What boiler do I need? The short answer comes down to three things: how many bedrooms and bathrooms your home has, how much hot water your household uses at once, and what type of heating system your property already has. Get those three right and picking a brand or model is the easy part.
In this guide, we’ve put together a quick sizing table by home type, explained what actually changes the recommendation beyond room count, and pointed you toward the right boiler type for your situation, including off-grid properties without mains gas.
What size boiler do I need? Quick guide by home type
Here’s a starting point based on typical Dorset and Hampshire homes we work on. Treat it as a guide rather than a final answer. Every property is different, and an accurate size always needs a proper survey.
| Home Type | Recommended Boiler Type | Typical Output |
|---|---|---|
| 1-2 bed flat or small house, 1 bathroom | Combi | 24-27kW |
| 3-4 bed semi or detached, 1-2 bathrooms | Combi or system | 28-35kW (combi) / 12-24kW (system) |
| 4-5 bed house, 2+ bathrooms | System | 24-30kW |
| 5-6+ bed house, multiple bathrooms | System or regular (older homes) | 30-35kW or higher, engineer assessment recommended |
| Period or larger home with existing tank and cylinder | Regular (conventional) | Varies, needs a site survey |
| Off-grid rural property, no mains gas | Oil combi or oil regular | Varies by hot water demand |
What actually determines the boiler size you need
Room count is a useful starting point, but it isn’t the whole story. A few other things change the recommendation:
- Hot water demand: how many showers or baths might run at the same time
- Number of radiators and how well insulated the property is
- Incoming mains pipe diameter, which affects flow rate for combi boilers
- Whether you’re replacing like-for-like or switching boiler types entirely
- Any renovation plans, such as a loft conversion or an extra bathroom
This is why two homes with the same number of bedrooms can end up needing different boilers. It’s worth talking through your household’s actual habits with an engineer rather than sizing purely off a table.
Sizing a combi boiler for your home
Combi boilers work well for homes with modest hot water demand at any one time, typically smaller households or properties with a single bathroom. Because they heat water on demand rather than storing it, the output needs to match your peak demand rather than your total floor area. For a full breakdown of what a combi is good for and where it starts to struggle, our combi boiler advantages guide covers this in more detail.
Sizing a system boiler for your home
System boilers are usually the better fit once you’ve got two or more bathrooms, or a household that regularly needs hot water from multiple outlets at the same time. Sizing here also depends on your hot water cylinder capacity, not just the boiler’s kW output. Our guide to choosing the best system boiler walks through model comparisons, installation costs, and how to match a system boiler to your home.
What if you’re off the mains gas grid?
Plenty of the rural properties we work on across the New Forest and wider Dorset countryside aren’t connected to mains gas. The same sizing principles apply, bedrooms, bathrooms, and hot water demand still drive the decision, but tank size, location, and delivery access come into play too. As an OFTEC registered engineer as well as Gas Safe, we size and install oil boilers across the area. Take a look at our oil boiler services if this sounds like your property.
What about commercial properties?
If you’re heating something larger than a big family home, such as a holiday let with multiple units, a small office, or a commercial premises, the sizing calculation changes significantly and needs a proper commercial assessment. Have a look at our commercial boiler installation page or get in touch directly.
Still not sure what boiler you need?
Tables and guides can only take you so far. If you’d like an honest, no-obligation assessment of what your home actually needs, give us a call on 07545 198220. As Gas Safe registered and Worcester Bosch Accredited Installers, we’ll size the right boiler for your home, not just the easiest one to sell.
