In my first article I briefly wrote about how the installation of my central heating system with a wood-fired boiler, a heat storage tank and a storage tank charger went. After the installation was finished, the system was filled with water to 1.5 bar, we lit the first fire and waited to see what would happen. Everything went smoothly, once the temperature in the boiler hit 30°C a thermostat switched on the circulation pump in the storage tank loop and the heating up of the boiler continued. At 60°C the charger began adding cold water from the bottom of the storage tank to the return feed (the loading of the heat storage began). We waited for a while for the top of the heat storage to heat up to 60°C, switched on the circulation pump in the heating loop and after the initial few minutes of heating, we wound up with cold radiators.
The problem was the fact that the installation crew didn’t really know what they should do, so they installed a four-way mixing valve (4WV) between the circulation pump, the hot feed and the cold return. Such a valve is usually installed in heating systems without storage and a charger. In such installations, the 4WV serves to balance the feed and cold return temperatures, or rather enables the mixing of hot feed water with the cold return from the radiators so as not to shock the boiler with cold water.
You can see what this looks like in this rough non-technical doodle depicting a four-way mixing valve open to one third.
When the valve is half-open, we can see that the hot and cold water are mixed in both in the boiler/storage loop and the radiator loop. The valve can also be fully opened, in such cases the hot feed and the cold return from the storage are directly connected to the radiator loop, or closed, when the two loops are fully separated. These are the only two cases when there is no mixing, in all other cases the valve additionally heats up the water that returns to the heat storage.
In order to clear up why this is wrong in heat storage installations, we will discuss how they function, as they are completely different in terms of return temperature. To retain proper layering in the heat storage, the temperature difference between the hot feed and the cold return should be as high as possible. The heat storage tank looks something like this.
The boiler is connected sketch left, sketch right are the hot feed and the cold return for the radiators. Both the loading of the storage and its discharging are done in exactly the same manner, on the left we draw cold water from the bottom of the tank and return hot water to the top. On the right, we draw hot water from the top and return water cooled in radiators to the bottom return feed. When natural circulation is present in the system (i.e. the circulation pump is off and the boiler room/storage tank loop is located below the radiator loop) this works perfectly, hot water is drawn into the radiators where it is cooled down and then returned via the cold return.
The following line chart represents the loading of the storage tank in ideal conditions, the circulation pump in the radiator loop is switched off, only the pump in the heat charger is on. The temperature sensors are located in four positions on the storage tank (number 1 is at the top of the tank, number 4 is at the bottom). Hot water circulates slowly through the system, cold water returns to the tank. The spikes in the return temperature occur when additional radiators are turned on.
As we can see, the layering is perfect and while the top of the tank is at 60°C, only 30 cm lower the temperature is 40°C, the bottom is a little over 20°C.
The line chart representing the discharging of the same system using natural circulation (circulation pump off) is as follows.
Although the fire in the boiler went out and the boiler was switched off, the charger circuit continued to transfer heat to the tank (grey line), and the storage tank is discharging, cold water from the bottom pushes hot water up. The layering is preserved. (Nota bene: the line chart is for a warm day with only a few opened radiators, a 500 litre storage tank is not enough to keep my system warm for 12 hours).
In less then ideal conditions, when the circulation pump is switched on, the water circulates through the radiator loop much faster, doesn’t have time to cool down in the radiators (even at the slowest speed of the circulation pump) and the cold return is significantly warmer. Adding warm water to the bottom of the heat storage ruins the layering and after just a few minutes the water in the entire tank gets mixed up. Instead of 60°C at the top and 20°C at the bottom, we get a whole tank filled with lukewarm water at temperatures which aren’t high enough to heat up the radiators.
This is exactly what happened when I first switched on the circulation pump. In just five minutes I was left with a tank full of lukewarm water. The effect a four-way mixing valve has in such a system is either bad (loops are connected directly without mixing), worse (loops are connected in such a way that the cold return is mixed with the hot feed) or the worst (the loops are separated and the hot feed from the heat storage is connected directly to its cold return).
The four-way mixing valve was the main culprit and had to be removed. We replaced it with a three-way mixing valve which operates differently.
When the valve is fully opened, the loops are connected directly. This case is shown in the following figure, and is unacceptable for the system when the circulation pump is running.
However, when the valve is half-open (or half-closed, depending on its outlook on life), we can send most of the cold return water back into the radiator loop and draw only a little hot water from the hot feed. By doing this, we will send only a small amount of warm water back into the cold return and the layering in the heat storage will hold out much better. In this example, the three-way mixing valve is half-open.
The following line chart represents a system with a three-way mixing valve.
Early on the circulation pump is not running, the three-way mixing valve is fully opened, the hot feed and the cold return are directly connected to the radiator loop.
At 17:52 the circulation pump was switched on and we can see the return temperature spike, while the hot feed temperature is dropping. The temperature difference between the two drops from almost 35°C to barely 5°C. The bottom of the tank is heating up.
At 17:58 the three-way mixing valve is closed to about 1/3, meaning that 2/3 of the water from the cold return is mixed with 1/3 of water from the hot feed and sent back into the radiator loop. Although it is not evident in this chart, the temperature of all radiators has dropped because the pump has mixed up the layered water in them (the radiators were hot at the top and cold at the bottom) – the same thing that happens in the heat storage occurred in the radiators, they are now lukewarm from top to bottom.
At 18:22 the three-way mixing valve is opened to about a half (half of the water is sent back to the radiator loop, half is returned to the bottom of the heat storage). We see that the difference between the hot feed and the cold return is now rising as the radiators are heating up the rooms. The system is stabilizing.
The last chart shows how the loading of the storage tank continued with the circulation pump running and with a three-way mixing valve at 50%.
The difference in temperatures of the hot feed and cold return is stable at about 10°C, the storage is loading, although the chart is not as smooth as it was when the circulation pump isn’t running. It is also evident that the system is much slower to react, it takes a lot longer for the radiators to heat up (obvious from the hot feed temperatures).
To sum up, in order for the heat storage to work well, it has to be connected to the radiator loop through a three-way mxiing valve, a four-way mixing valve is completely useless in this case. These charts also indicate a need for a control system which would monitor the temperatures in the loops, switch on the circulation pump when needed and control the mixing valve in order to preserve the layering in the heat storage whilst providing a high enough hot feed temperature to heat up the radiators.
This will be discussed in future articles, the next one with deal with the first building block of the system, the sensors and methods of recording temperatures. I’ll show you how to measure the temperatures, log them on a computer, monitor them via the internet and plot colourful line charts.
To be continued…
Hello,
Thank you for sharing your experiences, regarding your wood burning heating system, which have been very useful for me.
I’m designing and constructing a similar system myself although on a larger scale (50 kW wood boiler & 3000 l storage tank).
One comment is concerning the 4-way mixing valve. If the return temperature of the water from the mixing valve to the storage tank was quite high, why did you not try to connect it higher up in the storage tank, to say just above the middle of the tank? That way you would have mainly had mixing in the upper (hot) portion of the tank.
I notice that your storgae tank is also rather small (500 l) in comparison to the wood boiler you’re using (21 kW). Usually one uses the 50x rule, when sizing suitable storage. In other words 21 x 50 = 1050 l or larger would be more suitable for this size of boiler.
The point is, would the mixing of hot and cold water have been less pronounced if the return to the storage tank from the 4-way valve would have been much higher up in the tank, as well as the tank volume being over twice what it is now?
The only problem I can see with a 3-way mixing valves is that you then can’t add weather compensation (from outside and inside temperature sensors) to the system. Without weather compensation, the system can tend to struggle to provide the correct amount of heat required to the radiators, if the outside weather temperature fluctuates quite widely.
What are your experiences in the winter; is the system more or less stable or does the temperature tend to go up and down as the weather outside gets colder?
Have you tried to incorporate weather compensation into your control system in any way?
Thank you for your comments, Paul.
Regarding the first question, as I have probably mentioned in the article, the whole installation has been a learning experience both for myself and the guys that did the installation. I didn’t go for the 4-way-connected-higher option simply because I didn’t see the point in ever returning hot water back into the tank. 3-way valve is the way to go in this case, but what I should have done was to connect that return higher on the storage tank. That would help things.
The storage tank is indeed too small for my system, but the reason for that is very simple – there was no way I’d be able to get a larger one into my basement i.e. the boiler-room. The plan, though, was to get the 500 l one, and at some later date get another one installed in parallel. That part has yet to happen 🙂
In regards to your comment on 3-way mixing valve and weather compensation, I can’t think why weather compensation would not be possible with a 3WV. However, I don’t think I need any weather compensation because all my radiators (save one) have thermostatic valves, so they take what they need and the temperature (while the boiler is burning) is fairly constant. As for the weather compensation on the boiler itself, I do that when I take a peek at the external thermometer and decide how many logs to throw in. 🙂
What I never finished was a third article detailing how I got the thing to work semi-automatically. I did finally get the mixing valve motorized and managed to get it to maintain a fixed return temperature but found that a much simpler solution works almost as well. My circulator pump is very old and stopped working on the lowest speed where I kept it to further reduce the possibility of a hot return. When that happened, I’ve set it up to work intermittently – the circulator runs for 30 seconds which is enough to heat up the radiators, and then stops for 90 seconds to give the radiators a chance to cool the return down. This worked quite well, interestingly enough, and the chance of hot and cold water mixing in the tank was greatly reduced.
If you have any further comments or questions, I’d be happy to help.