What is a combi boiler?
A combi boiler combines a highly efficient water heater and a central heating boiler into one compact unit. As a result, the combi boiler is fast becoming the heating system of choice in new homes across the UK, comprising 50% of new boiler installations.
Combi boilers and storage space
Combi boilers provide central heating and hot water without the need for a hot water cylinder in an airing cupboard or a cold water tank in the loft. The combi boiler is not only an ideal space-saving solution, but it also limits the amount of wasted heat when compared to traditional models, as it does not have a hot water tank.
Combi boilers and heating hot water
Combi boilers take water from the mains and then heat it as required, but are on standby when there is no water demand. Therefore hot water is available 24/7 and also just as importantly, the water is delivered at mains pressure, so no pump is required to deliver the ‘power shower’ experience (provided your home has adequate mains water pressure).
To produce the hot water that comes out of the taps, the cold mains water passes over a highly efficient heat exchanger. The heat exchanger transfers the vast majority of the heat from the burnt gas (90% or more) to the cold water, and then delivers it to the taps as required. Since combi boilers do away with the hot water tank, they are much more energy efficient, since there are no heat losses associated with stored hot water, it is simply created as needed.
>>> Should I replace my conventional boiler with a combi? <<<
Combi boilers and your central heating system
On the central heating side, combi boilers pump the water round the heating system in a completely sealed system. The boiler incorporates an expansion tank internally, so there is no need for an external feed and expansion tank in the loft, which obviously saves a great deal of space.
One of the issues with a combi boiler is that they struggle to produce both hot water and central heating at the same time, so priority is given to your domestic hot water whenever a hot water tap is opened in the home. For example, if you are running a bath, during this time no hot water will be circulated through your heating system, but as soon as the bath is full, the central heating circulation will resume.
Combi boiler efficiency
Since combi boilers don’t have to store any hot water, the heat losses via the hot water tank are zero, helping save you money on your gas bills.
Most boilers installed today are condensing boilers, so if you choose a combi boiler, this will be a condensing model. These types of boilers use the heat contained within exhaust gases that would normally be released in to the atmosphere via the flue. They take advantage of heat exchangers to make the most of this latent heat, so less fuel is needed each time to heat the water, thereby further reducing your fuel costs.
The main 2 issues with combi boilers
The first issue is that since the hot water is produced at mains pressure, if the hot water is required in multiple outlets at the same time (e.g. a shower and the washing machine are running at the same time), there will be pressure drops at the different outlets. There are two solutions for this; firstly you can install a mains booster which recognises when pressure and flow are low and automatically boosts the performance of the incoming mains water. These cost in the region of £300 – £500. The second solution is to install an accumulator tank (approximately £500 – £1000 depending on the size), which is a steel vessel that stores mains water at the pressure it is supplied into the home. Air within the steel vessel (trapped in a rubber diaphragm) is compressed by the mains water. When a tap is opened, the water can rush out of the vessel to the tap faster than the mains can get into the property. The downside of the accumulator tank is that once all the water in the tank is gone, you have the same pressure issues that you would experience without it.
The other issue is that if your combi boiler breaks, you will have no hot water generating facility other than the kettle. Most hot water tanks (within the system or regular boiler systems) come equipped with an electric immersion heater, so even if the boiler breaks you can still produce hot water for showers and baths (albeit in a rather expensive way).
Final thoughts on combi boilers
In homes where hot water demand is not huge, then a combi boiler is the most effective central heating/hot water system. They are very reliable and simple to install, they save space (since they do away with cold and hot water tanks) and produce the hot water on demand as it is required in the home. As a result, in most situations we would recommend installing this type of boiler when your current boiler reaches the end of its useful life.
Benefits
- The hot water is available 24/7 on demand at mains pressure, giving that ‘power shower’ effect.
- Combi boilers are very compact and can sit fairly innocuously in your kitchen, or take up a small amount of space in your airing cupboard.
- Combi boilers only produce hot water when it is needed, so no hot water is produced unnecessarily, which can help lower your bills.
- There is no need for a hot water tank in an airing cupboard or a cold water storage tank in the loft.
Limitations
- The main issue with a combi boiler is that it will struggle to cope with high demand for hot water from multiple sources at the same time, because the hot water flow rate is divided by each of the water sources.
- If the combi boiler does break down for some reason, you will have no capacity to produce hot water other than the kettle.
- Combi boilers are currently not compatible with solar thermal technology.
Cost
- The cost of installing a brand new combi boiler in your home will obviously vary between suppliers, but prices will start from about £1800 + VAT to replace an old combi with a new energy efficient one. To replace a regular boiler or system boiler with a combi will cost an extra £500 approximately.
- Companies like Heatable offer finance options, allowing you to replace your boiler from as little as £10.28 per month
Installing a new boiler
Are you thinking about getting a new boiler? If you would like us to find you a local installer to install a new boiler in your home, just fill in the form below and we will be in touch shortly!
I presume the heating acts as a slave to the hot water when you install a combi boiler? i.e. if you have the hot water running, no hot water will be sent around the radiators? Is that correct? It is only that my small tribe at home tend to all shower in the morning so I guess we wouln’t be able to have the heating on at the same time as everyone jumps in the shower, so would need to set the heating on earlier in the morning?
Exactly, combi boilers struggle to provide hot water for heating as well as showers and baths at the same time. Therefore it would be sensible to just adjust the heating programmer accordingly.
I would like to know what is more efficient and/or cheaper over 5 years:
i) a Gas Combi Boiler System or a PV Electric System?
Thanks
Living in the south west, our water bill is very high. The initial hot water run off before a decent temperature is reached is a bucket full! even when the tap is only half turned on. Pipe work run off accounts for only ten percent of this, the rest is due to boiler heating up. Obviously when the heating is on, run off is greatly reduced. Our boiler is now 5 years old, have improvements been made in more recent models, and if so, how?
My water pressure is rather low, having had it checked 3 years ago it was 4.00 bar. In view of this could I fit a mixer shower taken off the boiler? Is there a particular type that is best for such a job?
No answers, no answers, no answers at all!
Hi Brian, sorry that you weren’t able to find any answers at all. What question do you have?
I am buying a house the gas combi boiler is 10 years old, it has passed its recent corgi check. How long is the usual life of the boiler before it needs replacing?
Hi Judith, a combi boiler will last about 10 – 15 years before it will need replacing. The combi boiler manufacturers have clocked on to the fact they need people to buy new boilers (the old ones lasted 40 years plus!) so they nowadays tend to work perfectly for about 10 -15 years then they need replacing!
do c0mbi boilers pumpop to the bed rooms if it is in the kitchen
Will it damage my combo boiler if whilst having a shower I keep turning it on and off in order to save water ?
They are designed to withstand a good amount of on/off switches, some lasting for years and years. I was researching quite a bit about combi boilers, found another blog post that is quite good too, check it out https://www.boiler-repairs-in-glasgow.co.uk/how-combi-boilers-work/
I don’t think that will be a problem! The combi boilers should be able to withstand this sort of behaviour!
Can l still put the heating on even though l have turned the water off on a combi-boiler?
Hi Mr Freeman, when you have a combi boiler installed it obviously both does your hot water and heating. Unlike conventional boilers, all the hot water is made on demand. If the heating is on, but you want to run a bath, or have shower, the hot water produced by the boiler is sent to the particular tap / hot water outlet where it is required. For the duration of the tap / shower being on, no more hot water will be sent around the central heating system.
So you can absolutely have your heating on and provided you don’t run the hot tap then there is no problem. Likewise the hot water travelling around the central heating system is completely independent from the hot water that gets produced, so therefore if you stop water entering your home, provided the central heating has not be drained you can still have your heating on.
If that is the case, why won’t my hot water come on, without me having to put the heating on! I can run a bath, or wash dishes when the heating comes on and I don’t have it on all day.
I have a combi boiler and have installed a water softener to reduce the chance of furring of pipes in the water supply. Do I need to do anything extra to ensure that the treated incoming water supply enters the central heating pipe system
Hi John, no that should be fine, although definitely recommend installing a Magnaclean on your combi boiler if you can since that pulls out all the ferous oxide in your hot water system before this enters (and potentially clogs) the boiler
I have set my combi boiler on a timer if I turn the timer off then put the heating back on will the timer go back to the original settings
I currently have a hot water tank in my home with a boiler too, but my neighbour installed a combi boiler a couple of years ago and she says it works great. From reading the material the combi boil heats the water as it is required, but we have three bathrooms. They are rarely all used at once, but if for example all three showers were used at the same time would there just be a dribble coming out of the shower? That is one of the things that makes me a little nervous! I guess the other thing is what happens if the boiler breaks down. Obviously I won’t have any hot water, but I assume the system will still allow cold water to run through it so at least water will come out of my taps? Final question, is there a minimum water pressure for which a combi boiler is suitable? I am pretty sure it should be fine in my home because the water comes out of the tap with seemingly decent pressure but just want to see if there is a hard and fast rule!!
Thanks very much for your help!
Have just had a pressurised hot water system installed, got rid of the loft tank and the poor flow from a gravity fed system but retained my existing boiler., great performance and very pleased with the flow to taps and shower. You need a plumber/heating engineer who
is qualified to install this type of system as it is more complicated than installing a combi boiler. Cost of removing old hot cylinder and loft tank, installing a 150 litre pressurised system £1100.
Hi All,
What is your opinion on getting a boiler changed in summer? Is it better to wait for the winter or will I get a better deal if I get my boiler installed now?
Thank you,
Andres
Hi Andres,
A lot of people will wait until the winter before getting their boiler changed, but if you think you might need to get one installed in the next 12 months, now is the perfect time. Boiler installers get very busy in the winter and you will probably waiting for weeks to get your job done. In the summer there is less of a wait, and less of a problem if the boiler breaks down, because you are onyl going to be short of hot water and central heating shouldn’t be a factor.
Hope that helps,
Alan
i have just had a combi boiler fitted,, at what tepriture does it need to get to before the central heating comes on ive never had a combi boiler before and not sure what to do can someone let me know how it works as the repriture is starting to fall i dont want to be sitting in the cold i am 75 yrs old
You fail to mention perhaps the biggest downside of combi boilers- the often significant lag between opening a tap and hot water being produced. To fill a kitchen sink it can be necessary to run off a significant volume of barely warm water until the water coming through is properly hot. I have found this to be an issue in just about every property fitted with a combi. Also, when the central heating is operating water temperatures in the boiler can be as high as 90 degrees. If a tap close to the boiler is opened an initial few moments of scalding hot water can be produced. General consensus is that combi boilers are NOT as reliable as conventional system or heat only boilers. In very hard water areas they can require regular cleaning or replacement of heat exchangers which is a professional and fairly costly job. As you may have gathered, I hate them! My personal advice is that if your existing system or conventional boiler fails, replace it with the same type. if you have a conventional boiler and an old fashioned cylinder with a cold loft tank it is usually straightforward to upgrade to a pressurised system and get rid of the loft tank, while still retaining the advantages of stored hot water.
Thank you for this comment! I can’t even run the hot water without putting the heating on as well! But yes, even then it takes a good 5 minutes or so for the water coming out of the tap to get hot. I’d have new heating put in if I could afford it.
When my hot water tap is opened I can stand and watch 7-10 litres of perfectly good drinking water pour down the drain before it starts to warm up! This has happened in two houses. That is not efficient.
Some boilers store a small amount of hot water to reduce the delay in hot coming through. Some boilers make this effect optional via an economy switch.
Set the switch to not ‘economy’ setting before opening the water tap – and set it back again, before closing the water tap – for the best of both options.
Hi. With this spell of good weather do I need to run the central heating pump now and again to stop it having problems or does running the hot water system only utilise the pump. I have a combi condensing boiler. Thank you.
Having just had a combi installed to replace a regular boiler + HW tank I am interested to know why combi’s have to operate at typically 1bar pressure on the CH side. It is only circulating the water through the rads etc and the old system didn’t operate at pressure. I have not received any sensible answer yet from people in the business of selling/installing them
if the gas is running out due to topping it up will it not heat the central heating just the water
Ehow long do combination boiler last
A combi boiler with the right water pump can boost water and give you hot water on cold weather days. Saw a list of water pumps and just wanted to know your thoughts on which can work best for my current home.
So am i right in thinking that you dont use any gas unless you turn a tap on or the heating
I need to drain my heating system down for a few days but will I still be able to get hot water,i have a combi boiler.
Hi Chris , I don’t how long ago you posted this question , you might have had many answers by now , I had to drain my central heating because of a leak and what I found was , because of the lack of pressure in the system , I had to operate the hot water on the ECO mode to prevent overheating in the preheating tank .it just means running the tap a little longer to get hot water . Did you get any professional answers to your question ?
Since having a combi boiler there doesn’t seem to be any hot water going into my dish washer. Before we had a hot water tank and wasn’t a problem.
Hi Suzette,
The only reason I can think of that this would happen is if someone was using hot water elsewhere in the house at the same time. If you have an older washing machine that you run at the same time, it could be that. If you don’t think that you’re using hot water anywhere else in your home at the same time, you’ll likely need to speak to a plumber to look at the problem.
Thanks,
Harri
I am interested in a good quality combi boiler for my three bedroom home plus one bathroom what would you recommend I live in Dublin Ireland
Intergas combi. The best available without doubt. Read the reviews.
My understanding is that whilst a flat’s heat losses are only about 5 – 6 kW, the boiler has to be rated at about 25-30 kW to produce hot water. So even in the middle of winter the boiler will ‘cycle’ on and off at high frequency because tye size of the heating syste is so small. What eefect does tgis rapid cycling have on the boilers compenet fopr example solnoid gas valve?