Heat only or system boiler?

[chris5335]

Hello, I currently have a baxi heat only boiler (pump in cylinder cupboard) supplying an unvented heating system and vented hot water cylinder coil. The boiler is about 20 years old now and due a change.

Was wondering should I do a straight swap for a new heat only boiler, or get a system boiler and remove the existing pump?

As I guess the new heat only boiler would need wiring into the existing pump. Whereas a system boiler would only need a live feed which is already there and could take care of any necessary pump overrun etc itself with the pump being self contained? Or am I best just leaving the pump where it is? Don't want to encourage any problems...

Anything else I should consider?

Many thanks, Chris

 

[Agile]

You seem to have little understanding of the differences or the rerquirements!

Most heat only boilers have the pipe connections at the top. Some need to power the pump to provide the over run so more that two wires are needed.

A system boiler needs a permanent live as well as a call for heat connection from the controls.

A heat only is cheaper and enables the existing or any generic pump to be used.

Tony

 

[chris5335]

Not surprising really seen as I'm not a professional! That's why I'm asking!

Ok well the current boiler is floor standing with the flow and return along the floor behind, so wall hung with pipe connections at the bottom would be best. I'm looking at a Worcester 30kw to power 21 radiators in total if that sounds about right?

I understand a new heat only boiler would need wiring to the existing pump, I meant new system boiler would surely take care of itself?

Yeah I thought the boiler would need connecting to the controls but if I get a Worcester with snap on front panel timeclock that's it sorted?

There's not much difference in price, just want which option is better and why

 

[dcawkwell]

For a normal domestic house.
System boiler every time.

Rip the old pump and anything else out. Package all in one box
under one warranty and no header tank needed in the loft.
Job done.

I don't install anything but system boilers.

 

[chris5335]

Well it's a 5 flat property, 21 radiators in the whole building. I'm the landlord and live in flat 2. The boiler, cylinder, pump, pressure vessel, filling loop etc are all in flat 1. The hot water cylinder is a vented setup and provides hot water for flat 1 only. There are no motorised valves on the system. The lady never requires hot water only from the cylinder though. She has electric shower, boils a kettle to wash the dishes and only takes advantage of the free hot water in the cylinder during winter.

Could you please explain what you mean by "no header tank needed in the loft" with a system boiler? As the current boiler is a regular/conventional boiler but there is no header tank

Thanks, Chris

 

[kevindgas]

I don't install anything but system boilers.

why?
i fit more heat only a year than i have ever fitted system boilers!
usually i fit what suits the system already fitted.

 

[kevindgas]

I'm looking at a Worcester 30kw to power 21 radiators in total if that sounds about right?

i usually calculate the requirements as 1.5kw per rad plus 2kw for cylinder so that's just over 31kw allowing for the fact that not all will demand heat simultaneously i would say your about right. although you may find it slow to heat in the dead of a harsh winter.

 

[dcawkwell]

I don't install anything but system boilers.

why?
i fit more heat only a year than i have ever fitted system boilers!
usually i fit what suits the system already fitted.

The main reason is that it then puts everything into the one box.
Keep everything clean and simple.
I put in built in timers so even that is in the box.
The only thing that is external are the zone valves and thermostat.

The only time I would use a heat only boiler would
be for larger systems where I have multizones and pumps.
Otherwise the system boiler is the ideal all in one box solution.

Can't think of an installation where I wouldn't replace a heat only boiler with a system boiler.

 

[dcawkwell]

Well it's a 5 flat property, 21 radiators in the whole building. I'm the landlord and live in flat 2. The boiler, cylinder, pump, pressure vessel, filling loop etc are all in flat 1. The hot water cylinder is a vented setup and provides hot water for flat 1 only. There are no motorised valves on the system. The lady never requires hot water only from the cylinder though. She has electric shower, boils a kettle to wash the dishes and only takes advantage of the free hot water in the cylinder during winter.

Could you please explain what you mean by "no header tank needed in the loft" with a system boiler? As the current boiler is a regular/conventional boiler but there is no header tank

Thanks, Chris

A system boiler has an expansion tank in the boiler and you fill the system with a filling loop. Therefore there is no header tank needed. However given you have 21 radiators you will would almost certainly need an additional expansion vessel in the system somewhere due to the amount of water. I would avoid Worcester. Overpriced in my humble opinion and I find them just plain annoying to work on and fault find.

 

[dcawkwell]

Well it's a 5 flat property, 21 radiators in the whole building. I'm the landlord and live in flat 2. The boiler, cylinder, pump, pressure vessel, filling loop etc are all in flat 1. The hot water cylinder is a vented setup and provides hot water for flat 1 only. There are no motorised valves on the system. The lady never requires hot water only from the cylinder though. She has electric shower, boils a kettle to wash the dishes and only takes advantage of the free hot water in the cylinder during winter.

Could you please explain what you mean by "no header tank needed in the loft" with a system boiler? As the current boiler is a regular/conventional boiler but there is no header tank

Thanks, Chris

What do flats 2,3,4,5 do for hot water? Electric?
Given you want to keep your tenants happy. I would go for two smaller cheaper boilers so that they work together in high demand and one works
as back should the main fail.
I would also suggest thermostats in each flat and 5 zone valves.
Only flats that are calling for heat will get any.

 

[doitall]

A heat only boiler is more reliable.

As dcawkwell. suggested.

Done right you can have two smaller boilers with one as a back up and extreme cold weather.

Zones will also save a bucket of cash in running costs.

 

[transam]

Allway's take into consideration that in most cases a systems boiler will be part of a sealed / closed central heating system , there fore all the existing heating equipment will be or maybe (?) (rads ect) under more pressure ! there have been a few cases on here in the past were this has caused problems ???

 

[chris5335]

Well it's a 5 flat property, 21 radiators in the whole building. I'm the landlord and live in flat 2. The boiler, cylinder, pump, pressure vessel, filling loop etc are all in flat 1. The hot water cylinder is a vented setup and provides hot water for flat 1 only. There are no motorised valves on the system. The lady never requires hot water only from the cylinder though. She has electric shower, boils a kettle to wash the dishes and only takes advantage of the free hot water in the cylinder during winter.

Could you please explain what you mean by "no header tank needed in the loft" with a system boiler? As the current boiler is a regular/conventional boiler but there is no header tank

Thanks, Chris

A system boiler has an expansion tank in the boiler and you fill the system with a filling loop. Therefore there is no header tank needed. However given you have 21 radiators you will would almost certainly need an additional expansion vessel in the system somewhere due to the amount of water. I would avoid Worcester. Overpriced in my humble opinion and I find them just plain annoying to work on and fault find.

Ah right I didn't realise all system boilers had expansion tanks in. Are you 100% sure about that? Not questioning, just checking! Cool so I'm fine just to ignore the existing pressure vessel and filling loop and leave them where they are?

What do flats 2,3,4,5 do for hot water? Electric?
Given you want to keep your tenants happy. I would go for two smaller cheaper boilers so that they work together in high demand and one works
as back should the main fail.
I would also suggest thermostats in each flat and 5 zone valves.
Only flats that are calling for heat will get any.

Yeah electric and definitely keeping it that way! The heating has always just been on a timed setup which is controlled from my flat and does the job fine

Allway's take into consideration that in most cases a systems boiler will be part of a sealed / closed central heating system , there fore all the existing heating equipment will be or maybe (?) (rads ect) under more pressure ! there have been a few cases on here in the past were this has caused problems ???

The heating system is already unvented!

So basically it really doesn't matter which I go for? One of my main concerns was 'moving' the existing pump (by fitting a system boiler) if the pump was positioned there for a reason? Don't want to mess with anything which may cause issues. Although I'm pretty sure the pump is on the flow, near the boiler, pumping away from it, then tees off to the coil and heating. So by fitting a system boiler it would essentially be in the same place, just a few feet nearer towards to boiler. Another concern was the overrun aspect. If the pump doesn't currently overrun and needs to with the new boiler to avoid overheating. A system boiler would surely take care of this itself whereas a heat only boiler would maybe need some additional wiring to the existing pump?

I don't know... just speculating and trying to make a final decision between the two boilers!

Thanks, Chris

 

[D_Hailsham]

It's worrying that so many posters did not bother to read the OP's original post properly before replying!!

He said:

I currently have a baxi heat only boiler (pump in cylinder cupboard) supplying an unvented heating system and vented hot water cylinder coil.

 

[transam]

Well must admit I did not read the post correctly ?? but none the less i wolud stick with a heat only boiler as opposed to a systems boiler , no water press switch on a heat only !!