. My ductless mini-split is a heat pump. I never measure the temperature, though.

OK, perhaps a rephrase, how can a heat pump be as powerful as a furnace but take longer. I’m not asking why it takes longer so much as that is easy, just use less power, but how can it be the same in BTU’s/h but take longer?

I think you’re getting confused on the definition of furnace “power”. Are you talking about the instantaneous Btu output of a furnace, or the hourly Btu capacity? Those are different things.

A fuel-fired furnace blows out air that is hot compared to the room; you can’t burn the fuel at only 5 degrees above room-temp; it’s always going to come out hot and therefore the systems are designed to come on, blow hot, shut down, then repeat. They can’t run at a low idle and blow luke-warm like a heat pump does.

In order to provide the same instantaneous output of a fuel-burning furnace, you’d have to massively over-size a heatpump. They aren’t designed to flick on and off at the same rate a furnance does, and need to run long and slow so to speak to be efficient.

I think it’s the instant vs hourly output ratings that’s tripping you up.

Perhaps, as far as I know a furnaces instant rating can be maintained indefinitely, so is also the hourly rating if run for a hour. But the same thing for a heat pump, if you run it for a hour. Though after that hour of run time the furnace should have warmed the room more because we are told that the heat pump is slower. So perhaps I don’t understand this instantaneous vs hourly BTU thing.

And no we are not talking about cycling the furnace nor multi-stage heat pumps, as we are talking about the situation where you want to raise the temperature from a low setting to a higher one, where both should run flat out till the temperature is reached (after which time the furnace can cycle and the heat pump can drop to a lower power level).

I just looked at this: Here's How to Pick the Right Size Heat Pump for Your Home - CNET. and perhaps I got my answer. For my home the recommended heat pump size is 36,000 BTU’s but my furnace is 65,000 BTU’s. So it appears heat pumps are underpowered, and why it takes so long compared to a furnace. I really would need to about double the heat pump capacity, after which I suspect the heating time would be the same.

So is this the case? And how can I get away with 36,000 BTU’s when in the coldest winters my furnace run times can hit 45 minutes per hour? Is there something about heat pump heating that could allow the same BTU’s to heat more? That seems to violate some laws of thermodynamics.

Umm, yeah.
It’s a pump.
That’s why they are so efficient - up to 3x the heat per input joule compared to a resistance or combustion heater. But, you can only pump so fast…

But the BTU rating is output, isn’t it?

A BTU is the amount of energy required to change the temperature of one pound of water by one degree Fahrenheit. (Its SI equivalent unit is the Joule, although the BTU is more than a thousand times larger.)

“BTU” in the HVAC context is actually BTU/hour. So it’s a measure of the ability to change temperture over a comparable period of time.

So a 75,000 BTU heater can raise temperature three times faster than a 25,000 BTU heat pump, because it can push in three times the heat energy in the same time as the heat pump.

So the recommendation is to underside heat pumps, and why they take longer to heat a room, and that’s all there is to why heat pumps take longer?

It’s really nothing more than “heat pumps deliver less heat per time than direct heating.” Less heat per time means it takes more time to raise the temperature to the same level. Like filling a bucket from a slower faucet rather than a faster one.

But the heat pump is hugely more energy efficient, and fine in general temperature keeping.

I think this is probably the right answer. The component of the furnace that actually generates the heat is a trivial part of it, so it can be sized to always output the maximum temperature (120 F, say) for a given airflow, even if the input is very cold. Operating under the same conditions, the heat pump gets the same airflow, but can only add so many degrees of heat at a time, perhaps outputting 60 F air when the input is 30 F. Eventually the system cycles the air enough to result in a comfortable temperature, but the furnace can heat it enough in one shot.

I don’t think it’s that heat pumps are undersized as it is that furnaces are typically oversized (because burners are cheap, whereas fans and ducts and so on are expensive).

Sure they can. You just turn down the flame size using the regulator in the gas control valve. Both of my garage spaces are heated with through the wall furnaces. The garage that I keep heated all winter is only 10’ x 22’. I purchased a used 20,000 BTU furnace which is way more then it would need for the well insulated space. When first installed it ran for a couple of minutes before shutting off. Not too efficient that way. I dialed down the gas pressure until the flame is quite small - a fraction of 20,000. Now it runs for much longer cycles and the air is cooler but still nice and warm.

I think a good part of the answer is that “heat pump” is the same mechanism as the A/C, and an oversized A/C creates major problems with humidity because it runs just long enough to cool but not long enough to properly dehumidify. Sizing a central air conditioning system correctly is very important.