Multi-Burner Combi Boiler (Furnace)

We have a Baxi combi boiler in our house for central heating and hot water. It has a very powerful 45 kW gas burner which enables it to produce hot water as and when required, fast enough to run a good shower or fill a bath. Hence no energy is wasted keeping a tank of hot water warm "just in case". And it takes the water direct from the mains, so no tanks are required in the house. So far so good.

The problem is that our boiler cannot heat just a little water. Below a certain flow rate (3.5 litres/min according to the manual), it switches off completely. This is a waste if you only want a small flow of hot water. And worse still, the mains-fed flow rate to our shower on the second floor is only just above the threshold to make the boiler work. Anyone using a cold tap or flushing the toilet downstairs reduces the available pressure (remember the boiler is fed by the cold mains), resulting in an "invigorating" icy shower until normality returns.

Presumably the problem here is that the large gas burner required to support high flow rates of hot water cannot sustain stable combustion at the low heat output required to sustain low flow rates. I calculate that ours cannot go below approximately 10 kW, or about 22% of its maximum rated output.

My solution is simple: have more than one burner, which can be independently controlled and used in tandem in the same heating loop. For example, replace the single 10-45 kW range burner in my boiler with two burners:

Now I have the same maximum 45 kW power using both burners in tandem as before. But I can also go all the way down to 1.8 kW, with well under 1 litre/min minimum flow rate with the big one turned off and the small one at minimum. I now have a much bigger dynamic range or "contrast" between minimum and maximum, and can reach any power level between the extremes.

Of course this generalises to more than two burners, or you could envisage a single device with multiple valves or sliding elements such that just part of it can be used if required.

No more icy showers?!

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