Combi or System Boiler for a Large Edinburgh Home with Two Bathrooms?
Bathroom count is only a prompt. The right choice depends on simultaneous use, incoming mains performance, available storage and the household's recovery expectations.
Direct answer
A system boiler with a correctly sized cylinder is often the more resilient choice when an Edinburgh household regularly uses two showers or other hot-water outlets together. A combi may still suit where simultaneous demand is low and measured mains flow and pressure support the appliance. Test demand and incoming supply before choosing.
What the survey should measure
Record normal and peak use: two showers, bath filling, kitchen draw-off and household timing.
Measure incoming mains flow and dynamic pressure at relevant outlets. A high boiler output cannot deliver water the main does not supply.
Inspect the existing cylinder, cold-water arrangement, distribution and space for an appropriately sized replacement; check access for removal and installation.
Where a combi can work
A combi avoids stored hot water and can free cupboard space. It can be effective for a home where outlets are rarely used together and the incoming main is strong.
Its hot-water delivery is constrained by the appliance and incoming main. Marketing flow rates should not be treated as guaranteed performance at every Edinburgh address.
Boiler space heating output and domestic hot-water output are different design questions; do not oversize space heating solely to chase a hot-water headline.
Where a system boiler and cylinder can work
Stored hot water can support overlapping demand more comfortably when the cylinder, primary circuit and distribution are designed together.
Cylinder volume is not selected from bathroom count alone. Occupancy, shower type, bath use, recovery time, heat source and available space all matter.
Unvented cylinders require appropriate competence, safety devices, discharge arrangements and servicing. Incoming flow still matters because storage cannot create mains capacity indefinitely.
Future choices
A renovation may create a better cylinder location or allow pipe routes to be improved before finishes go down.
If heat-pump readiness is a genuine future objective, assess emitter temperatures, heat loss and cylinder coil suitability now—but do not sell a generic 'heat-pump-ready' claim without a design.
Frequently asked questions
Can a combi boiler run two showers at once?
Sometimes, but only if the measured incoming main and selected appliance can support the combined demand at acceptable temperature and pressure. A site test is more useful than bathroom count alone.
Is a cylinder better for a large family?
Often when hot-water use overlaps. The cylinder, recovery rate and distribution must be sized from the household's demand, available space and heat source rather than a generic litres-per-person rule.
Who can install an unvented hot-water cylinder?
It must be installed and commissioned by a person with the appropriate unvented-hot-water competence. Gas work separately requires a Gas Safe registered engineer qualified for that work.
Quoted on Monday, fitted by Friday. Fixed price, no surprises, and the engineer cleaned up better than I did. House is finally warm in the upstairs bedrooms.
Sarah M., Morningside EH10 — Worcester combi boiler swap · 5/5 ·