I have an Ideal Isar HE24. It lights for 2 minutes once an hour to warm the heat exchanger and 'optimise delivery time for domestic hot water to taps or showers'...

The problem is that this is burning a small fortune in gas for a function I use once a day in the morning for a shower - all of my appliances heat their own water and I'm out of the house 14 hours a day 5 days a week.

Is the normal functionality for a combi boiler? (first one I've lived with) if not it could actually be worth replacing the boiler as this appears to be adding £400 per year to my energy bills.

  • Surely such a feature has an on/off switch. Have you looked for it?
    – wallyk
    Commented Dec 26, 2013 at 0:55
  • Hi @wallyk yes, I have looked, but it seems I have an early version of the he24. Much of the information online concerns later models and I wanted to know if this is expected functionality for combi's in general. Commented Dec 26, 2013 at 12:26
  • 2
    Maybe you can get a replacement for the control circuit board - expensive but cheaper than replacing the whole boiler. Obviously you'd have to be sure the replacement would bring with it the features lacking from your earlier one. Commented Dec 26, 2013 at 12:31
  • @RedGrittyBrick very good idea. Further research has shown this to be a known problem with this model, it was basically a bad model to begin with and they only sold it in the format I have for 10 months. I may have to resort to placing the boiler on a timer plug. Commented Dec 26, 2013 at 22:47

1 Answer 1


On page 5 of the boiler manual you linked to, it states:

PRE-HEAT DHW pre-heat will be disabled during DHW off periods.

So, you need to define some DHW "off" periods (most of the time, evidently) and a few DHW "on" periods when you expect to use it.

Programming those is described on Page 8.

The default is to run DHW "on" from 06:30 to 00:00 (page 11)

Edit: I have observed that most generalizations don't hold worldwide, so I did not start out with this one - but certainly the vast majority of North American "heating and domestic hot water" (what you presumably mean by "combi" - not a term we use) boilers simply run to keep themselves hot all the time - there may be a summer/winter switch which changes some setpoints, but the basic operation is to sit there being warm/hot as a default. So yes, I'd consider that "normal" operation for such a boiler, if it hasn't got the time controls as described. And @RedGrittyBrick's suggestion is a good one, if a newer control panel is compatible with an older boiler. The other change from older boilers we commonly see here is a LOT more insulation - according to the manual linked, your hourly burn is somewhat more frequent that expected, at least on the newer model - unless your boiler is located in a particularly cold place, that may well mean the newer models are better insulated (and/or, if you have not already done so, you may want to add insulation to the water pipes.) You might ask the manufacturer if there is a way you can add more insulation to (at least some parts of) the boiler itself without causing a hazard.

We had a period (not entirely extinguished, but somewhat tempered by observed results now) when "instant" hot water heaters, where only a pilot light, or sometimes not even that, was kept burning until water is called for, were all the rage. "Observed results" include far less savings (.vs. a well-insulated tank-type hot water heater) than claimed, and relatively delicate heat exchangers and control systems which become expensive to maintain after a few years. Some folks still love the things - they still hold a certain attraction for me, but being off the natural gas grid, not much - bottled/tank-trucked gas (propane) is far a more expensive fuel source.

  • Thank you, I noticed this too. It appears my boiler is an early version of the he24 and lacks many of the iTime controls described in the manual. I'll have to call the manufacturer after Christmas. Commented Dec 26, 2013 at 12:24

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