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What size boiler do I need? How to choose?

11 Answers

Answered by SDW Plumbing & Heating

27th June 2020

SDW Plumbing & Heating answered the question "What size boiler do I need? How to choose?"

Gas engineers use a few methods to size boilers when doing surveys. If you are requiring a new combi, then we measure the incoming mains water pressure. Combi boiler sizes refer to the amount of hot water it can deal with. If you have a good incoming water main then we need to size the boiler to be able to cope with that main. Otherwise you will have poor hot water performance. If we are surveying for a heat only boiler, then the amount of radiators and their size come into play, along with the load of the hot water cylinder.

Answered by Careys Plumbing & Gas Ltd

22nd September 2019

Careys Plumbing & Gas Ltd answered the question "What size boiler do I need? How to choose?"

A standard 3 bed property will only ever use up to 14kw for central heating. The reason a combi model will start from 24kw is the production of hot water. The higher the KW the higher the hot water output. A 24KW combi will produce around 9-10 litres of hot water per minute. Some models will go above 40KW producing over 16 litres per minute. However it is important to measure the incoming flow rate of your cold main supply before choosing a boiler size.

Answered by BoilerOne

25th May 2019

BoilerOne answered the question "What size boiler do I need? How to choose?"

Sizing a boiler suitable for your home is dependant on the system you have, with a combination boiler we would be looking at the flow rate on the incoming mains and the hot water demand for the property, then account for the radiators and any heat loss in the building. With a heat only boiler we would calculate the radiators then the size of the hot water cylinder and again any heat loss in the building. BoilerOne recommend you ask a gas safe registered engineer to size your boiler

Answered by LP Plumbing & Heating Ltd

19th May 2019

LP Plumbing & Heating Ltd answered the question "What size boiler do I need? How to choose?"

If you are looking for a combi boiler then sizing is down to flow rate (there are other factors involved but generally the main and most important factor is flow rate) . This is measured using a weir cup, most boilers will have a minimum flow rate at which they will work at. This is because they have a heated plate inside, the size of the plate denotes how many litres per minute can run through it to be heated efficiently and to the correct temperature. If this isn't sized properly the boiler could have heat loss and in some cases will not modulate properly. The boilers manufacturer instructions will state how much flow rate there boilers need, so if you have a heating engineer come to your home to survey for a new combi and they don't carry out a flow rate test I would be asking them how they can size it without it. Heat only boilers (traditional gravity system) are sized differently but to keep it simple using how many radiators the home has, and how big the hot water cylinder is, once you have this information ie hot water demand and heating demand you can calculate what size boiler you need. Again there are other factors involved such as heat loss but this is the nuts and bolts of it.

Answered by J. Wheatley Gas, Plumbing & Heating Limited

3rd May 2019

J. Wheatley Gas, Plumbing & Heating Limited answered the question "What size boiler do I need? How to choose?"

The actual boiler required will depend on your property type and its size. Here at J.Wheatley Gas Plumbing and Heating Ltd. we offer a FREE boiler quotation service in which we can help and advise on which boiler you choose with our technical knowledge and fully trained, Gas Safe registered Engineers.

Answered by Heat Trend

27th March 2019

Heat Trend answered the question "What size boiler do I need? How to choose?"

Boiler sizing will firstly depend on what type of boiler you are fitting. There are 3 types firstly and most commonly is the Combination or Combi Boiler. With a combi boiler they generally have a high output due to having to heat the hot water on demand therefore it is important to size a combi according to the water flow rate coming into the property and the needs of the customer. The other 2 types of boiler you will find are the System and Regular boiler, both working in a similar manner with the system having a pump built in and designed for sealed systems. They both heat water indirectly via a cylinder coil. To size these we calculate the heating demand of the radiators and add to that the KW that the cylinder coil is rated to. It is normal to upsize slightly allowing for additions to be made later on however be careful not to over size as this can cause the boiler to cycle.

Answered by Kingston Gas Services Ltd

23rd March 2019

Kingston Gas Services Ltd answered the question "What size boiler do I need? How to choose?"

To calculate the output of a boiler is a complex process, you should get a trained professional to size this correctly, choosing a boiler too big is a common mistake, customers think bigger is better, but sometimes this can lead to costly bills, too small and your heating or hot water isnt going to work properly, Its important to take everything into account and arrive at the correct output and then 10%

Answered by Nickthegas

20th March 2019

Nickthegas answered the question "What size boiler do I need? How to choose?"

Carry out heat loss calculations for the building This will tell you the KW heat loss. Add 10% for pipework. Find out your cylinders coils output and add this to that figure. This will tell you the size of boiler required for your property. If you live in an exposed area then you will require a bigger boiler same as if you don't have any insulation or double glazing If its a combi then the same calculation is used but ignore the hot water element. Carry out water flow test at main tap this will tell you what the maximum expected flow rate you can expect at the hot tap. However if anyone flushes the loo or another tap is opened then the rate will drop. So if you have multiple outlets then a combi would not be suitable, also of you have a combi, shower valves may need to be changed and even high pressure toilet valves may need changing also. So look for the hot water output for that boiler and that is what would be required.

Answered by W Heathcote Plumbing and Heating

19th March 2019

W Heathcote Plumbing and Heating answered the question "What size boiler do I need? How to choose?"

Depends on the type of system the boiler is serving and the size of the heating and whether it's stored hot water or not. If it's a combi a good guide is the flow rate of the mains cold water for example if it's a poor flow rate there is no point in fitting a high output boiler A survey of the system should be carried out by your heating engineer so he or she can advise on the best options.

Answered by BPH - Blakemore Plumbing And Heating

13th February 2019

BPH - Blakemore Plumbing And Heating answered the question "What size boiler do I need? How to choose?"

If installing a combi boiler then the cold water flow rate would determine the size of your boiler. Roughly a 35kw can give you 14lpm in hot water at an outlet where as a 24kw would give you around 9.5lpm both at a 35° rise. There would be no point in having a 35kw combi boiler if you haven’t got the flow rate. On a standard boiler or system boiler then you would size to your output requirements, the kw rating of the cylinder coil and radiator outputs would tell you this. The majority of boilers in the Uk are oversized, your heating engineer so size correctly and advise you.

Answered by Elements Heating Installations

8th February 2019

Elements Heating Installations answered the question "What size boiler do I need? How to choose?"

The way i size my boilers depends on the house size, everyday needs and amount of people living in the property. if the customer is looking for a system or heat only boiler then we look at the heat loss of the building. if a combi boiler is needed then we look more towards how many litres per minute of hot water is required. for example a 3 bed 1 bathroom with a shower would normally require a 30kw combi boiler which would give a total heat out put of 24kw if needed on the rads and a steady 11 to 13 litres per minute of hot water.

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