Author Archives: Tomas McGuinness

If you’re planning to upgrade your boiler or heating system, I would like you to read this post. I learned some valuable lessons since having a new heating systems installed in March 2021. I want to tell you what those lessons were. They might help you get better performance our of your new system!

New everything in our new house.

In 2021 I relocated from London to Solihull. We bought a house in need of renovation and during the first few months of that year we had the plumbing, electrics and some structural works carried out.

At the time, I knew nothing about plumbing, heating, boilers etc. We were giving a strong recommendation of a plumber and got them around to give us a quote.

I had heard about heat pumps and wanted to see if one of those was an option. When I first broached the idea, it was dismissed out of hand as my property was too old and unsuitable. We were on a tight deadline to get the renovations completed, so we agreed on a gas boiler. I knew that big radiators and big pipes were important to have with heat pumps, so I pushed to have larger than usual radiators installed.

We discussed a future extension and underfloor heating and the plumber recommended a 30kW system boiler. I nodded in agreement as I’d no idea if that was big or small. Trust the professional.

They did the installation. The work was very tidy, completed on time and budget and we were very happy.

First Winter and our first eye-watering bill

Fast forward six months. Winter had arrived and the heating was turned on. Towards the end of November, we had a cold snap and in one single day, the 28th of the month, we used about 200kWh of gas.

I had no clue if this was a lot of gas. It certainly seemed like a lot. I posted my question to Twitter and quickly got confirmation that it was an insane figure for a standard four bedroom house.

I checked my usage against friends and family and it became obvious to me that we were simply burning more gas than everyone else.

Down the rabbit hole

Given that I wasn’t happy burning gas in the first place, burning more than I needed was adding insult to injury. I set about trying to understand why our usage was so high.

The first thing about was how modern boilers have a condensing mode.

Condensing Mode

You’ll most likely have heard the term condensing boiler. I, for one, hadn’t a clue what that actually meant 🙂

As I now understand it, condensing is a way the boiler recovers heat from the flue (exhaust) of the boiler. It takes some of the heat that would otherwise be blown out the flue and puts it back into the water. It has a pretty dramatic impact on your boiler’s efficiency too; when it’s not condensing, efficiency might only be about 80% and when it’s condensing it will be up in the 90s. My boiler has a 93% efficiency.

So what’s the catch? Well, as I discovered, your boiler only works in condensing mode in certain circumstances. To help explain this further, I need to introduce two more terms; flow temperature and return temperature.

Flow Temperature: What is it?

Flow temperature is the temperature of water coming out of your boiler. It has nothing to do with your thermostat or anything like that. When your boiler is running, it will try and keep the water in your pipes at the flow temperature.

Return Temperature

If flow temperature is how hot the water leaving your boiler is, return temperature is how hot the water coming back to your boiler is.

As the water flows around your radiators, the heat in the water is released in your house. The boiler then heats this water up again and this happens around and around.

What does this have to do with my boiler’s condensing mode?

Everything, really. I don’t know exactly how all this works, but I know that a boiler will only operate in condensing mode when the return temperature is below 55 degrees Celsius.

This is really important to note because most boilers are installed to have a flow temperature of 70 and a return temperature of 50. You might think that this means your boiler is in condensing mode all the time, right? Unfortunately, you’d be wrong. The 70/50 rule is used to ensure your heating system is putting enough heat into your house when it’s freezing outside. But what happens to your return temperature when it’s, say, 7 degrees outside? Well, the return temperature goes up.

When it’s warmer outside, your house doesn’t lose heat as quickly. This means the water in your radiators don’t put as much heat into your house, so the water coming back to your boiler is warmer. When the temperature of the water returning to the boiler is warmer than 55, the boiler will stop condensing.

What can I do?

You can turn down the flow temperature of your boiler. If you drop this down to 55, for example, you can almost guarantee that the return temp will be lower, and the boiler will condense. You will also burn less gas since the boiler isn’t heating the water as hot.

That sounds great.

If you have a combi boiler, you can do this straight away! If your house ends up being cold, you can always turn the flow temperature back up! If you want to read more, I recommend this article from the heating hub: https://www.theheatinghub.co.uk/articles/turn-down-the-boiler-flow-temperature

If, like me however, you have a water cylinder for your hot water, things get a little more difficult!

After I learned about condensing modes and flow/return temperatures, I immediately dropped my boiler’s flow temperature down to 55. Almost immediately, I did see my gas usage drop whilst the house stayed warm.

My initial happiness was short-lived after wife complained about the lack of hot water one morning. She will tolerate a lot of my smart home shenanigans, but messing with the hot water is a no-no!

What was the problem?

System Boilers, like mine, typically have only one flow temperature. When I lowered the flow temperature to 55, it reduced the gas I burned, but it made it impossible for the boiler to heat the water in my tank to 60!! I hadn’t really understood that as a side effect.

I was really confused because in my previous house, I could set *two* temperatures on my boiler. One for heating and one for hot water. Billy, my plumber, had explained this to me, but as this happened in 2017 I knew even less about heating and was completely uninterested. Billy was, as I would come to realise, ahead of his time!

Fast forward to 2021 and the penny dropped. I wouldn’t be able to run a lower flow temperature. Not only would it result in a lack of hot water, but there were also other considerations, such as legionella, which I won’t go into. If you want to know more, read this amazing article by Heat Geek on legionella.

Given my Viessmann boiler in my old house let me have two temperatures, I couldn’t understand why my Worcester Bosch boiler didn’t. The boiler had both hot water and radiator buttons on the front, but the hot water icon was disabled.

I knew I needed to lower the flow temperature for the heating, but I also needed a higher flow temperature for my hot water. I put my boiler question to Twitter and, sure enough, received enlightenment!

Priority Domestic Hot Water

Priority Domestic Hot Water is what I was told. I had to google this. I learned it’s the industry name for a setup that allowed a boiler to have different flow temperatures when heating your house or heating your hot water. It will also ensure that your hot water needs come before your heating needs. That’s the Priority in PDHW. Exactly what I needed! Knowing the “technical” term, I started googling!

I soon discovered that the reason my boiler’s hot water symbol was disabled was because it required the installation of an “internal diverter value”. Without it, the boiler can’t enable the dual temperature function.

To install the Diverter Valve, however, required not only the the installation of the valve, but the alteration of the pipework! I rang around a few Worcester Bosch installers and found one (out of three I spoke to) that a) knew what I was talking about and b) was willing to do it. The cost: £400 and he couldn’t do it until February at the earliest.

Pseudo priority domestic hot water

As I needed a more immediate solution, I started looking for a digital solution. I’m a programmer, so my view is that you can solve everything with software 🙂

I had already found an open-source project online called ESP-EMS and, with a small piece of hardware purchased in the Netherlands, was happily monitoring my boiler’s behaviour.

I realised that I could use this to adjust the flow temperature of my boiler using Home Assistant. Using a Shelly i3 to monitor the call for hot water, I created an automation NodeRed that would just adjust the boiler’s flow temperature automatically. When my Mixergy tank needed heat, the EMS-ESP would increase the flow temperature to 70 and when it didn’t, the flow temperature would drop back to 45. (If anyone is interested in how I achieved this, drop me a note in the comments)

Whilst posting about on Twitter about how proud I was of my digital solution many months earlier this year, Richard Burrows of MWPHS pointed out an even simpler solution which involved a 10kOhm resistor.

Richard also shared this video with me, where he walks through how he converting a Worcester Bosch to PDHW without any plumbing changes.

I had to try it out the 10k resistor suggestion and I made a recording of it! You can watch how I became lost for words:

How has it worked so far?

I first installed the EMS-ESP board in November 2021 and implemented my homemade flow control in February 2022. I added the resistor in March 2023. Between both of these, I’ve been running dual flow temperatures for about 18 months, and it never failed.

It’s not perfect, don’t get me wrong. Its main drawback is that I’m missing the “priority” part. If the heating is on and the Mixergy tank needs to heat water, the 70 degree flow is used for the heating and the hot water. This is bad in two ways; it takes much longer to heat the water and the radiators are hotter than I need them to be, wasting gas. The same happens if the boiler is heating water and the heating kicks in.

As Richard mentions in his video, he needed to add relay control so the boiler only sends the 70 degree water to the tank and not the radiators. This I something intent to change in my system later this year as part of my project to move all heating control to Home Assistant. (I’ll document that here and on YouTube, so be sure and subscribe)

Weather Compensation

Once you can change the flow temperatures independently, you can take advantage of another piece of boiler wizardry: Weather compensation. This is the simple idea of changing your radiator temperatures based on the weather outside. When it’s subzero, you want loads of heat. When it’s 12 outside, you don’t need as much.

Boiler Sizing

Another aspect of gas boilers, which directly impacts your bills, is their size. I mention earlier in this post that my plumber recommended a 30kW boiler. I don’t know how he arrived at this figure, but I’ve learned that it’s WAY TOO BIG! It’s at least twice as big as it really needs to be!!! I will write up another post on this as it’s a very important topic.

Final thoughts

If I had known about these things back in 2021 I would certainly have pushed to have them incorporated into my system. They would have been worth the money.

I’ve got another post worth of thoughts on Smart Thermostats, related to weather compensation and smart TRVs, which I will post at some point in the future.

If you’re having a new system boiler installed, please, please, please speak to your plumber/engineer about priority domestic hot water and weather compensation. They should be able to advise you on what would work best (better than me anyway!)

Your wallet, and the planet, will thank you!