Central heating system

Pressurised, by all means. Combi - why??

Combi boilers are horrid things, they are they definition of a compromise. We currently have a combi oil fired boiler. In our last house we had a thermal store heated by an oil boiler. The thermal store performed much, much better.

I thought combis were the way to go now, efficiency being the main factor ?

No, this is not true. Condensing boilers are the way to go (wherever possible), and most combis are condensing, which is where the confusion arises.

Whats the theory behind a condensing boiler? making it more efficient

The difference is in the design of the heat exchanger. A non-condensing boiler exhausts the combustion gases at a temperature high enough for the water to stay gaseous. A condensing boiler's heat exchanger condenses the water in the exhaust gas, extracting more energy from it. This increases overall system efficiency.

They just extract more heat from the flame (by virtue of bigger heat exchangers) to the point where water "condenses". By lowering the exhaust temperature below the condensation point of water, it can extract more heat energy (from the latent heat of condensation).

This is why you get the visible plume with condensing boilers. It's not rocket science but it does mean that the inside of the boiler has to cope with being "wet" and have a drain and be made of corrosion resistant materials.

Tim

(wherever

environmental

efficient

... at least until the guarantee runs out

AWEM

In a few weeks time (April 2007) installing a non condensing oil fired boiler will only be possible in exceptional circumstances.

What makes you think that 'combi'=='efficiency'?

Reduced outflow tempeature== less heat going up the flue==more heat in the water.

Outflow temperature doesn't have to be reduced. Of more importance with regard to efficiency is reduced inflow temp. The cooler the return, the greater the amount of heat extracted from the combustion gases.

Tim

I think we are talking at cross purposes. I meant exhaust outflow temp - what gases goe up the flue, essentially.

I suspect you are talking about the water circulation, in which case I agree with you.

Got ya. Thought you meant water temp.

Tim

Richard

Consider a system with a cylinder rather than a combi boiler. This way, you can hedge against future rises in the cost of heating oil by supplementing your water heating with a solar water heater. Our calculations, you'd save over =A37,000 in oil bills over the life time of the panel*. Full details at

• Assuming heating oil rises as much in the future as much as it has in recent years.

You keep mentioning these calculations - it would be a good idea to show the calculation you're basing your statement on, since a well worked out calculation would lend your statement greater validity. (or is it top secret?)

Well... this is not a straight forward subject. Essentially there are trade offs between the storage losses of a non-combi system and the losses during the start up sequence of a combi.

Frankly in the list of criteria whether to select a combi or not the energy saving/losses is relatively minor.

Firstly, space heating is the major consumer of energy and there is more or less no difference in the performance of either system. Maybe the non-combi can do a little better if it is a smaller unit better suited to match the heating loads than a 24kW+ combi.

Secondly not having the water pressure/flow for a combi or not having the location for a vented cylinder/cistern would be absolute criteria whereas as efficiency would only be relative.

That might be a more important consideration in the case of oil fired boilers which I think are typically non modulating.

Well heating oil has quadrupled in price over the past 8 years - if it does the same again it might not be to difficult to get up to £7000 if the panels have a reasonable life time. - I just hope that the assumption is wrong!.

I suspect that assumption is incorrect, but one can hedge against it in a number of ways - for instance, by buying oil company stocks, or an oil based ETF.