Key Takeaways
- Air source heat pumps provide all your hot water through an insulated cylinder, typically kept at 48 to 55°C.
- A family of four normally needs a 200 to 250 litre cylinder; a slimline or loft-mounted unit solves most space problems in combi households.
- A modern cylinder holds heat for over 24 hours, so hot water is ready before you wake, not heated on demand.
- Heating water costs roughly a third less than a gas boiler for the same use once the system runs at heat pump efficiency.
- Cylinder size and placement are decided at the heat loss survey, measured against your actual bathroom count and usage.
Table of Contents
If you heat your home with a combi boiler, the hot water question is probably the single thing standing between you and a heat pump. Not efficiency, not winter performance, not cost. The tank. Where it goes, how big it needs to be, and whether four showers in a row still come out hot.
The honest answer is that hot water is a solved problem in heat pump design, and has been for years. What it needs is a cylinder sized to your household rather than a guess, and an installer willing to walk the house with you and find the spot for it. That is survey work. This guide covers what the survey will tell you, so nothing about the tank comes as a surprise.
Do Heat Pumps Provide Hot Water? Yes, All of It
An air source heat pump does two jobs: space heating through your radiators or underfloor heating, and domestic hot water through a cylinder. The same outdoor unit does both, switching between them through a diverter valve. There is no separate water heater, no immersion-only setup in normal use, and no compromise where the house is warm but the taps run cold.
The difference from a combi is when the water is heated, not whether. A combi heats water the moment you open the tap. A heat pump heats a full cylinder in advance, usually overnight and in the early afternoon, then the cylinder holds it until you use it. Open the tap and hot water arrives at mains pressure, same as any unvented cylinder system. In practice the change you notice is positive: no more waiting for a combi to wind up, and no pressure drop when two showers run at once.
A heat pump will also prioritise sensibly. When the cylinder needs a top-up, the system diverts to water for a short period, then returns to space heating. The house does not go cold while the tank heats; the cycles are short and scheduled around your usage.
Why the Cylinder Is the Point, Not the Problem
The cylinder is what lets a heat pump heat water efficiently. Heating water slowly, in advance, at heat pump efficiency costs far less than heating it instantly at gas prices, and the store means the expensive moments of your day, the 7am shower rush, run off heat that was made cheaply hours earlier.
Modern units are unvented, pressurised and seriously insulated. A decent cylinder loses so little heat that water heated at lunchtime is still hot the next morning. Pair the heating schedule with a smart electricity tariff and most of your hot water gets made at the cheapest hours of the day, which is a saving a combi physically cannot copy, because a combi can only heat water at the moment you demand it.
If your last experience of a tank was a 1980s vented copper cylinder in an airing cupboard, gravity-fed and lukewarm by evening, that is not what is being proposed. The comparison misleads more Scottish homeowners than any other piece of heat pump folklore.
Get a cylinder recommendation measured against your actual bathroom count and morning routine, not a brochure default.
“Outstanding service, difficult decision to make to change to air source heat pump especially give the level of misinformation. Sean/Aventus Eco were outstanding in helping me explore and decide that this was right solution. Additionally it involved full house re-plumb because of problems with the old system, This was handled sensitively and professionally by all the members of the team with Sean setting very high expectations for everyone.”
Robert Henderson, ★★★★★ Google Review
There is a lot of misinformation out there about heat pumps. Sean took the time to explain everything, answered every question, and the install was spot on. I could not be happier with the result.
What Size Hot Water Cylinder Do You Need?
Cylinder sizing follows people and bathrooms, not house size. The working rule is 35 to 45 litres per person per day, then round up for bathing habits and bathroom count. In practice:
- 1 to 2 people, one bathroom: 150 to 180 litres
- 3 to 4 people, one or two bathrooms: 200 to 250 litres
- 4 to 5 people, two-plus bathrooms or a big bath: 250 to 300 litres
- Larger households or high-occupancy homes: 300 litres and up
Two things push the number around. Baths beat showers for draw-off, so a household of dedicated bath-takers sizes up. And reheat time matters less than people fear: a heat pump can rewarm a 250 litre cylinder in roughly an hour to ninety minutes, so even an unusually heavy morning does not leave you cold until tomorrow.
Undersizing is the mistake that hurts, and it is the classic shortcut of a rushed install. The right number comes out of the same heat loss survey that sizes the heat pump itself, which is exactly how we approach heat pump installation across Scotland: measure first, specify second.
Where the Cylinder Goes When You Have a Combi
Combi households, by definition, got rid of their tank. The good news is a modern cylinder does not need the old airing cupboard back. Where they actually end up in Scottish homes:
- The old cupboard anyway. Many combi homes still have the airing cupboard the combi replaced, now full of towels. A slimline cylinder often fits it.
- The loft. Structurally checked and properly supported, a loft install keeps the footprint out of living space entirely. Common in bungalows and interwar semis.
- A utility room or garage corner. A 250 litre cylinder needs roughly a 60cm square footprint. Most utility rooms have it.
- An under-stair or hall press. Tenement and terraced homes frequently hide a cylinder in the press without losing usable storage.
Slimline and horizontal formats exist precisely for awkward homes, and placement affects pipe runs and heat loss, so it is decided at the survey, not on installation day. If you are replacing a combi wholesale, the practical steps are covered in our guide to replacing a boiler with a heat pump.
Find out where a cylinder would go in your home before you commit to anything.
“Great experience using Aventus Eco to install a heat pump at our terraced home. Very efficient from start to finish, it took 4 days and we’re really happy with how it’s working. Sean took the time to talk us through how it all works and has been on hand to answer any question post-installation.”
David Acheson, ★★★★★ Google Review
From start to finish the work was carried out with real attention to detail and care. You can tell it matters to them that the system performs at its best, not just that it is in.
Water Temperature, Comfort and the Legionella Cycle
A heat pump stores water at 48 to 55°C. A gas boiler typically stores or delivers at 60 to 65°C. That difference alarms people until they check what they actually use: a comfortable shower runs at 38 to 41°C, and a hot bath around 44°C. Nobody showers at 60°C. Stored water at 50°C still gets mixed down with cold at the tap, exactly as boiler-heated water does.
Running the store cooler is deliberate. Every degree lower improves the heat pump’s efficiency, so the system holds the cylinder at the temperature your household needs rather than a temperature tradition expects.
Legionella is handled by design, not luck. The system runs a scheduled sterilisation cycle, typically weekly, lifting the cylinder above 60°C using the heat pump and, where needed, the immersion backup. It is automatic, costs pennies per week, and is a standard requirement of a compliant installation. Any installer vague on this at the survey stage is telling you something about the rest of their design.
What Hot Water Costs to Run on a Heat Pump
Hot water is around 15 to 20 percent of a typical household’s heating energy, and it responds to heat pump efficiency the same way space heating does. Heating water with a heat pump at a coefficient of performance around 2.5 to 3 means each kilowatt-hour of hot water costs roughly a third to 40 percent less than the same water heated by a 90 percent efficient gas boiler at current Scottish prices, and the gap widens on a smart tariff.
The schedule is where the money hides. Set the cylinder to heat in the small hours on an off-peak rate and again in the early afternoon, and most of your hot water is made at the cheapest electricity of the day. The store does the time-shifting for you. The full picture across space heating and water is in our breakdown of heat pump running costs in Scotland, and system sizing, which drives all of it, is covered in the heat pump sizing guide.
Get a running cost estimate for heating and hot water based on your home and your tariff, not national averages.
“From the first phone call, I knew I was in good hands. Sean took the time to understand my needs and walked me through the entire process, from initial consultation to installation day. There were no surprises, the installation was done ahead of schedule, and I’ve already noticed a drop in my energy bills.”
Shereen Ackrim, ★★★★★ Google Review
From the first phone call I felt in good hands. Everything was explained, the work was finished ahead of schedule, and our bills have come down since.
Get Your Hot Water Sizing Answered
Hot water sizing, cylinder placement and running costs all come out of one engineer-led survey of your actual home.
Frequently asked questions
Do air source heat pumps provide hot water?
Yes. An air source heat pump heats your radiators and your domestic hot water through an insulated cylinder. The same outdoor unit does both, and the system switches between them automatically. Stored water is delivered at mains pressure, so showers and taps behave exactly as they do on an unvented boiler system.
Do I need a hot water cylinder with a heat pump?
Yes, in almost every domestic installation. Heat pumps heat water gradually and efficiently rather than instantly, so the cylinder stores hot water ready for use. Modern unvented cylinders are compact, pressurised and insulated well enough to hold heat for more than 24 hours.
What size cylinder does a family of four need?
Usually 200 to 250 litres, sized on 35 to 45 litres per person per day plus an allowance for baths and bathroom count. The exact figure is set at the heat loss survey alongside the heat pump sizing itself.
How hot does the water from a heat pump get?
The cylinder is typically stored at 48 to 55°C, mixed down at the tap exactly as boiler-heated water is. A scheduled sterilisation cycle lifts the store above 60°C regularly to manage legionella automatically.
Can I have a heat pump if I currently have a combi boiler?
Yes. The install adds a cylinder, and in most Scottish combi households the space exists in the old airing cupboard, the loft, a utility room or an under-stair press. Slimline formats cover awkward homes, and placement is confirmed at the survey.
Does a heat pump heat radiators and hot water at the same time?
It alternates. When the cylinder needs a top-up, a diverter valve sends the heat pump’s output to water for a short scheduled period, then returns to space heating. The switching is automatic and the house does not cool during water cycles.
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