Almost every guide you will read online answers this question the same way: count your bathrooms. One bathroom, get a combi. Two or more, keep your cylinder.
After thirty years on the tools I can tell you that advice gets it wrong more often than it gets it right. The number of bathrooms in your house is close to irrelevant. What actually matters is how many of them are being used at the same time.
The question that actually matters
Not "how many bathrooms have I got?"
But "how many showers or baths are ever running at the same moment?"
Why Simultaneous Demand Is the Real Test
A stored hot water system — a cylinder, and in older properties a couple of tanks in the loft — exists to solve one specific problem. It lets you draw a lot of hot water from more than one outlet at once, because the water has already been heated and is sitting there waiting.
That is genuinely useful in a busy house. Two teenagers showering before school while someone runs a bath is exactly the job a cylinder is designed for.
But plenty of houses have two bathrooms and never once use both at the same time. The second bathroom is an en-suite that gets used in the morning while nobody else is up, or a downstairs shower room that sees a shower a fortnight. In that house the cylinder is solving a problem that does not exist.
Households Change — Heating Systems Usually Do Not
This is the situation I see most often, and it is worth being honest about because it is probably the single biggest reason people convert.
A family moves into a three or four bedroom house. Kids at home, two bathrooms both going flat out every morning. The cylinder and tanks are exactly right for that household.
Ten or fifteen years later the kids have moved out. There are two of you. One shower runs in the morning and that is the extent of the hot water demand in the house.
But the system has not changed. You are still heating a full cylinder of hot water every day for a household that uses a fraction of it. The rest sits there, slowly going cold, and gets reheated. You are paying to heat water nobody uses.
A combi heats what you use, when you use it
There is no tank, no stored water and nothing being kept hot on standby. The boiler fires when you open the tap and stops when you close it. If your demand is genuinely one outlet at a time, you get the same hot water for less.
Pressure and Flow Rate Are Not the Same Thing
This is where most of the worry about converting comes from, and it is nearly always a misunderstanding of two different measurements.
Pressure is the force behind the water. When you convert to a combi you are running on incoming mains pressure. In most properties that is the same as, or better than, what you had before. You are not losing pressure.
Flow rate is the volume of water, measured in litres per minute. This is the number that changes.
| System | Typical flow rate |
|---|---|
| Unvented / pressurised cylinder | Around 25 litres per minute |
| Combi boiler | Roughly 12–15 litres per minute, depending on the model fitted |
On paper that looks like a big drop. In practice, for one shower running on its own, most people do not notice it — a shower does not need 25 litres a minute. That extra capacity only earns its keep when you are splitting it across two or more outlets at once.
Which brings it back to the same question: are you actually running two at once? If not, you are paying for flow rate you never draw on.
When You Should Keep the Cylinder
I am not going to pretend a combi is right for everyone, because it is not, and I would rather tell you that at the survey than have you unhappy afterwards.
Stick with a stored system if:
- Two or more showers genuinely run at the same time on a regular basis
- You have a large household where bathroom use overlaps every morning
- You have a bath and shower that are frequently in use simultaneously
- Your incoming mains flow rate is poor — this is something I check on the survey, because a combi can only ever deliver what the mains brings into the property
That last point matters and it is why I will not quote a conversion over the phone. The mains supply has to be capable of feeding a combi properly. It usually is, but it needs checking rather than assuming.
The Practical Benefits People Actually Notice
You get your airing cupboard back
Removing the cylinder frees up the whole cupboard. If the loft tanks go too, that is loft space back as well. For a lot of households this is genuinely the main appeal.
No waiting for a reheat
With a cylinder, once the stored hot water is used you wait for it to reheat. A combi does not run out — it heats continuously for as long as the tap is open.
Fewer components to go wrong
No cylinder, no expansion vessel for the stored water, no loft tanks and no associated pipework. Less to leak, less to fail.
What a Conversion Actually Involves
It is a bigger job than a straight boiler swap, and it is fair that you know that up front:
- The old boiler is decommissioned and removed
- The hot water cylinder is drained and taken out
- Any loft tanks are removed
- Redundant pipework is capped off or stripped out
- Hot and cold supplies are reconfigured to run from the new combi
- The new boiler is fitted, commissioned and registered for warranty
- Everything is cleared away and taken with us
In most cases we complete a conversion in a single day. It is priced differently to a like-for-like swap because there is genuinely more work in it — but you get a straight fixed price at the survey with no surprises added afterwards.
Not Sure Which Is Right for Your House?
I will come out, check your mains flow rate, ask how you actually use the hot water, and give you an honest answer — including telling you to keep your cylinder if that is the right call.
📞 01942 800158 Book a Free Survey →Frequently Asked Questions
Should I convert to a combi boiler if I have two bathrooms?
It depends on whether both are genuinely used at the same time, not simply on how many you have. Two bathrooms where only one shower ever runs at once is a good candidate for a combi. Two showers regularly running together is usually better served by a cylinder.
Will I lose water pressure if I convert to a combi?
No. Pressure and flow rate are different things. A combi runs at incoming mains pressure, often the same or better than a stored system. What changes is flow rate — an unvented cylinder might give around 25 litres per minute against roughly 12–15 for a combi. For a single shower that is not usually noticeable.
Does a combi save money compared to a cylinder?
A combi heats water on demand rather than heating and storing a full cylinder. If you are a smaller household heating a large tank you no longer need, you are paying to heat water that goes cold unused. A combi removes a running cost you are getting no benefit from.
Do I get more space by removing the cylinder?
Yes. The airing cupboard is freed up entirely, and if the loft tanks come out you get that space back too. For many households this is one of the main practical reasons for converting.
How long does a combi conversion take?
In most cases a single day. It is more involved than a straight swap because the cylinder, tanks and redundant pipework all have to come out, but it is very rarely a multi-day job.
The Bottom Line
Forget the bathroom count. Ask yourself how many showers or baths are ever running in your house at the same moment. If the honest answer is one, you are almost certainly heating and storing hot water you do not need, and a combi conversion will give you the same hot water, the same pressure, more cupboard space and a lower running cost.
If the answer is two or more on a regular basis, keep your cylinder — and I will tell you that to your face rather than sell you something that will not suit you.
We carry out combi conversions across Wigan, Bolton, Leigh, Warrington, St Helens and the surrounding areas. If you are weighing it up, give me a ring and I will come and have a proper look.
Book a Free Survey
Honest advice, fixed prices, and a straight answer on whether a conversion is right for your home. Gas Safe registered 928523.
📞 Call 01942 800158 Get My Quote →Lewis Heating & Plumbing Ltd · Company No. 16460026 · Gas Safe Registered 928523 · Hindley Green, Wigan · FCA FRN 1055713