Solar water heaters and washing machines

Many people wonder if they can connect their existing washing machines and dishwashers to the hot water supply. The reason is obvious, feeding with hot water would theoretically reduce electricity consumption and consequently the height of bills. The answer is not simple and certainly not what we would all like to hear. However, there is an approach that can work and you will learn it if you read the article to the end. Before we start examining the issue, it’s good to clarify from the beginning that there are washing machines that come from the factory with two separate water inlets (hot and cold). In this article we do not examine these washing machines but those with one (1) cold water supply which are also the most common.

Who hasn’t wondered if the surplus water from the solar water heater is suitable for use in washing machines?

In Greece and for most of the year, solar water heaters produce more water than we exactly need for use and this is not bad as solar energy is free. Even during winter, water can become lukewarm and compared to the cold water supply, the temperature difference can be quite large.

Most of a washing machine’s consumption comes from the process of heating cold water to the temperature selected by the device user

Since this happens, why not direct it to washing machines to reduce the electricity bill?

The answer is no, at least not before someone fully understands how washing machines work and what is good for washing clothes.

Hot water can be too hot

We mainly talk about the solar water heater (there may be surplus hot water from the central heating boiler or some wood water heater), but whatever the source of hot water, it can be hotter than necessary. The inlet water from these sources could easily exceed 60 degrees and this would endanger both our washing machine and our clothes.

There is no way to control the temperature of water entering the washing machine and tub. A washing machine designed for two supplies (hot and cold) will handle hot water either by mixing it with cold to reach the appropriate temperature or will use hot water only in phases that really need it. But this is decided by the brain
(the electronic circuit) of the washing machine which has been programmed to control two supplies and create the appropriate water for each phase of a wash.

Wash phases are affected by water temperature

Most wash programs need warm water at 30-40 degrees Celsius. It makes no sense to feed such a program with water at 60 or more degrees. Let’s not forget that water from solar heaters can reach much more. Beyond possible damage, washing will not be performed according to program. The thermostat will close very early and the current wash phase will be completed prematurely. This will shorten the total time but also degrade the result.

Hot water is not good for biological detergents

Biological detergents contain enzymes. These enzymes are killed if the water is too hot. So in this case too, the supply of irregularly hot water has negative effects since in most cases their mode of action requires initially colder water and then to be gradually heated to the degree desired by the wash program.

Spinning with hot water can destroy some clothes – fabrics

If a washing machine is connected only to hot water supply, then reasonably when the time comes (wash phase) for spinning, the water will also be hot. If the contents are spun while having high temperature, wrinkling will be very intense and ironing… a nightmare! If we save some energy from heating water, we will probably give it to an even greater degree to the electric iron. This is not what we want to achieve. Also, hot water activates the detergent more and this practically makes the final phase and spinning difficult.

Simple mixing of hot and cold water before giving it to the washing machine?

Giving hot water directly to the washing machine supply was ruled out as an option for the reasons mentioned above. Making a manual Y-type mix with hot water and cold water “joining” and continuing without control to the washing machine is an option? No again. Very bad for nothing in a situation where essentially no control is feasible and the gain is probably minimal. Indeed, this way the water will not be very hot but someone will have to manually adjust the pressure of each supply each time to achieve… the ideal temperature. It is also known that the washing machine draws water at time intervals which are short, so until the hot water runs in the piping network, it is most likely that it has already taken the water it needed as a quantity. In reality, this is also one reason why cold water washing machines have ultimately prevailed, since practically dual-supply washing machines did not make the expected use of hot water, but considerably less. During washing there would be less hot water and during rinsing more since more quantity is needed there. Not very good synchronization – timing.

Thermostatic valve, does it solve the problem?

A thermostatic valve is nothing more than a mixer that receives two supplies (hot and cold) from one side and provides water at a constant temperature from the other. Depending on the product, the output water temperature can be adjusted with small or greater accuracy. It’s good for the temperature to be checked manually by the user (with a thermometer) in case of unexpected operation. This way we solve two basic problems concerning feeding the washing machine with hot water:

• We have hot water within safety limits
• We have a steady supply of this water and not with gaps due to heat losses from piping

A thermostatic valve has low cost relative to what it offers. We install it before the washing machine supply and solve the issue that concerns us? Again not absolutely. We achieved hot water regulation, but the last problem is that in the final phase of a wash… we don’t want it! The vast majority of washing machines are single cold water supply, so at the end they simply use cold water as they should. On the other hand, dual-supply washing machines are equipped with a corresponding brain which knows how to handle water according to the phase the wash is in. Single cold water supply washing machines have a brain that doesn’t concern itself with the issue since whatever water we give it, it considers it cold. So it will proceed to spinning normally even if it has very hot water for this phase.

Initial conclusion, secondary thoughts

The thermostatic valve solves the majority of problems caused by supplying hot water from the solar water heater to the washing machine. However, it cannot be used simply without understanding how the washing machine operates. If we have it set to give us water at 40 degrees for example, we will achieve great savings from water heating, but we will most likely have problems and “destruction” to our clothes.

It should therefore be set to the lower operating limits to avoid heating the cold network water (especially in winter) from the minimum temperatures approaching 0, but giving room to the washing machine brain to heat the water a little when needed and to have control over the maximum temperature and not the thermostatic valve.

A solar water heater during the year in Greece for the most part has the ability to heat water to very high temperatures, higher than what a washing machine needs. Direct supply will certainly at some point, sooner or later depending on circumstances, destroy the washing machine. The thermostatic valve protects us from this case. By setting it to lower temperature levels we also protect our clothes (user tests will prove the real situation regarding clothes). During winter the solar water heater may not heat water sufficient for our bath, but as a supply for the washing machine it is a very beneficial source since it has a noticeable difference from the cold network water. Even this difference can save energy. Whether it’s winter or summer, if the water in the solar water heater is hot or lukewarm doesn’t concern us since the thermostatic valve takes control.

So far we have seen enough information about how single-supply (cold water) washing machines work, why we shouldn’t feed them with water directly from the solar water heater or other hot water source, what thermostatic valves are and how they can be used in the case of washing machines. In practice, however, can we calculate if there is and how much is this saving (in energy and therefore money)? Let’s look at the data below.

The electricity consumption profile from a washing machine