Gas hot water — stored, or on demand
Domestic hot water
There are two genuinely different ways to make hot water with gas, and the choice is not really about the boiler — it is about whether your house has somewhere to put a cylinder, and whether more than one shower runs at once.
The two answers
System boiler and cylinder, or a combi
Both are Vaillant ecoTEC plus. The difference is what happens after the boiler.
System boiler + cylinder
- The boiler heats a stored cylinder through an indirect coil
- Several outlets can run at once without the flow dropping
- Needs somewhere for the cylinder to live — 180 to 500 L
- Recovers fast, so a smaller cylinder goes further than it would on a heat pump
- The same boiler runs your heating at the same time
Combi — no cylinder
- Makes hot water on demand as it flows through, 34.4 kW instantaneous
- No cylinder at all, so no storage space and no standing loss
- Flow rate is the limit — two showers at once reduces the flow to each
- Suits apartments, small houses, and homes with no space for a cylinder
- Hot water takes priority; heating pauses for the minute or two you are in the shower
Gas boiler to stored hot water
Swipe the diagram sideways to follow the full circuit →
The boiler charges, it does not store
Gas in, flue gases out, condensate to the drain. The boiler's job is to push warm water round the primary circuit whenever the cylinder asks for it.
Heat crosses at the coil
Primary water runs through a coil inside the cylinder and hands its heat to the stored water. The two never mix. A sensor in the cylinder tells the boiler when to fire.
A full tank, quickly reheated
Cold water in at the bottom, hot out of the top to every tap. A boiler recovers a cylinder fast, which is why sizing is about recovery as much as litres.
The boilers we fit
Click a boiler to see its full specifications.
Sizing the cylinder
Stored hot water
Cylinders are more often undersized than oversized. Our calculator uses the same method we use at design stage.
Gas or heat pump
Gas and heat pump are not rivals in our shop — we install and service both, every week. What follows is how we actually decide.
A hot water heat pump suits you if…
- Hot water is your biggest single power bill — it usually is
- You already have or want a heat pump for the heating
- There is outdoor space for the unit and room for a cylinder
- You want the cheapest hot water available and can wait for recovery
Gas suits you if…
- Gas is already at the house
- You want fast recovery, or no cylinder at all
- Space is genuinely tight — a combi needs no storage
- The same boiler is doing the heating anyway
If gas is already at the house and the boiler position is easy, a condensing boiler is normally cheaper to put in and more expensive to run. A heat pump is the other way round. Which wins depends on how long you are staying, what the gas connection costs, and whether you also want pool or hot water off the same plant. Ask us and we will price both.
Questions
Gas hot water, answered
Combi or cylinder?
If you have somewhere for a cylinder and more than one bathroom gets used at once, take the cylinder. If you are in an apartment or a small house and the cupboard space is worth more to you than simultaneous showers, take the combi. There is no universally right answer.
Can a combi run underfloor heating as well?
Yes. It heats the house and makes hot water on demand, prioritising hot water. On underfloor you never notice the pause, because the slab does not cool measurably in two minutes.
What temperature does the cylinder need to be at?
Hot enough to keep the stored water safe, with a tempering valve on the outlet so nothing leaves the cylinder at scalding temperature. That valve is not optional and we fit it as standard.
Will it work with my existing cylinder?
If it has an indirect coil and the coil is in good condition, often yes. If it is a direct electric cylinder with no coil, no — there is nothing for the boiler to heat through.
Free, no obligation
Request a quote
Send the plans, or tell us what is there now. We will price the gas option and the heat pump option side by side.