Boiler Sizing – Choosing the Right Boiler

Is your engineer giving you the right advice for a suitable boiler?

Looking around the internet, there are many reviews on plumbing as well as heating products and services. A lot of these are written by heating engineers. As a heating engineer has in depth knowledge on the subject it’s sometimes a mine field for technical jargon and overly complicated explanations of what your home requires. A lot of other reviews seem to be written by either the manufacturer or retailer of the products and services. Ultimately the best source of relevant information is going to come from a heating engineer who has personally assessed your home and heating system.

A traditional way of finding a heating engineer is to get multiple quotes. In this modern age there are many ways of finding online reviews on trades people. This is truly an invaluable service, but word of mouth and personal recommendations are always the best way of finding a trusted trades person. The purpose of this article is to try and help you as a consumer to ensure your heating engineer is giving you the right advice. I will now break this down into brief categories.

So you already have a combi boiler – its simply a case of deciding what brand and what kind of warranty you can budget for. If you have the older type system with a boiler and a seperate hot water cylinder, converting to a combi boiler can be an upgrade worth considering. So if you’re going to change to a combi boiler what are the pros and cons?


The cylinder is removed giving you some extra storage space. A cylinder has stored hot water so ultimately sometimes your heating up water that wont ever be used, causing inefficiency All the old pipe work in the loft is removed – handy if your thinking of a loft conversion The new system is what’s called a sealed system, this means that without the tanks the pipework is less likely to draw air and dirt into the heating system, The old tired heating controls which can be quite technical and costly when they fail are removed .


If you want to run multiple showers you probably wont have the same hot water draw off. Combi boilers can be pretty complex and wont last as long as the simple kind of boiler you have now. A cylinder usually has an immersion heater so if there is a problem with your gas supply you can heat the water in your home through your electric (it works a bit like a kettle)

Boiler sizing

A common conception is “I have a big house, I must need a huge output boiler” Ok this is going to get fairly complicated but stay with me here. Boiler output is measured in Kw (kilowatt) Its basically just a way of measuring how much output the boiler has. To give you an example of how much heating power you need, in the late 90’s a common boiler of choice for a 4 bedroom house with a cylinder was an “Ideal Classic HE 15”. This boiler would typically run an average size water cylinder and approximately 12 radiators. Its output was 15kw. A 24kw boiler usually does heating at 18kw. So wait a second, your telling me a 24kw boiler will run my 4 bedroom house? Well aslong as the engineer is confident the pump is the right size and you don’t have massive hot water requirements, yes!

Hot water demand

A factor when sizing a boiler up for hot water demand is your mains water pressure. If you have the older conventional heating system and wish to change to a combi boiler, you must ensure you match your boiler size to the mains pressure. For example if you currently have a cylinder and 2 bathrooms, with the right system you may find you can run both showers at the same time and have a steady stream of hot water. If you are to remove the cylinder and fit a combi boiler, you probably are not going to be able to run 2 showers, unless both your showers have a flow rate matched to the mains pressure and your boiler is sized to accommodate this.

For example one of the biggest hot water flow rates on a combi boiler is a Worcester, Bosch 42 cdi. This boiler has a flow rate of 17.1 litres per minute. To explain this further, A cheap way to test your flow rate is a 99p bucket with measurements on the side. Simply place the bucket underneath a tap / shower and run cold water for 1 min. If you have 18 litres or above then a 42 cdi maybe the boiler for you. If you only have 15 litres a minute then perhaps you should think about the 30kw version and save yourself a few hundred pounds. If you have around 12 litres there’s absolutely no point in going higher than a 24kw unless your engineer has some specific reason for doing so (pump size etc).

My plumber has told me I need new radiator valves (TRV’s)

Firstly if you don’t have them, your engineer is absolutely right you should fit them. People think a TRV (Thermostatic Radiator Valve) turns up and down just like the old style valves but with a larger head making it easier. For a more detailed explination, (Wikipedia link click here) all you need to know as a customer is that its estimated you can save up to 17% on heating bills.

Condensate pipe? What’s that?

Basically the old boilers did not burn efficiently and what seems surprising to most people is when you burn gas one of the by-products from this is water vapour (just like your dryer at home) This water needs to be channelled out through a waste pipe and into a suitable drain. When condensing boilers first came about we had a few mild winters back then. Manufactures and engineers thought a 22mm plastic pipe would be suitable, then we had the bad winters, everyone’s pipes froze and boilers stopped working. Now the standard size condensate waste pipe is 32mm (inch and a quarter) As long as it’s fitted with a suitable fall you wont have any freezing issues.

Room Thermostat?

A room thermostat is a very simple device that will ensure you heat your room up to the desired temperature and then knock the boiler off until the temperature has dropped enough that it needs to fire back up and reheat the room. This cheap little device will save you from wasting gas, ultimately saving you money.


I will go more into detail about powerflushing here (insert link) If you have heavy system sludge, radiators that don’t work and generally poor performance from your heating a powerflush is more than likely required. Its not cheap, and it shouldn’t be, sometimes flushing can take longer than it takes to install the actual boiler. With boiler company’s competing for longer and longer warranties its super important that you have nothing that will invalidate this. The Ideal vogue and Baxi Platinum range both come with an astounding 10 year warranty. The last thing you want 7 years into the warranty is to find out that it’s invalid because your radiators were never cleaned properly. I strongly advise on getting a system with older radiators and pipe work, if your going to install a boiler with a big warranty then have a powerflush done at the start and a few years later as part of your servicing and maintenance schedule.

Gas pipe size

Your existing boiler has a 15mm gas connection and your engineer has quoted a few hundred quid to replace the pipe with a bigger one. Its important to understand here that your not having a made up charge, if your gas pipe isn’t suitable it needs to be upgraded. Not only will an incorrectly sized gas pipe cause your appliance to be fitted against the manufactures instructions, it also conflicts with the gas safety requirements.

To summarise

Your engineer should be recommending you a boiler that is not only suitable for your heating system but it is sized correctly for your hot water.

If you don’t have suitable radiator valves and controls for them, then you need to be upgrading. Penny pinching at this point is going to cost you money on your gas bill in the long run. Do your research on the chosen company. Regularly I have customers come to me through a recommendation, then they can see my website, my reviews and my social media photos which builds a level of trust between us. If you’re wanting a job done safely and to a high standard, you shouldn’t always go for the cheapest price you can find. Ask who your engineer is insured with and check them out on the gas safety register. This shouldn’t be a question your engineer feels sketchy about, they should be eager and willing to disclose all the relevant details. Get your quote on paper before any work starts, this covers you and the engineer. Often when going through multiple options exactly what you have agreed on can become cloudy.

I hope this guide has helped you to make sense of some of the common questions my customers seem to have confusion over. On a final note I would say it’s also important to feel comfortable with the engineers you’re having in your home. An engineer should be smart professional and approachable.

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