Water heating is traditionally carried out mainly by burning Fuel (LPG or condensed hydrocarbon gas) The thermal energy generated during combustion. This is a heating method Wasteful Performance Coefficient (COP) Relatively low (value 9.0). Furthermore, this method is environmentally harmful, Both because of the combustion products emitted into the environment - carbon dioxide, The greenhouse gas which negatively contributes to the warming Global, and air pollutants) sulfur oxides, nitrogen oxides, Particles and the like (which are detrimental to our health - and following the feasibility of soil contamination In fuels. On the other hand, similar to the principle of action Of air conditioning system, heat pump One intermediate heat pump (environment) And transfers it to a second broker, for example for the purpose Water heating. In fact, the energy produced in the process (heat) Exceeds the amount of energy invested) electrical energy in operation The heat pump compressor (thanks to the "free" pumping of energy) Which is a penny in the air. This is due to the high energy efficiency Of the heat pump in relation to that of the high COP heaters: 5.2-4 value (about 80% of the thermal energy produced) It is obtained from the air, and only about 20% is a product of the electrical energy Which is invested in the heat pump compressor, making the heat pump the most popular means of heating water today. This form of work yields high system efficiency and is the solution Preferred for heating water in small outputs (home application) and outputs Major (institutional / industrial application).

Not in the UK winter climate Benyamin, where ambient temperatures drop way down near, or below, zero in the winter months, I guess you live in a warm climate, or have been talking to a good heat pump salesman!

Since adding this post, its reminded me about evaporator superheat, and now I'm guessing that it's getting that high enough in the winter months to sufficienly boil off refridgerant in evaporator to ensure only gas gets back to compressor in swimming pool applications, where the circulated water temperature is lower than a similar house heating application.

My dear friend John, 

I explain the principle of operation and for that you need to know physics (we are in a professional association and I hope you understand me), how a heat pump works At any temperature exceeding minus 273 degrees C, there is heat in the air, the pump knows how to extract the heat from the air at any temperature and transfer it to a desired location. Also, these pumps take advantage of the natural air feature to balance by moving hot air to colder places. On the other hand, using a small amount of energy the heat pump makes this process, by injecting heat from the cold air outside the interior of the house more hotter. To do this, water, pumps, compressors and piping are used. The outside air comes in contact with water-filled pipes with a lower temperature than the air, which is constantly moving inside them by means of a pump. As a result, the air is drawn to the cold water in the cold pipe. Subsequently, the water is heated and transformed into a gas that passes through a compressor that compresses the gas, helping it to warm up. The heated gas works its way through the piping until it is a heat exchanger, which comes in contact with the house's water piping and greenhouse. This causes the gas to cool and return to the liquid that makes its way out again and then back again.  

In summary I'm sorry you don't understand physics (because the post you uploaded is not at all related to hot weather). I would also love to have a deeper understanding of heat with a mathematical formula. have a good day.

My colleague Malcolm Davies, time III I explain.

I understood you well. You are looking for a solution for cold weather (below 0 degrees Celsius) an ideal solution for heating the water.

As an engineer, I know how to research solutions according to the laws of physics (if you want to teach you some general rules), according to a physical solution you can tell which heat system you would like to install.
You have an important and useful note and are:

In principle, water temperature is a function that is determined by personal preference. Some people prefer high water temperatures while others prefer cool, refreshing water, so in the bath and so in the pool. However, in pool climate planning, external influences such as weather conditions, humidity in the air and water temperature must be taken into account and it is important that synergy between them be created that will produce a pleasant water temperature. It is also important to note that at too high a water temperature in a humid space, microorganism and bacteria can develop, so it is important to be careful about temperature control and of course the cleaning and purification of pool water, heating pumps and all other pool systems.

Not as simple as you suggest  Benyamin.

Real refrigerants do not obey the simple secondary school gas laws and they have a limited temperature range over which they can be used, because at some upper temperature compression no longer liquefies the gas, and equally at some lower limit, rarefaction will no longer cause evaporation, even at near vacuum.


In practice with '410 systems are run with the cold 'suction' side at  perhaps 50-100 PSI, and the hot compression side at a few hundred PSI. Much more or less than this and the  flow rates and rates of heat transport are not sensible with practical pipe sizes and pumps. This in turn tells you  the temperature limits for liquefaction and evaporation.  some info on R410A



The theoretical best is indeed that the heat transfer is in the ratio of the absolute temperatures - consider for an example  pumping between  300K (a rather toasty 23 degrees for indoors in the UK) from an outside temperature of 260 ( -13  which is  cold for an outside air temperature, but not incredibly cold for the cold parts of a heat pump working hard if it is air source against a frosty night.) the expected heat might be in the ratio 300/260 - so we put 40 watts in to suck 260 watts in from outside and push 300 into the room.  That would be a COP of 300/40, or about 7.
It is never that good, or indeed anything like that good. Half that would be very good.


In configurations  where the input icing up is a consideration ,the unit has to periodically reverse the heat flow to heat up and de-frost the outdoor unit, normally fans are stopped during this time, to avoid wafting cold air into the building and to stop sucking cold air over the frozen input heat exchanger. Sensing ice build up accurately, so as to neither perform unnecessary defrost cycles, nor to try and run with a frozen heat exchanger is an ongoing challenge. (i.e. there is as yet no 'one size fits all' reliable way)


There are plenty of good articles on the web, explaining various sources of non-ideality  Nordic heat pumps for example   .

Can I suggest that you may benefit from some background reading before commenting much further.

re-reading my post that the last bit may have come over as a bit condescending.

That was not the intention.  It was really to say 'this  is not so simple, and the school explanation is not really good enough' .

And if ever it gets so cold or over pressure that the pump sucks in liquid instead of gas, then  it has had it !


(Note that most of the modern refrigerant gasses are blends and have a range of temperature over which liquefaction/ evaporation occurs - rather as plumbers solder goes through a phase of being a bit pasty in between fully solid and fully liquid, - the exact gas blend 'slides' so that on expansion  the vapour is slightly richer  in the more volatile part, and in compression the liquid has more of the less volatile)

Assuming the designers have done their stuff there is no need to worry about that.