Do-it-yourself heat pump for home heating

Recommendation points

• A bit of theory
• Where is it better to “take away” heat
• Schematic diagram
• Evaporator and Condenser Manufacturing
• Installation of the circuit

Unlike alternative energy devices such as solar panels and wind turbines, the heat pump is less well known. And in vain. The most common ground-water scheme works stably and does not depend on weather or climatic conditions. And you can make it yourself.

A bit of theory

It is easiest to use the natural heat of the earth to heat a home if there is geothermal water in the region (as is done in Iceland). But such conditions are rare..

And at the same time, thermal energy is everywhere – you just need to extract it and make it work. This is what the heat pump is for. What it does:

• takes energy from low-temperature natural sources;
• accumulates it, that is, raises the temperature to high values;
• gives it to the coolant of the heating system.

In principle, a standard compressor refrigerator circuit is used, but “vice versa”. A natural heat carrier circulates in the first circuit. It is closed to a heat exchanger, which acts as an evaporator for the second circuit.

1 – earth; 2 – brine circulation; 3 – circulation pump; 4 – evaporator; 5 – compressor; 6 – capacitor; 7 – heating system; 8 – refrigerant; 9 – choke

The second circuit is the heat pump itself, inside of which there is freon. A heat pump cycle consists of the following steps:

1. In the evaporator, freon is heated to boiling point. It depends on the type of freon and the pressure in this part of the system (usually up to 5 atmospheres).
2. In a gaseous state, freon enters the compressor and is compressed to 25 atmospheres, while its temperature rises (the greater the compression, the higher the temperature). This is the phase of heat accumulation – from a large volume with a low temperature to a small volume with a high temperature.
3. The gas heated by pressure enters the condenser, in which heat is transferred to the coolant of the heating system.
4. After cooling, the freon enters the throttle (aka flow regulator or thermostatic valve). In it, the pressure drops, freon condenses and returns to the evaporator in the form of a liquid.

Where is it better to “take away” heat

In principle, there are three media from which heat can be “removed”:

1. Air. At normal pressure, all types of freons boil at negative temperatures (for example, R22 – about -25 ° C, R404 and R502 – about -30 ° C). But for circulation in the system, it is necessary to create excess pressure already at the first phase – evaporation. The same 4 atmospheres in the evaporator require the outside air temperature to be at least 0 ° C for R22 and -5 ° C for R404 and R502. In our regions, this type of heat pump can be used for heating in the off-season and for hot water supply in the warm season..

2. Water. This is a more stable source of heat, provided that the reservoir does not freeze to the bottom in winter. But the house should not just be next to a lake or river, but be on the first line.

3. Earth. The most stable source of heat energy. You can use two schemes – horizontal and vertical. Horizontal seems easier because it doesn’t require drilling. But a large amount of earthwork will have to be done to dig a system of trenches to a depth below the level of freezing of the soil (for mid-latitudes, it ranges from 1 meter in the west of the European part of the country and up to 1.6-1.8 closer to the Urals, in Siberia the situation is “even worse “The vertical scheme is more versatile and efficient, but requires drilling to a considerable depth. Although several shallow wells can be used instead of one deep one..

Schematic diagram

The heat pump circuit itself is simple: evaporator – compressor – condenser – choke – evaporator.

The heart of the circuit is the compressor. You can buy a new one, but it’s cheaper to find a used one. Naturally, we are not talking about low-power compressors for household refrigerators, but about models installed in split systems. It is necessary to focus not on power consumption, but on power in heating mode (which is 5–20% higher than in cooling mode).

The compressor model is chosen according to the ratio of 1 kW per 10 sq. meters of heated area.

Attention! The power can be indicated not only in kW, but also in BTU (English unit of measurement of thermal energy, adopted for climate technology). The conversion is easy – divide the BTU value by 3.4.

When calculating the parameters of a heat pump, including heat exchangers, software is used for modeling, calculating and optimizing cooling systems, for example, CoolPack

Already at the stage of calculations (or rather, when specifying the “input”), you can optimize the system by choosing the optimal thermal conditions.

Using a heat pump is effective for low-temperature heating systems, for example, for underfloor heating with a temperature not exceeding 35–40 ° C. By the way, the same temperature is recommended for medical requirements for the DHW system.

For each type of freon there are optimal temperatures of “inlet” and “outlet”, more precisely, boiling and condensation, but the difference in all of them is no more than 45-50 ° C.

It would seem that an increase in the temperature at the outlet of the heat pump will have a positive effect, but this is not the case. The temperature difference will also increase, which will lead to a decrease in COP (conversion factor, or efficiency of a heat engine). In addition, this will require the use of a more powerful compressor and additional energy consumption..

The ideal COP cannot be achieved (losses in the compressor, power consumption, heat losses during transportation within the system, etc.), so the actual values ​​usually lie in the range from 3 to 5.

There is another way to increase efficiency – the use of a bivalent heating circuit.

In reality, the operation of the heating system at full capacity is only needed for 15–20% of the entire season. During this time, you can use additional heating devices (for example, a ceramic heater or convector). Reducing the design thermal power up to 80% will save on the compressor, reduce the depth of the well or the length of horizontal pipes, and reduce energy consumption for servicing the heat pump itself.

The design of the horizontal or vertical ground heat exchanger depends on the given rated output of the heat pump and COP. On average, 20 W is removed from each meter of the “horizon” (with a pipe-laying step of at least 0.7 m), and from the “vertical” – 50 W. But the specific values ​​depend on the type of rock and its moisture content. The best values ​​for groundwater.

Interesting! There are also other ground heat exchangers – “spiral” or “basket”. In fact, it is a vertical probe made of a pipe in the form of a spiral, which allows to reduce the drilling depth..

After determining the length of the horizontal circuit or the depth of the vertical probe, calculate the dimensions of the evaporator and condenser.

Evaporator and Condenser Manufacturing

You can buy ready-made heat exchangers for both the evaporator (at low pressure) and for the condenser (at pressures up to 25 bar). But it is cheaper to make them from a copper tube for air conditioners (which is designed specifically to work with refrigerants at high pressure) and improvised containers.

Important! Plumbing copper pipe is not so clean and flexible. It is worse to solder and roll it during installation.

The surface area of ​​the heat exchanger is calculated, which is directly proportional to the heat release power and inversely proportional to the temperature difference of the heat carriers at the inlet and outlet of each connected circuit (ground and heating system).

Knowing the pipe diameter and surface area, determine the length of each coil for the evaporator and condenser.

It is better to make a container for a capacitor of stainless steel (the temperature of the incoming freon vapor can be quite high):

• take a ready-made tank of a suitable capacity (so that a spiral from a copper tube fits);
• place a coil in it (entrance at the top, exit at the bottom);
• remove the ends of the copper tube for connection to the compressor and expansion valve (by soldering or flange);
• make a tie-in of adapters in the tank for connecting pipes of the heating system;
• weld the lid.

The evaporator operates at lower temperatures, so you can take a cheaper plastic container for it, into which adapters are cut to connect to the ground loop. It also differs from the condenser in the location of the heat exchanger coil – inlet (liquid phase of freon from expansion valve) from below, outlet to the compressor from above.

Installation of the circuit

After the heat exchangers are manufactured, the gas-hydraulic circuit is assembled:

• the compressor, condenser and evaporator are installed in place;
• braze or flange copper pipes;
• connect the evaporator to the ground circuit pump;
• connect the condenser to the heating system.

1 – circulation pump of the soil circuit; 2 – evaporator; 3 – exit of the soil contour; 4 – thermostatic valve; 5 – compressor; 6 – to the heating system; 7 – capacitor; 8 – heating system return

The electrical circuit (compressor, ground loop pump, emergency automation) must be connected via a dedicated circuit, which must withstand fairly high starting currents.

It is imperative to use a circuit breaker, as well as an emergency shutdown from the temperature switch: at the water outlet from the condenser (with overheating) and the brine outlet from the evaporator (with overcooling).