HVAC Heat Pump

[Isaac Zachary]

Originally Posted by redpoint5

My air handler fan is 700 watts, single speed. If AC or heat is called, 700 watts. No idea what the AC condenser fan draw is. There's a tiny motor in the furnace to move the exhaust.

Ok, but what is the power of the entire system? And without any heat pump, wouldn't the air handler blower still be needed?

On the other hand, I've always thought that efficiency could be gained by heating or cooling the floors, walls and ceilings simultaneously. The greater the difference in temperature, the greater the efficiency of a heat exchanger. If everything is heated then they only need to be heated to 70 °F. This is different than in a forced air system where the hot part needs to be at least some 110 °F in order to be effective.

[freebeard]

On the other hand, I've always thought that efficiency could be gained by heating or cooling the floors, walls and ceilings simultaneously.

The home my parents built in 1980 had radiant floor heat and a shape that conserved energy. My brother said he let himself in after it had sat vacant for two weeks in the Winter, and it was comfortable; just from passive solar.

[Isaac Zachary]

Originally Posted by freebeard

The home my parents built in 1980 had radiant floor heat and a shape that conserved energy. My brother said he let himself in after it had sat vacant for two weeks in the Winter, and it was comfortable; just from passive solar.

I knew a family that made a house like that, and in a town where not far from there I experienced temps as low as -52 °F. It was also very windy all the time. The house had that same geodesic design, and the bottom was a huge sun room with a cement floor. They could leave the heat off all winter long and not have frozen pipes.

[redpoint5]

Originally Posted by Isaac Zachary

Ok, but what is the power of the entire system? And without any heat pump, wouldn't the air handler blower still be needed?

My AC runs about 5kW total. My furnace 120k BTU 80% efficiency, plus the 700 watt blower. The blower adds on regardless of AC or heat.

On the other hand, I've always thought that efficiency could be gained by heating or cooling the floors, walls and ceilings simultaneously. The greater the difference in temperature, the greater the efficiency of a heat exchanger. If everything is heated then they only need to be heated to 70 °F. This is different than in a forced air system where the hot part needs to be at least some 110 °F in order to be effective.

There's efficiency of heating everything, and efficiency of heating what needs to be heated.

Our comfort depends on the micro layer of air closest to our skin. We can maintain total comfort if that micro layer is at ideal temperature, which is why we layer clothing based on temperature.

It seems a simple matter to me to have an IR emitter track subjects in a room and deliver a custom "dose" of heat. Heating everything is stupid when the subject is known.

[Piotrsko]

Never thought about it that way, but makes sense.

Figure my heat pump system is the same at max heat as redpoint5 cooling system because it's pretty much the same design just routed differently. Using the heat generated by cooling the already cold outside. For example: it is 44 outside front porch but the temp outside the heater fan is 5 degrees according to the heat pump outside thermostat. That delta means 72 degrees radiated into house

[Logic]

These fifty-eight square meters of mirrors produce the same amount of thermal energy as fifty square meters of similar solar collectors, but my solar station is ten times cheaper.

https://www.youtube.com/watch?v=JB_i0z-C_tI

[Isaac Zachary]

Originally Posted by redpoint5

My AC runs about 5kW total. My furnace 120k BTU 80% efficiency, plus the 700 watt blower. The blower adds on regardless of AC or heat.

So assuming the other fans amount to 300W, that's 20% for blowing air, which means the system is at most 80% efficient.

On the other hand any inefficiency, including the eventual slow down and stop of the air movement due to friction, means the fan system is 100% efficient for heating purposes.

Originally Posted by redpoint5

There's efficiency of heating everything, and efficiency of heating what needs to be heated.

Our comfort depends on the micro layer of air closest to our skin. We can maintain total comfort if that micro layer is at ideal temperature, which is why we layer clothing based on temperature.

It seems a simple matter to me to have an IR emitter track subjects in a room and deliver a custom "dose" of heat. Heating everything is stupid when the subject is known.

Yes, you are right, but that's not what I was talking about.

You could heat each person individually, but if you're heating the whole room or house anyway, then talking about heating individuals is irrelevant to your central heating or cooling system.

If the system is centralized it means you need a greater heat difference, which is less efficient for heat pumps. It's much more efficient to pump heat out of 30 °F outside air and feed that into 70 °F walls, floors and cielings than to pump heat out of 30 °F air to heat up a centralized heat exchanger to 110 °F or better. The lower the heat difference the more efficient for the heat pump. But centralized heating requires higher heat differences between different points along the system.

[JSH]

Originally Posted by Piotrsko

Pretty sure a heat pump doesn't do anything near 100% efficiency because moving air with a fan is horribly inefficient. Think they quoted 85%. In my case I waste 2500 watts just running air through with the all 3 of the fans on: 2 outside, one in the heat exchanger. Add another amount for the compressor if it's needed and your talking serious wattage

Heat pump efficiencies vary based on the temperature difference between the outside air and the desired inside temperature. The larger the difference the lower the efficiency This efficiency is called COP or Coefficient of Performance.

At a temperature delta of 20F a typical air source heat pump has a COP of 3.7 which means it is 370% efficient. That steadily drops as the difference increases. 30-40 years ago a heat pump would hit a COP of 1 at about 20F. That is why they had electric resistance coils in them that would kick in at cold temperatures.

Heat pumps have steadily become more efficient over time and that COP of 1 dropped to 0F and new modern cold weather heat pumps can reach -10 or -15F before getting to a COP of 1. That is why heat pumps no longer have electric coils in them. Most modern heat pumps are still at a COP of 2 at 0F

(Heat pumps are more than 100% efficient because they are using the phase change of a gas to MOVE heat from outside to inside.)

1000 watts = 3,413 BTU / hr This is one for one for an electric resistance heater.

For a heat pump with a COP of 3.7 you are getting 12,624 BTU / hr from 1,000 watts input.

When you look at a energy star label for a heat pump you will see a Heat Efficiency Rating. Divide that by 3.41 to get the COP. This window unit with a HSPF of 10 has a COP of 2.97 and is 3x more efficient than running a 1000 watt space heater.

https://www.mideacomfort.us/packaged.html

[JSH]

Originally Posted by Isaac Zachary

So assuming the other fans amount to 300W, that's 20% for blowing air, which means the system is at most 80% efficient.

That assumes a heat pump's maximum efficiency is 100% but it is not. The theoretic maximum efficiency of a heat pump for 10C to 60C is 670%. When we look at heat pump efficiency ratings that includes 100% of the electrical draw, pumps, fans, wifi, little light that says the temp.


Both you and Freebird would likely come out ahead adding a window heat pump. It wouldn't help you at night in the dead of winter but it would be much more efficient than gas or electric resistance heating during the shoulder season and when temps are at freezing or above. For Freebird here in Oregon a heat pump would pretty much always outperform electric heat.

It comes down to what you pay for a therm of NG vs a kWh of electricity.

[redpoint5]

Originally Posted by Isaac Zachary

So assuming the other fans amount to 300W, that's 20% for blowing air, which means the system is at most 80% efficient.

The sneaky thing is that blower motor consumption isn't factored into efficiency ratings whatsoever, which is why I have a grossly inefficient blower motor that can only run at a single speed. The 80% rating is for the heat exchange of burning the natural gas only. 20% is lost because the combustion air comes from inside the house, and gets tossed out as exhaust. An efficient system doesn't use already warmed air that has to be replaced by drawing in outside cool air, but instead draws the cool air from outside and combusts it in isolation of the heated air inside.

If the system is centralized it means you need a greater heat difference, which is less efficient for heat pumps. It's much more efficient to pump heat out of 30 °F outside air and feed that into 70 °F walls, floors and cielings than to pump heat out of 30 °F air to heat up a centralized heat exchanger to 110 °F or better. The lower the heat difference the more efficient for the heat pump. But centralized heating requires higher heat differences between different points along the system.

Our microclimate isn't attached to walls and the ceiling, it's the air we're surrounded by. The point is to maintain comfortable air temperature, not comfortable wall temperature. The fact that walls and the ceiling reach room temperature is simply an outcome of having heated the air within.

An ideal envelope would have a vacuum space so that convection and conduction was completely eliminated as pathways for heat loss, and a reflective layer that traps radiative heat.

Instead of many feet of insulation, we should be building with thin layers of vacuum panels, but then we couldn't have monkeys with hammers and nails banging around everywhere willy-nilly.