Energy squeezer - Blog #3

20 Feb 2022

In this post, I will provide some details about the first project for this challenge: a device to recover some otherwise-wasted heat from the boiler fumes

1. Fume energy extractor

The heating system in my house has two main zones. Ground floor (where we typically live) has a floor heating system, whereas first floor (where there are two rooms we use rarely) has an heating system with water radiators.

A floor heating system has a major main advantage: low temperature (up to 40 °C) water can be used to heat the ambient. This is an efficient approach to heating, because many highly-efficient technologies can be used to generated low temperature water required by the system. The optimal solution in terms of efficiency would be probably an electrical heat pump. However, at least here in Italy, this is not the best solution from a financial point of view, because electricity cost is quite high and, when I built my house, photovoltaic panels were very expensive. So, at the time, the best compromise was a condensing boiler. When used to generate low-temperature water, a condensing boiler has a very high efficiency because it can extract extra heat from fume by condensing water created as a result of the combustion of methane. Thanks to the condensing process that takes place before the flue, fumes have a low temperature (60 - 75 °C according to the boiler installation manual)

However, as I said, I also have some radiators, which require hot water (at about 65 °C). When the boiler has to generate hot water, condensing process can not take place and fumes are much hotter (well above 100 °C). This is obviously a waste of energy. The problem is: letting the "cold" fumes pass through the heat exchanger will further cool down the fumes, causing them to stagnate because they may not have enough energy to go up through the chimney

The idea for the first project is to build a control circuit that can detect fumes temperature and, in case they are too hot, it diverts hot fumes to an heat exchanger so that some of the otherwise wasted heat can be sent to the room where the boiler is installed. Since we use to dry clothes in the boiler room, this extra heat would be welcome during cold and wet days.

2. Building the fume energy extractor

I built a prototype of the fume energy extractor with 2.5mm plexyglass. The size of this prototype is 20 x 30 x 10 cm

The "doors" are driven by a DC motor and leverages

The Thermorite OHD1-90M switch has been installed on the fumes intake

The control circuit is shown in the following diagram

Here is a brief explanation of how the circuit works. The OHD1 thermal switch is connect to a relay whose purpose is to select the direction of the DC motor.

When fume temperature is below 90C, the thermal switch is open and relay coils are in their normal position. This makes the DC motor rotate in clockwise direction.

The DC motor runs until the limit switch is reached, then the limit switch (which is normally closed) opens and power to the DC motor is cut. Pictures below shows the path of current in this case

When fume temperature is above the sensor threshold, then the thermal switch closes and energize the relay coil, causing the DC motor direction to be reversed. When DC motor direction is reverted, the "opposite" limit switch is closed, causing the DC motor to run until the limit switch is reached by the door.