Combi boiler with cylinder

[takacs]

Hello All,

(Similar questions have already been asked here, but I have slightly different circumstances.)

I have a Baxi system boiler with a 170 litre cylinder (santon premier plus pp170b), and boiler must be replaced. I have only one bathroom, and 3 bedrooms, so all sites recommends combi boiler. However, the cylinder is in place and we do not immediately need the space that the cylinder uses. The cylinder is 10 years old.

So, I am thinking of three options:
1. Buy a system boiler (18kw), because the system is already built, and the cylinder will be okay for next 10 years.
2. Get rid of the cylinder, and have a new combi boiler, and rebuild the hot water system.
3. Somewhere between: get a combi boiler (30kw), but keep the cylinder and the existing system. We can rework the system to use the combi boiler's hot water later if needed.

Do you have any advice in this regard? Would be my cylinder possibly okay in the next 10 years?

 

[takacs]

Sorry, wrong thread: Old thread but it is relevant for me: what about the COP of the combi boiler? Will it be the price to run it?

 

[CBW]

Hard to give an answer regards the cylinder as we don’t know the water quality, but in general they’re usually ok. Go for system or combi, whichever is cheaper and best suited for your needs.

 

[Madrab]

COP of the combi wouldn't really come into it. It would be the balance between the cost of producing a given volume of daily use instantaneous HW under full power from the the Combi against heating up a HW store @ system temps over a given period of time, that could only be calculated over time. That balanced against the versatility of stored HW against the restrictions of a combi when it comes to using more than one HW outlet at a time and the ability of a backup HW supply should the boiler fail.

Lots or pro's and cons to consider over and above the COP.

Can I suggest starting your own thread if this means to go further.

 

[Madrab]

Apologies, didn't realise you had started your own post -

Re the other post - COP of the combi wouldn't really come into it. It would be the balance between the cost of producing a given volume of daily use instantaneous HW under full power from the the Combi against heating up a HW store @ system temps over a given period of time, that could only be calculated over time. That balanced against the versatility of stored HW against the restrictions of a combi when it comes to using more than one HW outlet at a time and the ability of a backup HW supply should the boiler fail.

Lots or pro's and cons to consider over and above the COP.

Re the options - if everything is in place and there is no requirement to change anything then leave it the way it is. No reason not to use a combi as a system boiler and they can be cheaper.

 

[flameport]

No reason not to use a combi as a system boiler

Unless the property is vast, a combi would be grossly oversized.

 

[Madrab]

Unless the property is vast, a combi would be grossly oversized.

Not if it's properly range rated

 

[takacs]

Thank you, all. I posted my second message into a wrong thread, apologies for the confusion. So, I am mainly interested in the options, thank you for directions above.

 

[takacs]

What is not clear for me, why is there a system boiler category, if a combi boiler can do the same job.

 

[CBW]

A system boiler is easier to work with, and possibly wire up and probably existed before combi boilers, in the U.K. at least.

 

[stem]

What is not clear for me, why is there a system boiler category, if a combi boiler can do the same job.

Heat water only boilers still have their place, especially where a property has more than one bathroom, or the incoming cold mains supply isn't very good. But here are some other things you might want to consider:

With a heat only boiler, if you like DIY (& most folks on here do) then outside of the boiler, you can carry out most repairs yourself. For example need a new pump? Programmer? Motorised valve/s? Cylinder Thermostat? This can be done DIY with parts available from plumbers merchants or DIY outlets (great if something fails on a Sunday afternoon). Combi boilers usually have many of the components that carry out these functions located internally instead, so can't be fixed DIY. A visit from a gas safe engineer would be required to fit parts that are generally specific to the boiler.

Combi’s can be slow to get hot water to the taps if the boiler is starting up from cold. This is because the boiler has to heat an internal heat exchanger first before it can heat any water and send it to the taps. I sometimes hear complaints from folks on a water meter about the quantity of water wasted whilst the tap is running and waiting for the water to get hot. Some boilers have a pre-heat function to keep them warm in case you want hot water more quickly, but this isn't so good from an ECO point of view.

Combi's usually have a larger heat output than is required by the radiators in order to have the capacity to heat mains water from cold, so often a larger diameter gas supply pipe installed from the meter is required to supply it. (These can sometimes be seen routed around the outside of houses-Yuk!)

Using a hot water cylinder means, you can have an immersion heater to provide hot water for when the boiler fails, and a cupboard you can use to 'air' clothes, or make beer or yogurt etc.,

Generally the cold water main supply to a combi is via a single 15mm pipe. That same pipe also feeds the cold taps, WC’s, washing machines, dishwashers, etc., if any of these are used at the same time as the hot water, the hot water flow will be reduced accordingly. With a hot water cylinder fed from a cold storage tank, the hot water will be unaffected by the use of the cold water anywhere else in the house.

Combi boilers cannot provide hot water and heating at the same time. When running hot water the heating will go off. This is not normally likely to be a noticeable or create a problem unless perhaps you live in a house where several people are running water or showering one after another.

 

[bernardgreen]

Stem has made an excellent and accurate statement about the reality of the combi boiler.

 

[gas112]

yep you can have a combi with it the way i done it in one of my houses was separate hot from combi to kitchen sink / bathroom sink and the shower , Cylinder purely done the bath . So no need to have 170 litres of water sitting there constantly with the cost of heating that just to sit there .

 

[oldbutnotdead]

The only real beartrap with having a cylinder driven by a combi boiler is the flow temperature on the heating circuit. Modern boilers/systems are configured for a flow temp of 50-55 deg C (so condensation occurs at the boiler to achieve the impressive efficiency ratings). Your cylinder really needs to hit 65 plus degrees weekly to prevent Legionella. You can get round this without a complex control system (assuming the cylinder has an immersion heater in it) by setting a timer on the immersion heater for a 3 hour blast to whatever temp the immersion heater is set to (70 deg C) once a week. And after that, combi with cylinder can work really well, specially if the combi is in the kitchen and the cylinder is near the bathroom

 

[gas112]

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