Heat Pumps for dummies (beginners guide)

[AC_Hacker]

Originally Posted by student

...once i know that i will re-ask the question of how long a ground loop i need and what size push pull pumps i need, but a great many thanks to all who write here, and the site operator.

If you are considering a heat pump system, you should consider including PEX tubing in the insulated foundation slab.

Radiant heating is quite inexpensive at the time of construction, but is fairly expensive (and less efficient) as a retrofit.

-AC

[jeff5may]

Now that you have assembled a plan and have the money, it is time to begin the project. If you are having the work done, simply pay the money and watch as your plan is implemented. There may be minor setbacks and decisions to make, but a competent firm or contractor will have considered nearly all of the unknowns during the estimate and padded their budget accordingly. Once the project is completed, operation should be demonstrated as well as any maintenance or upkeep needs of the system. The project will be guaranteed for a certain amount of time, and you may receive free follow-up visits. Enjoy!

For the rest, now is the time to do the work. Depending on skill level, this may be just work, or this may become a nightmare. Having had your plan inspected previously by experienced professionals, most of the traps and guesswork should have been eliminated by now. But every job is different, and details will pop up that were never even considered. For this reason, unless you are yourself an experienced building professional, advice should be sought from previously designated mentors who helped devise the plan.

Just "winging it" on your own has been proven to be deadly in some cases, and costly in most. Unless you have a lifetime to complete the project, or simply enjoy the pain+pleasure of learning a new skill set the hard way, some of the work will have to be contracted out. Skilled tradesmen have a way of making the supposed impossible look easy, and can make otherwise difficult decisions seem trivial. They get paid well to do what they do for a reason.

If you run into an unplanned issue that looks like a trap, it probably is. At least have someone with experience come look at it and offer advice before you destroy something. As stated before, there is a right way to perform any given task. There are many unseen hazards present in building science and the HVAC trade, and professionals know how to identify and prevent unsafe conditions in realizing an end product. Injuries come in many flavors, and avoiding one is priceless. If you do something dumb, you may not be tough enough to survive.

At this point, it becomes vitally important to ensure the work being performed is as planned. If not done correctly, problems will pop up in the future, necessitating rework. If the work does not perform as planned, either the plan must be revised or the quality improved to match the plan. Material and labor cost should take a back seat to completion of the design. A couple extra dollars or hours invested to do the job right is cheap insurance against a poorly performing or failed end product.

When the hard labor and equipment is put in, the system may then be tested and commissioned. Depending on the system, some trials, programming, and training may still be required to ensure the system does what it should, when it should, how it should. Most likely, the completed project will need to be inspected and signed off by at least one licensed inspector.

[charlesfl]

Dumb question.

Given the Carnot cycle limits how is the efficiency achieved with lower temperature difference for gshp?

Is it by lower compressor amperage used, shorter run times or something else?

[AC_Hacker]

Originally Posted by charlesfl

Dumb question.

Given the Carnot cycle limits how is the efficiency achieved with lower temperature difference for gshp?

Is it by lower compressor amperage used, shorter run times or something else?

Could you please elaborate on your question.

I am somewhat familiar with Carnot and his efficiency theorem, but I'm not sure I understand how you are considering the Carnot cycle in asking your question.

Best,

-AC_Hacker

[mejunkhound]

Originally Posted by charlesfl

Dumb question.

Given the Carnot cycle limits how is the efficiency achieved with lower temperature difference for gshp?

Is it by lower compressor amperage used, shorter run times or something else?

Carnot is inversely involved with a heat pump cycle.

Look at it this way: it takes work to pump water uphill, in a like manner it takes work to move heat from low temperature to higher temperature.

Thus, lower temperature difference (as in GSHP from 55F earth to 100F air vs. 20F air to 100F air) means less work to pump heat to higher temperature, just like less work to pump a given quantity of water fewer feet uphill.

[charlesfl]

I understand that less temperature differential means less work.
Less work means less electricity used. How is the electricity used less, less amperage use by the compressor or run for a shorter time.

Sorry, I am not being very clear.

[jeff5may]

Originally Posted by charlesfl

I understand that less temperature differential means less work.
Less work means less electricity used. How is the electricity used less, less amperage use by the compressor or run for a shorter time.

Sorry, I am not being very clear.

With respect to total energy consumed, it can go either way. All the compressor really cares about is compression ratio, as far as the energy it consumes. With a constant-displacement, constant-speed, residential HVAC compressor, the current consumed is closely related to the difference between (absolute) suction and discharge pressures. These two pressures determine the compression ratio the motor must work against. When the CR is low, so is the current consumed. As the CR rises, so does the load current.

Run time is more dependent on mass flow of refrigerant. The mass flow versus temperature gradient determines the system capacity and the amount of heat transferred by the phase change cycle.

In cooling mode, the compressor will see moderate suction pressure and high discharge pressure. The condenser pressure follows outdoor temperature, so when it is "only warm" outside, it sheds heat well, keeping CR low. Lots of mass flows, and lots of heat is transferred at high efficiency. The hotter it gets outside, the higher the discharge pressure rises, the more current the compressor draws, and the less mass that flows through it. Run time increases accordingly.

In heating mode, the compressor will see low suction pressure and moderate discharge pressure. The evaporator pressure follows outdoor temperature, so when it is "only cool" outside, it gathers heat well, keeping CR low. Lots of mass flows, and lots of heat transfers at high efficiency. As outdoor temperatures drop, evaporator pressure drops, load current drops, and mass flow through the compressor drops due to reduced suction pressure. Runtime increases accordingly.

If the outdoor temperature drops enough, the compressor will be starved of refrigerant due to very low suction pressure. There will not be enough refrigerant flowing through the compressor to cool the motor windings, causing CR to skyrocket along with discharge temperature. This condition can burn motor windings, valves, and/or oil, causing compressor burnout and system failure.

If the evaporator coil freezes up, the same "starvation" condition is forced on the system due to the drastic reduction in heat flow through the frozen heat exchanger. If a heat pump is not designed for cryogenic conditions, it is equipped with some sort of defrost control to protect the compressor. Some units merely stop the compressor until the evaporator warms enough to support operation again. Others have an active "reverse cycle, hot gas defrost" function. Others have some sort of electric defrost heater built in. Most units will not resume cooling mode of operation until the evaporator is well above freezing temperature.

The ground-source (aka geothermal) heat pump does the best when there is a supermassive outdoor heat source that remains mainly constant in temperature, regardless of outdoor (weather) conditions. Due to the relatively constant temperatures of both outdoor and indoor heat exchangers, the performance of the unit is always very predictable, so heating and cooling circuits and controls can be optimized for maximum energy savings. In extreme weather conditions, the energy savings compared to a standard air-source unit add up very quickly.

[charlesfl]

Excellent, exactly what I was looking for. Thanks so much, Charlesfl

[buffalobillpatrick]

Jeff5May: your:

"As outdoor temperatures drop, evaporator pressure drops, load current drops, and mass flow through the compressor drops due to reduced suction pressure. Runtime increases accordingly."

Wouldn't load current go up?

I thought it went like this in heating mode:
Hotter Evaporator -> Higher suction pressure -> Lower CR -> Higher mass flow -> Lower Compressor Amps -> Higher COP
Colderer Evaporator -> Lower suction pressure -> Higher CR -> Lower mass flow -> Higher Compressor Amps -> Lower COP

[jeff5may]

Nope, It doesn't work that way.
In heating mode, it goes like this (even with a cap tube):

Hotter Evaporator-> Higher suction pressure -> Higher mass flow -> Higher CR -> Higher compressor Amps -> Higher COP and discharge temperature

Colder Evaporator -> Lower suction pressure -> Lower mass flow -> Lower CR -> Lower Compressor Amps -> Lower COP and discharge temperature

Frozen Evaporator -> Lowest suction pressure -> Lowest mass flow -> Highest CR -> Highest Compressor Amps -> Lowest COP and Highest discharge temperature