The tool works from simultaneous and sequential draw-off during the peak period, the reheat rate achievable from your heat source through the coil, and the storage temperature with blending – which together give a far better answer than occupancy tables.
Why occupancy tables are inadequate
The traditional approach sizes a cylinder from bedroom count or occupancy. It produces an answer, and it ignores everything that determines whether the answer is right.
- Simultaneity. Two showers at once is a completely different requirement from two in sequence, and occupancy does not capture it.
- Shower type. A high-flow mains-pressure shower can use several times what a gravity one does. A rain head with body jets is a different order again.
- Reheat rate. A cylinder that reheats quickly effectively has more capacity over a morning than a larger one that reheats slowly.
- Storage temperature and blending. A cylinder stored hot and blended down at the outlet delivers more usable litres than its nominal capacity.
- Usage pattern. A household that showers over ninety minutes has a different requirement from one that showers over twenty.
- The heat source. A boiler recovers a cylinder quickly; an immersion heater does not; a heat pump needs a much larger coil.
The questions the tool asks
- How many showers on a typical weekday morning, and over what period?
- Are any of them simultaneous?
- What type of shower – gravity, mains-pressure mixer, electric, high-flow?
- Is there a bath, and is it used regularly?
- What is the heat source – oil boiler, gas boiler, heat pump, immersion?
- What is the coil surface area, or which cylinder model is proposed?
- What storage temperature, and is there a blending valve?
- Is there a second peak – an evening bath routine, for instance?
- Is a heat pump planned now or later?
Two cylinders of identical volume can perform completely differently depending on coil surface area, because the coil determines reheat rate and reheat rate determines how much water you effectively have over a busy morning. If you run out of hot water, the coil is as likely to be the answer as the volume – see sizing a hot water cylinder.
Heat pumps change the arithmetic
A boiler delivers to the coil at a high temperature, so a modest coil transfers plenty of heat. A heat pump delivers at a much lower temperature, so the temperature difference driving heat through the coil is far smaller.
To transfer the same heat with a smaller difference, you need substantially more surface area – which is why heat pump cylinders have very large coils, frequently several times the area of a standard boiler cylinder.
Two practical consequences:
- Fitting a heat pump to an existing boiler cylinder is a common cause of disappointment, because the cylinder becomes the bottleneck.
- If a heat pump is in prospect now or later, specifying a heat-pump-rated cylinder at replacement is worth considering. It works perfectly well with a boiler and it removes a future obstacle.
Getting it wrong in either direction
| Undersized | Oversized | |
|---|---|---|
| Symptom | Runs out during the morning peak | Higher standing losses, continuously |
| Cost | Quality of life, and the household adapts by staggering | Energy, every day of the year |
| Discovered | First winter | Rarely noticed |
| Legionella consideration | None | Slow turnover in stored water is a genuine risk factor |
| Reheat from cold | Fast | Slower |
| Space and weight | Less | More |
| Fixable | Only by replacement | Only by replacement |
Oversizing is the milder fault and it is not free. The right answer is a calculation from peak demand, not generosity.
What the tool produces
A recommended volume range, with attention to coil surface area, and a note of the assumptions it used – so you can see what it based the answer on and challenge it if your household does not match.
It will also flag the situations where the cylinder is not the constraint:
- Where the incoming mains cannot feed an unvented cylinder of the size indicated – in which case the supply is the issue, not the cylinder.
- Where the heat source cannot reheat the indicated volume in the available time.
- Where the physical space cannot accommodate it.
- Where a combi would actually suit the household better, which occasionally it does.
The architecture for it is defined and it is not yet live. When it is, it will be free, require no email address, and store nothing. The method described on this page is the same one we use in practice, and you are welcome to work through it with us in the meantime.
Running out of hot water? Check the cylinder thermostat and the reheat rate before concluding it is too small.
Questions we get asked about this
How big a hot water cylinder do I need?
It depends on peak demand – how many showers, whether any are simultaneous, what type, and how fast it reheats between them. Bedroom count and occupancy are poor guides.
Why does coil size matter?
It determines the reheat rate. A cylinder that reheats fast has more effective capacity over a busy morning than a bigger one that reheats slowly. It is the most overlooked factor in cylinder selection.
Do I need a bigger cylinder for a heat pump?
You need a different one, with a much larger coil. Heat pumps deliver at lower temperatures and need far more surface area to transfer the same heat. A standard boiler cylinder will cripple a heat pump.
Is a bigger cylinder always safer?
No. Oversizing means higher standing losses continuously, and in a low-demand household it means slow turnover in stored water, which is a legionella consideration.
Can I get more from my existing cylinder?
Sometimes – storing at the correct temperature with a blending valve at the outlets delivers more usable litres than storing cool. Check the thermostat setting and the reheat rate first.
Is the tool available now?
Not yet. The method is what we use in practice and you are welcome to work through it with us on the phone.
Need this doing?
Call and describe the problem — you will speak to an engineer, not a call centre. Written quotes, no estimates over the phone.