efficiency of balanced flue gas heating

[js.b1]

As mentioned Drugasar are very good, but also take a look at Rinnai.

Rinnai are Japanese, electronic timer, forced fan to distribute heat evenly, self diagnostic, compact & high output (start at 3.3kW so able to actually heat a house if pushed). Price wise they were =A3378 online pre 2008, I fear they may be north of =A3500. Reliability is good, but check on the price of spares and installer familiarity - Rinnai can probably advise.

If you added GCH later they are great as a backup, colleague uses one for just that and it bailed them out in both 2009 winters (too small condensate drain) and 2010 winters (boiler failed outright). Cheaper and on-demand unlike a storage heater which demands really good insulation, adequate sizing which is rarely achieved, cost a fair bit each (=A3320-400) and more suited to people at home 24/7 (retirees, homeworkers, tagged uk.d-i-y members).

Balanced flue get their air from outside and can be 86-89% efficient which is far higher than an open flue fire - realise that whilst a radiant outset fire boasts good efficiency in reality they draw more cold air in through the house the higher they are turned which can result in "draught runaway" such that running them low with a door sausage can halve your bill. Balanced flue are far superior, simple, I think there are even flame-type rather than "metal convector blob" at not a lot more money. Gas is still cheaper than E7, Balanced Flue is probably the cheapest capital+running heating of all.

[Robin]

Can you or anyone please point me to calculations of the effect of a convention flue drawing cold air into the room on overall efficiency?

I've tried to do the sums but my thermodynamics has been lost entropy over the near-40 years since I had to do it so I'm more than struggling with the calculation of enthalpy and Mollier diagramme. The nearest I came to an answer suggested heating incoming air by 20 degrees reduced the overall efficiency by only 1 or 2 percentage points. But that doesn't seem to fit with the generally held view.

I do also appreciate that there is more to it than just the efficiency given the cold air tends to flow at floor level chilling tootsies.

[Andrew Gabriel]

When I had scaffolding up to the top of the chimney, I tried measuring the efficiency of my open flued decorative gas fire. I couldn't because the value of excess air (above that needed for combustion) was so high that the flue gas analyser could barely detect there was any flue gasses present at all. However, this is going to depend very heavily on the chimney height and flow restriction generated by the gas fire.

When mine was put in (by the then gas board), I provided ventilation from the subfloor right under it, so it didn't draw the cold draft across the room. That sort of thing is going to make a significant difference too, but virtually impossible to measure. (The fire is actually just under the rating where external ventilation is required and the instructions say it only needs to be provided if a match smoke test fails.)

[Robin]

Thanks. I had wondered if more than just combustion products went up the chimney but that amount of excess buggers any attempt at calculation by me.