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Advice on new Heating System - System boiler Vs Heat Only

Postby Mattlehat7 » Sun Nov 10, 2019 12:52 pm

Unfortunately my boiler needs replacing and my heating engineer has recommended I replace my existing boiler with a 'System' boiler. Not knowing nearly enough about this subject I need some advice please?

Its for a four bedroom / two bathroom detached house, approx 13 years old. We have a megaflo cylinder so no tank in the loft. I believe the existing boiler is heat only. Please can someone explain the difference and advantages of a system boiler over a heat only boiler? Is there much difference in price?

Many thanks

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Postby ericmark » Tue Nov 12, 2019 12:30 am

[quote]Like regular boilers, system boilers work with a hot water cylinder. Unlike regular boilers, the system is not open-vented, i.e. there are no tanks in the loft, and all of the components that would normally be outside a heat only boiler (pumps, valves etc) are integrated into the boiler.[/quote]
This is what it says on the internet, however when fitting items like thermostats and setting them up, both seem to be called system boilers, water expands when heated, and when there is no header tank you need a tank to allow for this expansion, this would be inside a system boiler but there is a limit to how much expansion the internal unit can work with, so it may need an extra one outside, pumps today are not fixed flow, but like everything else with central heating modulate.

Megaflo and other modern storage can have many functions, some are for domestic hot water only, some convert the digital output of oil boilers into an analogue system. It allows the boiler to run longer every time it switches on.

Gas boilers can modulate, in fact likely any new gas boiler will modulate, that is it has a variable output according to load, it has to do this in order to keep the return water cool enough to use the latent heat.

There is a control problem with the gas boilers, heart of winter they work well, but as summer arrives the boiler reaches the minimum output and starts to cycle, but using just return water to control it can never turn fully off.

So we use some form of thermostat, the traditional method was fit it in a ground floor room with no outside door or alternative heating, however today we have progressed to using modulating thermostats as well which connect direct to the boilers ebus.

So systems line Nest, Tado and EvoHome (latter is an add on module) have a system called OpenTherm, this allows you to mix and match thermostat and boiler, and still have an integrated system.

Some boilers don't support OpenTherm so your forced to use that manufacturers modulating thermostat or use some thing like Hive that uses the return water to modulate the boiler, but the wall thermostat turns off the boiler if non of the Hive electronic TRV heads have sent a demand for heat.

Some systems are easier to use to others, I have Nest and it worked great with my android phone, but when I sent the invite to my wife's android phone it took her round and round in circles, so when I go out with my phone the house auto turns to Eco mode and she is cold. Also it should work with MiHome Energenie TRV heads, using a follow command, but it worked intermittent, so had to program them independently so could have used much cheaper versions.

I know at the moment you likely only want a new boiler, but I would advise think about the future and that you may want independent control of each room and remove hysteresis so unless the Megaflo does it, then get a boiler that supports OpenTherm even if at the moment your not going to use it.
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