Not just to agree with the client, combi often pushed because it suits the installer.
Is switching to a combi boiler in a large-ish house really such a bad idea?
- Thread starter Spanky2k
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It suits the installer for a whole raft of reasons...
It's a single package, with most of the likely problems inside that one package, there is money to be made on the scrap, and money to be charged for the time needed to convert. Going forward, from the conversion to combi, much of what can go wrong with the heating system, is contained within that combi, so it needs a gas safe engineer to repair it - it is no longer strictly a a DIY repair, or plumbing repair.
One of the problems with big output combi boilers is the minimum output in heating mode. This will often be above the minimum required to heat the building so causes cycling (switching on and off in short bursts) and loss of efficiency. It would be worth getting a proper heat loss survey done to determine the requirement for heating.
My place had a 24Kw system boiler that couldn't heat the building due to undersized radiators. I have re piped the heating circuits and upgraded the radiators. The new boiler is happily running at 10Kw, and can probably be set lower once more improvements have been made to the house.
It's also worth upgrading controls to weather or load compensation. High temperatures to radiators are only needed at the coldest outdoor temperatures.
My place had a 24Kw system boiler that couldn't heat the building due to undersized radiators. I have re piped the heating circuits and upgraded the radiators. The new boiler is happily running at 10Kw, and can probably be set lower once more improvements have been made to the house.
It's also worth upgrading controls to weather or load compensation. High temperatures to radiators are only needed at the coldest outdoor temperatures.
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And it might suit the customer. Proper design is the key, not prejudging the best solution.
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Correct, and once the open vented is gone, it will cost far too much to put it back in.
That's right, but it's often the installers who do the prejudging.
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Well, perhaps. "Far too much" is subjective.
Installing a combi, has become the modern, default solution.
Quite, I've occasionally listened to radio discussions about energy saving, and the "experts" talk as if combi is the only type of boiler available. After I've finished shouting at it, I switch it off.
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So far no one has actually explained what the reason for no combi boiler is - It seems to me gospel that a large house needs a system boiler just in case two people use the bath at the same time and the kitchen sink is switched on - Even under those circumstances a bit of low pressure is not dramatic
And dont get me started on the excuse that at Christmas your boiler might pack up - Thats not an item that factors into it
I have a system boiler Ideal Mexico 2 beast on a one pipe system and even I considered replacing it with a combi
And dont get me started on the excuse that at Christmas your boiler might pack up - Thats not an item that factors into it
I have a system boiler Ideal Mexico 2 beast on a one pipe system and even I considered replacing it with a combi
It's not that a combi can't be used but it does need a need a bit of cooperation between the occupants. The issue is that anything but the largest of combi boilers will normally struggle to supply more than 1 HW normally flowing outlet at a time with a suitable flow of hot water. I.e. if someone is in a high flow shower and the HW is turned on other parts of the house closer to the boiler then the flow to the shower can be drastically reduced.
It can be done with everyone working together, less occupants or water saving outlets but can be very difficult to balance with a larger family with several kids and multiple bathrooms/higher flow HW outlets and to get it all to work well together.
That's when a suitably sized, mains fed stored HW cylinder comes into its own.
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Quite. It isn't the end of the world. Sometimes a combi comes out on top, sometimes it doesn't.
That does make me laugh. Some of the back ups proposed, such as an electric shower, just in case, are plainly nonsense.
Unless the combi has a large Kw rating, then the hw flow, has to be reduced, so it has time to be heated by the boiler. Even so, a combi has to be kw rated, to supply the hw, ignoring the probably much lesser kw needed for heating in the house. Boilers have a limited range of modulation, therefore a larger Kw boiler, only providing heating to the house, even on it's minimum modulation, will still be generating too much heat, so they begin short cycling, which is not good for economy, nor for wear and tear on the system.
A none-combi, can be kw sized to perfectly match the needs of the house's heat loss, My heat only is 18Kw and can modulate down to 6Kw. For a combi, I would need a 34/36Kw. My cylinder takes around 30 minutes to heat up, to provide a tank full of piping hot water. When I turn the hot tap on, because it is open-vented, it flows into the bath at a greater rate than the cold, but I can run the cold at the same time, without affecting the hot flow rate at all. I turn the hot tap on, and the hot water flow is near instant - a combi wastes much more water, until it achieves temperature. An hw cylinder also has the advantage that it can include an immersion heater, which can serve as a backup to those rare occasions when the boiler breaks down, or can use surplus energy from solar panels.
A stored water system, costs considerably more to install, than a instant combi system, and it does have some disadvantages. You have to wait while the water heats up, and to some extent need to plan your use. There is also some level of inefficiency, in that heat can be lost from the cylinder, and you may not use all of the water you have heated, but in winter, the heat is lost into the house, so perhaps not entirely wasted. The cylinder also takes up some space.
It would be a factor, if you had to wait a month, for an engineer to come and fix your combi. To be fair, in the 40_ years since my system was in stalled, I have never been without hot water long enough to need to put my immersion heater on, but then most of it's issues have been solved by me, within hours..
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All considerations and part of the system design process.
A large output Combi boiler is often massively oversized for the heat load of a house. Particularly if it is of modern construction.
The ideal place for a boiler isn't always where the hot water is used. A cylinder can be placed to minimise hot water wastage. A boiler needs to go where the flue, gas pipework and condensate pipework can be run. A combi boiler can have a large gas requirement making long runs of gas pipe prohibitively expensive. Long flue runs can also be very expensive, and can cause further problems if they get boxed in.
If you must have a combi the kitchen is a better place for it to minimise wasted water on short draws of hot water.
Consideration must be given for maintenance and repair. I've seen many boilers that cannot be worked on due to inadequate clearances, or in places where it is not safe to work. (eg over stairs)
Modern unvented cylinders are very well insulated, so the heat loss is minimal.
It's also worth considering fitting a solar or heat pump ready cylinder for future upgrades. If not a cylinder with an immersion heater could be heated by PV solar panels.
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