Heat only Vs System boiler Vs Combi (running costs)

[boolie]

I'm trying to work out what the most economical heating system set up is.. i.e. which is the cheapest to run (i.e. which uses the least amount of gas / electricity). For the purposes of this question please assume that all three setups are perfectly adequate for the users needs - I'm just interested in running costs. Also please ignore installation/materials costs. All three will supply just Central heating and hot water just a very simple system (don't worry about zones, multiple showers or underfloor heating, etc)

So the four options are

1. Vented cylinder with heat only boiler
2. Unvented cylinder with System boiler
3. Combi boiler
4. Heat only boiler with unvented cylinder

Which of the above uses the least amount of gas and electricity?

By the way not sure about the fourth option.. can you have a heat only with unvented cylinder or does it have to be a system boiler?

 

[Harry Bloomfield]

Don't ignore the water bill too. The answer to that question, largely depends on how you would use the system and your lifestyle. Usually there will be someone at home here, 24/7, so our cheapest to operate system is an open vented one, it is also the most convenient. We use hot water throughout the day and can quickly draw a hot bath. The boiler doesn't need to fire up and waste gas, water, wear and tear each time hot water is needed.

A combi suites someone who may spend much of their time out of the house, where it would be pointless to store a tank of ready to use hot water. A combi also takes up much less space than either of the other systems.

Unvented, comes in the middle.

 

[bernardgreen]

Vented cylinder with heat only boiler

Is my choice. with a shower coil in the cylinder for hot water to the shower.

A system boiler has all the pumps and valves etc crammed into the case, difficult for access and repair.

A tank of cold water in the loft gives more than enough constant pressure to supply taps and cistern without the problem of fluctuating mains water pressure

Edit withOUT the problem

 

[muggles]

A simple question with a complicated answer, if ever there was one. It depends on the components selected, the quality of the system design and installation, usage patterns, and so much more.

 

[boolie]

@bernardgreen in your diagram why are you supplying the cold water taps from the header tank rather than directly from the mains?

 

[bernardgreen]

why are you supplying the cold water taps from the header tank

because the taps and toilet cistern are operating at low pressure which prolongs the life of the valves and means they are silent in operation,

 

[boolie]

because the taps and toilet cistern are operating at low pressure which prolongs the life of the valves and means they are silent in operation,

Do you drink from the tap? I guess if your header tank is clean and well sealed then the water quality is about the same as what you get through the mains

 

[Harry Bloomfield]

Do you drink from the tap? I guess if your header tank is clean and well sealed then the water quality is about the same as what you get through the mains

I would not advise that, no matter how clean the tank is.

 

[bernardgreen]

As regards cost. I am told that boilers waste some gas each time they fire up. With a combi boiler this occurs every time a hot tap is turned on. How much gas is actually wasted each time remains a mystery but ( I am told ) the extra wastage in a combi boiler is not insignificant

 

[bernardgreen]

Do you drink from the tap?

No.... only from the kitchen tap which is mains is considered as potable water, though the water from the shower could could be consider as being potable

 

[boolie]

@bernardgreen didn't know you could get cylinders with a separate coil for a mains fed shower.. I guess the difference is you get a nice powerful shower? Is the shower hot water a high temperature with that set up? I imagine it's not as hot as the water for the taps

 

[Harry Bloomfield]

How much gas is actually wasted each time remains a mystery but ( I am told ) the extra wastage in a combi boiler is not insignificant

Whereas a heat only boiler will only run to heat water once a significant amount has been drawn and if it is set to do so. Once it lights up, it will have a good, long and economic run.

 

[bernardgreen]

I imagine it's not as hot as the water for the taps

The shower is hot enough with a gentle shower. There is a compromise:- The larger the shower coil the less water there is to heat it

 

[Gasguru]

The question of gas and electricity cost is irrelevant and insignificant compared to other considerations...ie. do you want an immersion heater backup, do you want high flowrates, do you want to be tied into a boiler manufacturers pump and vessel (system boiler) etc.
How much do you waste on poor value phone contracts/car leases etc...I guarantee it will be vastly more than any gas savings between different heating/hot water systems.

 

[Mottie]

because the taps and toilet cistern are operating at low pressure which prolongs the life of the valves and means they are silent in operation,

Ours wasn’t! Before we went unvented, every time the loo was flushed or the bath/hot water taps were used, we could really hear the cold water tank filling up in the loft. Our bathroom basin tap has always been mains fed. Since we’ve been unvented - perfect!