The History of the Combination Boiler

When did combination boilers first appear in the UK? I remember it vaguely — it was around the early 1980s.

My very first combi installation

I remember the very first combination boiler I ever installed. It was a Saunier Duval SD620. The reason I remember it so well is that every single instruction that came with it was written in French — nothing in English at all.

My local plumbing and heating merchants, which I used constantly, were always busy with sales reps from different manufacturers promoting their latest products, and Saunier Duval was one of them. These boilers were being offered at a special introductory price for around three months, purely to get them selling. Within that window, the merchants gathered feedback from the engineers who’d actually installed them, which decided whether they kept stocking the brand or dropped it.

I remember talking to other boiler engineers about these so-called “two in one” boilers at the time. Some were genuinely excited to try them. Others were certain they’d never take off — that they’d be a passing fad, gone within a couple of years. How wrong they were.

One of the first things I noticed was just how light this boiler was compared to what I’d been installing — roughly half the weight of the Potterton Netaheat, which was the popular choice at the time, connected to a separate hot water tank to provide both heating and hot water.

Translated overnight, by typewriter

My very first SD620 installation was in a property in Romford, Essex. The property owner was French, which turned out to be a stroke of luck — she helped translate the installation instructions for me. She sat up most of the night doing it, typed out in English on a typewriter. Remember, this was long before laptops or computers — a typewriter was about as modern as writing got back then.

Combination boilers had actually existed in Europe since the early 1970s, but weren’t permitted in the UK due to water regulations at the time. The main sticking point was that they connected directly to the incoming mains water supply, which simply wasn’t allowed.

Getting to work

With the instructions finally translated, I got started. The boiler was replacing an old multi-point water heater, which meant reducing the flue hole from a large square opening down to a small circular one, around 5 inches across.

The SD620 came in two parts. First was a fixing frame — a “jig” — that mounted to the wall and held all the pipe connections: two for the central heating flow and return, two for the hot and cold water, and one for the incoming gas supply. There was also a separate pipe for the pressure relief valve (PRV), which had to be routed through the wall to the outside.

With the jig fitted to the wall, I spent the next six days installing eight radiators throughout the property. Once all the main pipework was completed and connected to the jig, I lifted the boiler itself into place and connected the two together — fiddly at first, but I got there eventually.

The flue itself was surprisingly straightforward to fit — it came with an aluminium bend that sat on top and clipped into place, with the flue pipe pushed in and sealed using a rubber sleeve. Genuinely state of the art, for the time.

Filling, testing, and firing up

With everything in place, it was time to fill the system with water. There were no inhibitors or water filters back then — it was all very basic. Before starting, I spent a good hour making sure I properly understood how to fill a pressurised system correctly.

Eventually, the system was full, every radiator vented, and I’d checked thoroughly for leaks before connecting the wiring.

Commissioning back then started with a gas tightness test at the meter, done using U-gauges — there were no electronic manometers yet. With the gas check passed, it was time to power it up and see whether this was going to become a regular fixture in my installations.

The SD620 had a simple summer/winter lever arm on the front, which you switched between depending on what you needed. I set it for both heating and hot water.

First impressions

The first thing that struck me was how quickly the radiators heated up. I went round checking each one, bleeding any trapped air, and keeping an eye on the boiler’s pressure gauge, topping it up where needed.

Then came the real test — the hot water. I’ll be honest, I was genuinely surprised at how good it was, and so was the property owner. We tested the bathroom taps, including the bath — filling it took nearly 30 minutes, which was about normal for the time, since most showers back then were just an attachment fitted to the bath tap, nothing like the showers people have today.

Once everything was up and running, I asked the customer if she was happy with the installation. She was — a genuine relief, especially since she’d specifically requested this boiler by name.

Because this was my very first combination boiler installation, I asked if I could pop back from time to time to check on it, since I was genuinely curious whether this type of boiler was going to be the future of the industry.

Other brands entering the market

Before long, other brands started appearing — Chaffoteaux and Vaillant among them. I tried the Chaffoteaux combi but wasn’t particularly impressed; before combis, they’d been known for producing very good water heaters, which made the combi feel like a step down.

I later got round to installing a Vaillant combination boiler, and the size of it genuinely took me by surprise. But once it was up and running, it became, in my opinion, the boiler to use — the build quality was noticeably above the other brands I’d worked with.

Interestingly, if you look at the Vaillant Group today, Saunier Duval is now part of it.

Looking back from retirement

Combination boilers today are far more efficient, with noticeably better hot water flow than the early models — that was always the one weak point of a combi. Whatever brand people prefer, and people do tend to have a preferred brand, every major boiler manufacturer now produces their own combination boiler.

The technology has come a long way since the early 1980s — not just for combis, but boilers generally.

Before I retired, my two favourite brands were Vaillant first, Worcester second. Not for the reason you might expect — yes, they’re both excellent brands, but for me, it was also because both produce LPG (liquid petroleum gas) versions. My later working years, before retiring, were spent mainly on LPG boilers in the mobile home world, where they were extremely popular.

I hope you enjoyed reading about where the combination boiler actually came from.