Modifying a thermostat to make our heating more efficient

It's December 2nd and it snowed today. That was the first time that it snowed this winter, and it reminded me to write about a very cheap modification to our thermostat which made our heating system both more efficient (using less gas) as well as giving us better control of the temperature in our home.

Our central heating system is quite old and inefficient, dating from the 1990s. I've been meaning to replace it since we moved in, but first we did quite a bit of insulating, including the walls, roof, floor and triple glazed windows. All of this dramatically reduced the energy input required to stay warm and as a result we've gone from a home which was expensive to run in winter even while we were still cold to one which is very efficient so the heating doesn't come on very often. Much of our gas usage now is actually due to our even more inefficient and old water heater. We now burn far less gas than average because even though our central heating boiler remains old and inefficient it doesn't come on very often and last December we used less than half the amount of gas that average homes of our size required for heating, less even than an average apartment. Insulation works.

This shows gas usage last December. Our bill for electricity and gas together averages around €45 per month over the year. In part this is because the energy company pays us for our excess solar power.

One of the things that put me off buying newer gas appliances was wanting to get rid of them altogether. Unfortunately, when I first looked into doing this I was getting five figure quotes for heat pumps which made it impossible to justify them on economic grounds. Insulating saved us more money more quickly, and the solar panels were also far easier to justify economically. But the price of heat pumps has come down and I do now want to switch. Unfortunately, the absolutely awful government which this country currently has has ensured that the covid pandemic has already gone on for nearly two years and it'll probably go on for a while yet. I'm not keen on having people come into our home while this disease is spreading so the heat pump will have to wait. However I did think of a way of making our existing system more efficient:

Uneven temperature due to too much insulation and an overenthusiastic central heating system

Our boiler and radiators were designed for a leaky house. The radiators are large and the boiler likes to generate lots of really hot water. I turned down the temperature setting on the boiler a very long while ago and there's no problem with the house heating up (this bodes well for replacing it with a heat pump which will produce cooler warm water) but we still had a problem with excessive heat.

What happens is that thermostat switches on, the radiators heat up and because it takes quite a long time for convection to transport heat from radiator to the thermostat the heating would continue to push out heat for far too long resulting in the temperature overshooting sometimes by 3 C over our selected temperature. Setting the thermostat at a lower temperature doesn't fix this problem because that means that the lower temperature has to be reached before we get any heating at all. We were a victim of our good insulation.

My first thought was to replace the thermostat with an Arduino programmed to turn the heating on only for short bursts and I started working on doing this before I realised I was overthinking it: Couldn't I instead do something to convince the existing thermostat to switch for short bursts ? At first I thought perhaps this could be done by adding something like a 555 timer circuit which would change the state of the relay in the thermostat more often, but then I thought of an even simpler solution:

The solution which costs just a few cents

The thermostat operates by closing a relay contact between two contacts attached to wires from the central heating boiler. Those wires have 24 V AC on them when they're open. When they are shorted that powers something within the central heating boiler which turns on the gas and the pump. I found that a dead short wasn't required. Actually, any low value resistor across the two wires worked just as well as a dead short to make the boiler start up. Trying different values allowed me to calculate that the boiler consumes a constant current of about 80 mA for any low value of resistor across the contacts. A 47 ohm resistor drops about 3.7 V and consumes about a third of a watt itself which is enough to make it slowly warm up. I realised that if I installed this small "heater" inside the thermostat next to the temperature sensor it would give just the desired effect of short bursts of heat from the system before the thermostat thought the room had warmed up and would switch off again.

The temperature sensor is easy to spot. It's mounted such that ambient air can easily influence its temperature.

It was easy to find the temperature sensor inside the thermostat and easy to confirm that that is what it was because holding it between your fingers results in the temperature on the front panel of the thermostat rising quickly.

A 47 ohm resistor wrapped in self-amalgamating tape. This is now installed inside the thermostat next to the temperature sensor in the photo above

I attached a couple of wires to a 5 W 47 ohm resistor from my collection of parts, wrapped it in self-amalgamating tape to ensure that it doesn't cause a short and have installed this next to the temperature sensor in the thermostat. As I didn't need to actually buy anything to make this modification it cost more or less nothing to make it. If I'd had to buy the parts the most expensive thing would have been the roll of tape.

It works !

Now the thermostat can turn on the heating only for a couple of minutes before the resistor has warmed up enough that it thinks the target temperature has been reached. It then switches off again and the thermostat slowly returns to room temperature. If this is still below the target temperature then the heating will switch back on again for a few minutes. It takes a little longer than before to warm from a cold room, but we never overshoot by more than a fraction of a degree. Though the radiators never really get hot any more, there is enough energy in them to heat the room without burning more gas. This results in much more consistent and comfortable temperatures in the room and we hope also to see a lower gas bill due to less gas being burnt.

Next year perhaps we'll look again at replacing our central heating boiler with a heat pump. It's important that we all stop using fossil fuels but for now, with covid raging, the step of using a bit less is still worthwhile.

The result

We consumed 148 m3 of gas in December 2021 vs. 147 m3 in both December 2019 and 2020, the two previous winters with full triple glazing downstairs. Clearly there's no dramatic change there. January, February and March looked a lot better: We consumed 183 m3, 144 m3 and 107 m3 in Jan, Feb and March 2021 vs. 130 m3, 107m3 and 50m3 in 2022. There's still little data and this could be because those months in 2022 were milder. As more time passes there will be more data. But even if this makes no difference to gas consumption it does at least make our home more comfortable.

Our gas consumption in March was really low. Our home is a "2 Onder 1 kap" type so we used under a quarter of the average Dutch home, not only because of the other measures we've taken but also because we turned the thermostat to an even lower temperature than usual in order to avoid funding Putin's war in Ukraine. March was also unusually sunny so the energy company owes us €100 for the electricity that we supplied to the grid in March.

Nine years of solar power. 30 MWh generated.

Today is an anniversary. Our rooftop solar power setup was installed on this day nine years ago. When the system was first installed I had two concerns, about the inverter and about the panels themselves.

Inverter
The biggest concern was about the inverter. I expected that this would have a limited life because an inverter is inevitably a box of power electronics which has to work quite hard. Power supplies, especially in my experience switched mode supplies, can be quite unreliable after a few years. I've fixed quite a few of them in the past, and switched mode supplies can be quite tedious to work on. An inverter is like a large switched mode power supply which works in reverse so I didn't have huge expectations for longevity and sadly the inverter has actually failed twice, first in 2018 and then in 2020. In both cases it was quite simple to fix and because I know which end of a soldering iron to hold onto I did that myself.

Panels
The other concern was the less well known longevity of the solar panels. There are two main ways in which solar panels degrade. The first is due to corrosion should they become damp and the second due to the sun light falling on the panels degrading them. I've had solar panels on homes that I lived in since the mid 1980s, starting with several 30 cm x 30 cm panels which I used to charge batteries. Those panels were not sealed from the weather so they got damp, there was visible corrosion in some places, and their output dropped markedly over time. But around the turn of the century I bought a 12 V sealed panel to replace them. I still have that on my garage roof and it's output is still close to the specification so I hoped that the panels for the roof be similarly long lasting.

The guarantee said that the output of the rooftop panels would still be at 90% of the initial level after 10 years of use. I realised soon after the panels were installed that their peak output was significantly higher than it the specification suggested. The rated peak output for the installation was 3760 W and the installers suggested that we'd probably not see more than about 3600 W because of the angle of our roof. But within a few days I was seeing ~3990 W. I had some concerns at that time that perhaps there had been a little slight of hand on the part of the manufacturer, who perhaps under specified the panels in order to protect themselves from guarantee claims. i.e. Perhaps we'd see the panels degrade by more than 10% over ten years but that they'd still be within 10% of 3760 W or even within 10% of 3600 W in ten years time, meaning that a loss of almost 20% of output would be possible without being able to claim on the guarantee. But this has not turned out to be the case at all. There is no degradation. I can happily report that the observed peaks last summer were still around 3990 W. What's more, we're not quite at ten years yet, but the total output from the system also seems not to have dropped even slightly. After the first three years passed I noted that the system had generated 10200 kWh in total. Six years further on the total is almost exactly three times this, at 30556 kWh. What's more, if not for the inverter glitch in 2018 which costs us about 250 kWh of output, we'd actually be ahead of the first three years by now.

Graph of output over time

The orange line shows our excess production over time. It goes up in summer and down in winter, and by March we just about break even which is why the last part looks flat. When the panels were installed we had teenage children at home and our consumption was similar to our rate of generation. You can tell how long ago our children left home from this graph (some lower power appliances also made a difference). The glitch three summers ago was caused by the inverter failing and my taking a few days to fix it. The second failure was a year ago in February and lost only one day's winter output so that isn't visible on the graph.

Finance
When the panels were installed I estimated that it'd take about ten years for them to pay for themselves. Because electricity prices change, we've changed our electricity meter once and supplier more than once, it's quite difficult to work out exactly where we are now so far as the return on investment is concerned. However it's quite easy to make an approximate calculation so that's what I'll do. I know that we'd generated 30442kWh up to the end of March, and of that we'd consumed 27767 kWh and exported 2589 kWh. Electricity costs us just under 20c per kWh so we've saved about €5300 from our electricity bill. With our current tariff we get pretty much the same for what we export so that's worth about €500. This leaves us with about €5800 of our original €8000 investment returned, and at the same rate the system will have paid for itself in about 13-14 years after installation. This is about the same result as I came to about three years ago. Just four or five more years to wait, then.

Note that in reality over a whole year we use about 40% of our own generation directly from the panels and export the rest of what we produce. The electricity company doesn't bill based on this, though, so for a purely financial calculation we don't need to take that into account.

Conclusion
I'm happy with the system. The inverter fault was disappointing and the manufacturer's response even more so, but I fixed that. The panels are faultless. Solar panels with lower performance were much more expensive when I first started experimenting with them in the 1980s but when we installed this system it was clear that they'd pay back within a reasonable amount of time. They're now not far from half the price that they were then so someone considering installation these days can expect to see their money back very quickly indeed.