Question 1

Should I size the boiler with a margin?

Accepted Answer

Yes, 20–30% above the calculated figure. At −30 °C the boiler should run at no more than 70–80% of capacity, which extends burner life. A 16 kW boiler in place of a 13 kW one is a sensible choice.

Question 2

Single-circuit or combi (dual-circuit)?

Accepted Answer

A single-circuit boiler handles heating only. A combi also provides domestic hot water. If you have an indirect-heating cylinder, pair it with a powerful single-circuit boiler. A combi is convenient for low hot-water demand (2–3 people).

Question 3

Gas, electric or solid fuel?

Accepted Answer

Mains gas is the cheapest to run. Electric is expensive but needs no flue and is easy to automate. Pellets and firewood give independence where there's no gas. A heat pump has low running costs but high upfront investment.

Question 4

What is boiler efficiency and how much does it matter?

Accepted Answer

Efficiency is the share of heat passed to the water. A conventional boiler runs at 88–92%. A condensing boiler reaches 104–109% (it recovers the latent heat of the flue vapour), saving 15–20% on gas, though it costs more to buy.

Question 5

Do I need a hydraulic separator (low-loss header)?

Accepted Answer

With a boiler of 35 kW or more and several heating circuits (underfloor heating plus radiators), yes. It separates the primary and secondary circuits and protects the boiler from disrupted flow.

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