Solar water heater and washing machines

By | December 26, 2022

Many people wonder if they can connect their existing washing machines and dishwashers to the hot water supply. The reason is obvious, supplying hot water will theoretically reduce electricity consumption and therefore the amount of bills . The answer is not simple and certainly not the one we all want to hear. But there is an approach that can work and you will learn about it if you read the article to the end.

Important ! Because the nature of clothes and dishes as materials differ, if you are interested in learning about dishwashers read this article.

Before we start examining the issue, it is good to clarify from the beginning that there are washing machines which 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 the most common.

Who has not wondered if the surplus water of the solar water heater is suitable for use in laundry?

In Greece and for most of the year the solar water heaters produce more water than we actually need for use and this is not bad as solar energy is free. However, even during the winter the water can become lukewarm and in relation to the cold water supply the temperature difference can be quite large. Most of the consumption of a cheese maker comes from the process of heating the cold water to the temperature chosen by the user of the appliance. Since this is happening, should we direct it to the laundry to reduce the electricity bill?

The answer is no, at least not until you fully understand how laundry works and what is good for washing clothes.
Hot water can be very hot
We are mainly talking about the solar water heater (there may be excess hot water from the central heating boiler or a wood water heater), but whatever the source of the hot water may be, it may be hotter than it should be. The incoming water from these sources could easily exceed 60 degrees and this would endanger both our laundry and our clothes.
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To find out what the ideal washing conditions are, read here
There is no way to control the temperature of the water entering the laundry and tub. A laundry designed for dual supply (hot and cold) will either handle the hot water by mixing it with cold to reach the appropriate temperature or will only use the hot water in the phases that it really needs. But this is decided by the brain
(the electronic circuit) of the washing machine which is programmed to control two supplies and make the right water for each phase of a wash.
The phases of a wash are affected by the temperature of the water
Most washing programs need hot water at 30-40 degrees Celsius. It makes no sense to supply water with 60 or more degrees in such a program. Don’t forget that the water from the solar system can reach quite a bit more. Apart from a possible damage, the washing will not be carried out according to a program. The thermostat will close too early and the current wash phase will end prematurely. This will shorten the overall time but also degrade the result.
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What are the phases of a wash?
Hot water is not good for organic 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 colder water at the beginning and then gradually heating it to the degree desired by the washing program.
Spinning with hot water can damage some clothes – fabrics
If a laundry is only connected to a hot water supply, then reasonably when the time comes (washing phase) of drying the water will also be hot. If the content is ironed at a high temperature, the creasing will be very intense and ironing… a nightmare! If we save a certain amount of energy from heating the water, we will also give it to a greater extent to the electric iron. That 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 laundry?
Providing hot water directly to the laundry supply was ruled out as an option for the reasons previously stated. Is manual Y-mixing with the hot and cold water “combining” and continuing unchecked into the laundry an option? not again. Too bad for nothing in a situation where virtually no control is possible and the gain rather minimal. In fact, this way the water will not be too hot, but each time someone will have to manually adjust the pressure of each supply to reach… the ideal temperature. It is also known that the washing machine draws water at intervals which are short, so by the time the hot water runs into the piping network, it is likely that it has already taken the amount of water it needed. In fact, this is one reason why cold water washing machines have finally prevailed, since in practice the double supply laundry did not use the intended hot water, but rather less. So during washing there would be less hot water and during rinsing more since a larger amount is needed there. Not very good timing.
Thermostatic valve, solves the problem?
A thermostatic valve is nothing more than a mixer that accepts two supplies (hot and cold) on one side and supplies water at a constant temperature on the other. Depending on the product, the outlet water temperature can be adjusted with greater or lesser precision. It is good for the user to check the temperature manually (with a thermometer) in case of unexpected operation. In this way we solve two main problems concerning the supply of hot water to the washing machine:
We have hot water within safe limits
We have a constant supply of this water and not with gaps due to heat losses from the piping
A thermostatic valve has a low cost compared to what it offers. Do we install it before supplying the washing machine and solve the issue that concerns us? and again no. We succeeded in setting the hot water, but the last problem is that in the last phase of a wash… we don’t want it! The vast majority of washing machines are single cold water supply, so in the end they just use cold water as they should. On the other hand, the double supply washing machines are equipped with a corresponding brain which knows how to handle the water depending on the washing phase. The washing machines with a single supply of cold water have a brain that does not care about the issue since whatever water we give it, it thinks is cold. So it will proceed to spin normally and let it have very hot water for this phase.
Initial conclusion, secondary considerations
The thermostatic valve solves the majority of problems caused by the supply of hot water from the solar water heater to the laundry. But it cannot be used just like that without understanding the way the bread machine works. If we have it willing to give us water at 40 degrees, for example, we will achieve great savings from heating the water, we will probably have a problem and “disasters” in our clothes.
It should therefore be set to the lower operating limits in order to avoid heating the cold water of the network (especially in winter) from the minimum temperatures that are close to 0, but giving room for the brain of the washing machine to heat the water a little when needed and have it controls 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 at very high temperatures, higher than what a washing machine needs. The direct supply is sure that at some point, sooner or later depending on the circumstances, it will destroy the washing machine. The thermostatic valve protects us from this situation. By setting it to the lowest temperature levels we also protect our clothes (user tests will prove the real situation in terms of clothes). During the winter the solar water heater may not heat enough water for our bathroom, but as a power supply for the washing machine it is a very useful source since it has a noticeable difference with the cold water of the mains. Even this difference can save energy.
So far we have seen enough information on how single supply (cold water) washing machines work, why we should not feed them with water directly from the solar water heater or other source of hot water, what are thermostatic valves and how they can be used in the case of laundries. In practice, however, can we calculate if there is and how much this saving is (in energy and therefore money)? Let’s look at the details below.
The power consumption profile of a washing machine
The power consumption of the washing machine for the cotton and synthetics washing programs at 40 • C.
As can be seen from the graph of the electricity consumption of a washing machine, most of the consumption takes place during the initial phase and when the cold water is heated based on the selected washing program (40 degrees in the example). Then there is consumption from the turning of the bucket, which is very small and increases momentarily during the rinsing-spinning process, but is of much lower demand compared to heating the water. Note that the electrical resistance of the washing machines is 2000 watts (2kw). According to the energy star standard, a new appliance that follows the standard needs an average of about 20 liters of water during the energy-consuming phase.
The solar water heater in winter
Too many households during the winter “turn off” the solar water heater on the grounds that the water is heated by the central heating system (boiler), so you don’t have to deal with the “unstable” solar water heater in the winter. There are many days during the year when the solar water heater cannot provide us with the approximately 38 degrees that is the average acceptable temperature for people to bathe. However, there are very few that cannot provide us with water at 21 degrees, the ideal temperature to supply water to the washing machine through a thermostatic valve. Even if it is lower, for example 15 points compared to the 5 of the network, there is a benefit.
The temperature of the mains water in Greece
According to the data ( LINK ) the average temperature of the water in the network in climate zone D is 5-7 degrees Celsius during winter and early spring, while in the other climate zones it does not exceed 12. Taking into account the actual these data We will consider two cases of heating the water from the washing machine during the “cold” months of the year. For a specific washing program at 40 degrees we will calculate the energy required by the washing machine to heat about 20 liters of water. We will also consider this scenario for a final program temperature of 30 degrees and come to some conclusions.
Use of water from the mains directly at 6 degrees
Use of water from a thermostatic valve at 21 degrees (water that a solar can give us almost all year round in any climate zone)
Amount of energy required to heat water
Example 1
Quantity of water 20 liters, program temperature 30 degrees Celsius from 6 degrees directly from the network and from 21 degrees through a thermostatic valve.
As can be seen from the figures above, saving energy is very important. They will need 0.56kwh from the resistance of the washing machine, while with the supply of “warm” water from the solar water heater 0.21kwh. The difference and saving in the 30 points program is very significant and over 60%. We avoid quoting prices in € as this is a very volatile parameter, especially with the energy situation in Europe and in our country. Each reader can make calculations according to the program-contract he has entered into with his supplier.
Example 1
Quantity of water 20 liters, program temperature 40 degrees Celsius from 6 degrees directly from the network and from 21 degrees through a thermostatic valve.
And in the case of 40 points, there is a significant savings. From 0.79kwh, 0.44kwh will be needed if we supply water from the solar water heater through a thermostatic valve. This means over 40% savings.
In general, the calculation for the required energy needed to heat a quantity of water from an initial temperature to a final temperature uses the following formula as an example:
20 liters * (40°C – 6°C) * 4.18 J/g°C = 2842j * ( ? )
* 2842 joules is equivalent to 0.79kwh and because it is easier for the reader to understand the energy as kilowatt hours rather than joules, this unit is used. In addition, electricity bills are charged by the kilowatt hour (kwh).
Very cold water and bleach on clothes
What are these white stains left after washing our clothes? : r/whatisthisthingAnother advantage of using water from a thermostatic valve is the fact that during the winter it is very likely that it will end up directly in the laundry water at a temperature lower than 15 degrees Celsius. At low temperatures, the detergent is quite difficult to dissolve properly. This has consequences, the washing does not have the expected result while clothes can end up with light colored spots (especially on dark colored fabrics) which are nothing more than detergent residues that did not dissolve during the initial phase and were trapped between the folds. The water supply from a thermostatic valve helps to deal with this problem.
Test in practice with a power consumption meter
Results of the experiment will be posted here soon
Ecological approach
Saving energy does not only mean saving money, in times of energy shortage it means much more. It is a skill to be able to accomplish the same task using less energy and without compromising quality. By 2022, the world entered a situation where energy was becoming too expensive, while in some cases opinions and predictions were made for Europe that there would be an energy blackout. Of course, no one wants that.
The content of the article and the issues examined concern more those who live off the grid ( OFF GRID ) in homes and facilities that produce the energy they need by themselves. These people are already engaged and have probably found the ideal middle ground in their energy requirements. With the situation shaped by 2022 in Europe due to the Ukrainian issue, and energy supply prices set by the energy exchange is an opportunity for all of us to think more in terms of energy savings and not just euros.

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