Thursday, 2 December 2021

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

It's now January 2022 so I have a 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. I can't say how this December compared with the previous two. But even if this makes no difference to gas consumption it does make our home more comfortable. 

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