Keeping a boiler in condensing mode

lstar337

Cardew wrote: »

Incidentally your combi display is indicating 55C for hot water only? That is sufficient to scald in a shower the maximum recommended is 43C. - I appreciate it can be mixed with cold water.

Yes, I changed it to that recently as it was set at 60c which is only 4c off of the boilers maximum.

Thanks for the tip about 43c, I have been slowly cranking it down over time to see when SWMBO notices in the shower. It may need to stay higher than 43c to keep up the high flow rate at the shower head, but I'll have to wait and see.

It shouldn't be possible to scald in our shower because it is a thermostatic mixer and has an additional safety switch at 38c to stop the knob from being accidentally turned up further. Every tap in the house is a mixer too and though they can be set to hot only (far right position) it hasn't been a problem in 6 months.

Pincher

Cardew wrote: »

Well just how is 'condensing mode' achieved by home owners?

Discuss!!

Gas Central Heating
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The simple answer is, the "home owner" DOES NOT achieve condensing mode by fiddling with the settings. You need to have a boiler controller that has the multiple temperature sensors necessary for it to manage the water temperature to "optimise" for condensation "where possible".

If the user puts in some sensors, and monitors them manually, and tries to adjust the boiler output manually, it will be a full-time job, like a plant operator of an oil refinery in Grangemouth.

The sizing is to do with the lowest running scenario. Let us say you are heating two rooms, and the steady state heat loss is 4kW, what you actually want is for the burner to modulate down to 4kW, so the boiler output is 50 degrees, and returns from the radiators at 40 degrees, thus getting condensation ALL THE TIME. If the boiler's minimum setting is 10kW, you get cycling, where the boiler goes to 70 degrees, which heats the room up quite quickly, but the returning water at 55 degrees gets hardly a drop of condensation. The room is warm in say 15 minutes, so the boiler switches off. The boiler should have a sensor that measures the returning water, so it says, "TOO HOT, must take it down a notch." but it's ALREADY at the lowest setting. No condensation happened, because the boiler's minimum setting is not low enough.

Funnily, if your heat loss is 10kW when heating the whole house, you will get better efficiency because that suits the 10kW minimum boiler down to a tee.



Domestic Hot Water
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http://www.rinnaiuk.com/products/domestic/infinity-k26i/

See the diagram which has a condensate out near the right.

The Rinnai K26i gas water heater claims 107% efficiency due to the EXTRA pre-heat coil that runs the incoming cold mains water through the plume, so you get GREAT condensation using COLD water. The cheaper domestic model Infinity 16i does not have the pre-heat coil, so only claims "High Efficiency".

Have look at ATAG's A Series Combi boilers, the A325EC has a "built-in gas saver module to increase DHW production", and has a DHW efficiency of 95.1%, compared to the 85.9% of A325C, which does not have it. I think you'll find the mainstream boilers in the UK claim their Band A high efficiency based on their GCH performance, and simply don't bother with the DHW efficiency, because they don't have a Pre-heat coil.

So, the home owner BUYs a combi with a Pre-heat coil to get condensation when heating DHW, not by fiddling with settings.

Or just buy a separate water heater, and a 12kW GCH boiler, to get lots of condensation all the time.

PaulBowen

Well for me - it'd simply be whichever mode saved me money on my gas bill and kept the home warm too.

nxdmsandkaskdjaqd

So the boiler keep switching on and off is not good for the boiler. Is there a rule of thumb that says how many times it comes on is reasonable? i.e 70 times in a 16 hour period is excessive, but 20 would be optimum?