27 Nov 2012

The Gas Saver

Condensing boilers are no longer news. They first appeared in the 1980s and for years were known as energy-saving boilers. They work by having a secondary heat exchanger which is used to take a load of heat out of the flue gases from the main boiler. This makes them more expensive to build and a little more expensive to install. Hence, they were seen as something of a green curio, an expensive option for those who cared about the planet, or the size of their gas bills. By 2004, they had achieved a market penetration of just 3%. Although a condensing boiler could be shown to have a very short payback (less than 5 years), people didn't want to spend an extra £250 for a condenser.

But then the building regs changed. In April 2005, Part L (England & Wales) was amended to make condensing boilers all but compulsory. Today, condensing boilers have over 95% of the new boiler market. That, ladies and gentlemen, is a prime example of government intervention raising the bar. What is more, you can source an A-rated Baxi Solo condensing boiler for just £450 (plus VAT). We were paying that much for ordinary boilers back in the 1990s. So competition and economies of scale have eroded the price differential (though some cynics will argue that quality has also been engineered out).

All in all, it makes a powerful argument in favour of legislation as the best way of increasing energy efficiency. If condensers hadn't been made compulsory, I bet that they would still cost £250 more and that market penetration would be sticky at around 3%.

Which brings me on to Zenex's Gas Saver. This is a new(ish) device which you attach to a condensing gas boiler and it draws even more heat from the flue gases and uses it to pre-heat the water going into the boiler. It is, if you like, a tertiary heat exchanger. The generic name for this is Passive Flue Gas Heat Recovery Devices (PFGHRDs) and the inventor, Chris Farrell, started Zenex to develop and market it.

In some ways, it's been a splendid success. Fitting a PFGHRD to a boiler has been demonstrated to bring about gas savings of up to 40% for hot water generation and consequently PFGHRDs are a ground-breaking British technology, recognised in Building Regulations, recommended by the Energy Saving Trust and recognised in the Green Deal.

Better still, two of our major boiler manufacturers, Baxi and Alpha, now market Zenex's Gas Saver with their own offerings. Indeed Alpha have a built-in version (The In Tec GS) so that you can install the boiler and the Gas saver in just one box.

But the Gas Saver costs. On it's own, it's over £600 to buy. Coupled with an Alpha boiler, it doubles the price of the boiler. And just like the condensing boiler pre 2005, it has a tiny market penetration. There are around 1.5million new boilers fitted in the UK each year (about 90% of these are replacements) and only a tiny fraction have Gas Savers fitted to them.

How best to change this situation? Though the savings appear to be real enough, and the payback period is again probably less than five years, the uplift in price between a condensing boiler and a condensing boiler plus Gas Saver is enough to put most casual buyers off — that's if they ever get to hear about it in the first place. Maybe the Green Deal will come into its own here, as PFGHRDs appear to meet the Golden Rule. But I can't help feeling that the answer lies in building regs enforcement.


  1. In Sheffield virtually no-one seems to be fitting weather compensating controls; indeed most of the condensing gas boilers fitted are the dumbed down UK market versions that can't accept weather compensation. Surely a much cheaper option to save 15% on the CH than PFGHRD?

  2. Surely the reason people aren't fitting weather compensation is because it's gone out of fashion as it's proved to be over complex and of limited value. If you have a heating system with boiler interlock and TRVs fitted to all rads, it's very hard to see what added benefit weather compensation brings.

  3. The flow temps on my system boiler range between 37C and 55C with the return 5C lower. I assume that most of the time I am achieving maximum condensation.

  4. Yeah, hard to see that there's any more delta-t to be extracted from a condenser's exhaust that's set up properly and well matched to the spaceheating medium. Maybe for preheating stone cold incoming MCW when it's in DHW mode, but no hope, I'd have thought, of adding any heat to spaceheating return water.

  5. PFGHRD works by pre-heating the boiler feed - from stone cold, as you put it. The boiler therefore has much less work to do. It has nothing to do with the temperature outputs from the boiler.

  6. "It has nothing to do with the temperature outputs from the boiler"
    True - it's to do with spaceheating return water temp, which is nowhere near as low as 'stone cold' MCW. Must be above room temp anyway, in fact above the design temp of the floor screed.
    I suppose, at expense of an additional heat exchanger, exhaust gas temp does end up as 'somewhat above' return water temp rather than 'somewhat above' flow temp. Fair enough.

  7. Boilers can't condense at even low temps due to residency of flue gas. That's why boilers are tested at 30 deg retun at their slowest fan speeds.

    Also, seasonallity and dynamic demand profiles of energy use show that its bests to addrtess these issues through simple designs such as super condensing pfghrd and supeflow, temporal stores for peak use hot water demand.

    Chris Farrell

  8. I've got an Alpha Boiler with the gas saver GS-1 fitted above. All was going well until the gas saver burst internally and flooded the boiler below it.

    I the boiler has a five year guarantee and, guess what, its been in five years and two months ! £800 for a new unit as no way to get inside and fix the faylt. Not happy :(

  9. I bought the Baxi Multifit HeatSaver package, the gas saver now has a leak, and Baxi tell me anything external to the boiler is not covered, even though it was a package. It is two years and ten days old. Also not happy. I can no longer afford to even try to be energy efficient.