Back Up Heat Source and Boosting Loop Temps

[AC_Hacker]

Originally Posted by Blue Bomber Man

The idea was brought up to put say a 1 kw electric heat to prewarm the loop temp and was believed by the original poster to allow the heat pump to perhaps get 2 kw heat back from this extra heat in the loop.

This would violate conservation of energy.

I can't tell if you are joking or not with this post.

The whole idea of a COP > 1 with a heat pump is that it is implicitly understood that the heat pump can 'increase the quality' of low grade heat and make it usable to us. It increases the quality by raising the temperature.

So, with a heat pump, 1 watt in usually results in a minimum od 2.5 watts out, sometimes as much as 5 watts out. One of those watts is the heat produced by the compressor running, the rest is due to the utilization of low grade heat that otherwise would not be usable.

If you live in a land where the ground temperature is 0 degrees K, I would suggest that you move to a warmer, sunnier climate... like maybe Siberia.

-AC

[Daox]

Hes simply trying to clarify why adding 1kW of electric resistance heat to the ground loop will not result in a boost of more than 1kW to the house loop.

[jeff5may]

Originally Posted by wewantutopia

My thought of preheating the ground loop would be for "emergency heat" only insead of the resistance heater.

I undersand they have a COP of 5 or higher. If I understand correctly, if you add an additional 1kw to raise the temp of the water you would get 4kw of heat out of the pump. I you use 1kw resistance heater you only get 1kw heat.

Again, this is only me pondering these questions and looking for feedback.

The sentence describing the resistance heat is correct. With resistive heating, the effect is an increase of sensible heat from the energized device. A piece of wire, when energized, experiences electric "friction" and the temperature rises. You cannot get more energy radiated from the wire than the current that runs through it. Watts = volts x amps. The end.

The heat pump is a whole different animal. It doesn't generate most of its heat. The input energy moves the refrigerant around a loop with a compressor. If the compressor shell isn't insulated, a decent portion of the generated electric heat leaks out of the shell.

With a hermetic electric compressor, most of the energy consumed is turned into rotational mechanical kinetic energy, which spins a scroll, piston or drum to move refrigerant gas from low to high pressure. As the gas is pressurized, it naturally heats up, following the ideal gas law. Its condensation temperature (dewpoint) also rises due to the pressure increase. Yes, some of the electric resistive heat ends up in the gas, but as the gas pressure increases, the heat flow is pinched off by the natural increase in gas temperature.

At some point, the compressor pushes so much gas into the high side of the loop that it cannot exist as a gas below the temperature of the plumbing that contains it. At this temperature and pressure, the gas drastically reduces in size, condensing from a gas to a liquid and heat is released. Latent heat. Lots of it. The heat moves through the walls of the plumbing wherever it can and is released into the environment on the other side of the walls.

In reality, the "plumbing" is mostly a high-efficiency heat exchanger (condenser), designed to aid this transition of gas to a liquid. What happens is that the heat generated by compressing this gas is harvested as it condenses. The gas keeps on giving up heat ,at its raised transition temperature, until it is all a liquid.

Without a method to escape, this high pressure would build up, and the plumbing would eventually burst. Heat pumps are designed with a metering device at the liquid end of the condenser that keeps this from happening. As the high pressure liquid travels through the metering device, it experiences a massive pressure drop. At the exit end, the liquid is above its boiling point.

The liquid quickly begins absorbing massive amounts of latent energy, because it cannot exist as a liquid above its boiling temperature. The resulting temperature drop is harvested in another heat exchanger (evaporator), and heat flows into the refrigerant to complete the boiling process. The refrigerant leaves the heat exchanger as a low-pressure gas, saturated with latent heat absorbed in the boiling process.

The latent heat I describe is the "magic" in the system. Just like water, it takes much more heat to boil the refrigerant than it does to change its temperature. If you put a glass of water in a vacuum chamber, it will boil off as the pressure drops below its reduced boiling point. As the water boils, its temperature will drop to its ever reducing boiling point until it is all vapor or it freezes. At a certain low pressure, the ice will "sublimate", skipping the boiling process entirely and evaporating spontaneously like dry ice.

If this leaves you with confusion, the subject is covered in the sticky at the top of this forum.

[jeff5may]

I found a diagram that illustrates the associated heat transfer values in a typical system:

[wewantutopia]

Hi everyone.

I got my gas bill for the period of 12/20/2013 - 1/22/2014; this period includes the polar vortex and the rest of the extra cold month.

In that period we used 72.42 therms or 2.19 therms per day. That comes to 9125 btu/hour average.

What would you guys suggest as a GSHP size?

[NiHaoMike]

Originally Posted by wewantutopia

Hi everyone.

I got my gas bill for the period of 12/20/2013 - 1/22/2014; this period includes the polar vortex and the rest of the extra cold month.

In that period we used 72.42 therms or 2.19 therms per day. That comes to 9125 btu/hour average.

What would you guys suggest as a GSHP size?

Something like 12000BTU/hr or so. You can go a bit higher (18000BTU/hr or so) for an inverter unit.

[wewantutopia]

Thanks for the reply.

If we get a 12,000 btu unit does that mean in the coldest months it will run 45 minutes of every hour?

That seems like a lot. Is it?

[Mikesolar]

Don't forget that the HP will not give full output most of the time. Size it up to give some extra capacity for when the temp is below freezing. It is not a simple equation as far as the running time is concerned as the pot is running most of the time, sometimes faster, sometimes slower.

[NiHaoMike]

Geothermal heat pumps aren't affected much by outdoor temperature.

[Mikesolar]

ahhh, I missed that. I figured he was using a minisplit as it was to be 1-1.5 ton