Combi vs system boiler installation cost: what actually changes (and why)
People get stuck on the label: “combi” or “system”. But installers don’t price labels — we price the work. And the work changes for very specific reasons: cylinder set-ups, pipe routes, hot water demand, controls, and how tidy (or chaotic) the existing system is.
If you’re here because you’ve seen two wildly different quotes, good. You’re asking the right question. This page gives you the “why”, so you can compare like-for-like without getting lost.
The real difference is demand (and how the home actually behaves)
A combi heats hot water on demand. A system boiler works with a hot water cylinder. That’s the textbook bit — but here’s the lived-in bit: I’ve been in homes where a combi was perfect… and I’ve been in homes where the family basically had to “book” showers. Same boiler brand. Same size property. Different lifestyle.
If your mornings are two showers, a kitchen tap going, and someone washing up while the heating is ramping, it’s not about “what’s cheaper in theory” — it’s about what works without annoying you every day.
If you want the full market context and how installs are scoped, go back to the new boiler cost & installation guide. This page stays narrow on combi vs system so intent stays clean.
What changes the installation cost between combi and system
Cylinder work (system boiler)
A system boiler usually means a cylinder, additional pipework, controls, and space planning. If the cylinder area is awkward (packed airing cupboard is the classic), labour and tidy-up time can rise.
Pipework changes (conversions)
Swapping layouts isn’t always “remove this, fit that”. Conversions can uncover old pipe sizing, odd tee-offs, or controls that were barely hanging on — and a proper install corrects that, not hides it.
Hot water expectations
If the household expects strong hot water from multiple outlets at once, you’re into real-world constraints: mains pressure/flow, pipe sizing, and whether the home needs storage to feel consistent.
Controls, zoning, commissioning time
The difference between “it heats” and “it feels good to live with” is often controls and commissioning. Bigger or busier homes benefit more here, and that work is part of the install cost.
If you’re comparing quotes, use these as reference points
For a size-based baseline, check cost guide by bedrooms and property type. For the “why is it higher here?” layer, use London install cost drivers.
Two quick real-world stories (because this is where people choose wrong)
The “combi is always cheaper” assumption
Someone wanted a combi because they’d heard it’s “cheaper”. But their hot water use was high and the existing layout meant the conversion would be more involved than expected. We talked it through, matched it to how they actually use hot water, and avoided the classic regret: “We saved a bit now… and now it’s annoying every single day.”
The “system sounds old-fashioned” myth
System boilers aren’t “old”. They’re practical. In the right home, the cylinder makes the place feel smooth: hot water is there when you need it. The install can include extra components and labour, but the payoff is comfort and consistency.
What we do (and what we won’t pretend)
At ARA Services, we install, repair, and diagnose boilers across London — and a lot of our work is fixing the consequences of rushed installs. If you’re comparing quotes, we’ll help you compare scope, not just totals.
If you’re still on the fence, it’s worth reading the core cost hub first, then coming back here for the combi vs system split. (Keeps the decision clean.)
If you’re here because your current boiler is limping along, start with boiler repair options before you commit to replacement.
I can usually point you the right way in a 2-minute call
Tell me: how many bathrooms you have, whether you run two showers at once, and if your mains pressure feels “strong” or “meh”. That’s usually enough to say whether combi or system makes sense — and what kind of install complexity to expect. Call 07727 154746.
FAQ (combi vs system boiler installation cost)
Is a combi always cheaper to install than a system boiler?
Not always. A combi can be simpler in some homes, but conversions can mean pipework and control changes. The cheaper option is the one that fits your demand without extra work (or compromises) showing up later.
What makes a system boiler installation more complex?
Cylinder work, associated valves/controls, pipework layout, and space planning. Also: if the existing cylinder setup is old or messy, correcting it properly takes time.
We have two bathrooms — does that automatically mean system boiler?
Not automatically. It depends on simultaneous use, your incoming mains pressure/flow, and your “tolerance” for flow dropping when more than one outlet runs.
Can a combi handle two showers at once?
Sometimes — but it depends on boiler output and the property’s incoming mains flow rate. A lot of disappointment comes from assuming the mains will deliver more than it actually can.
Where should I start if I’m still unsure?
Start with the main new boiler cost guide, then come back here for the combi vs system split. If the quote is being pushed up by access/parking/constraints, read London factors that affect installation complexity.
If you’re choosing between combi and system, don’t guess
Call 07727 154746 and tell us your bathrooms + hot water demand. If you’d rather send it, use the quote form and we’ll reply with a clear recommendation.