There are small water heaters, Webasto and Eberspacher are very popular, however to make them so small they are not so economic to run, in both cases they use an external calorifier, I would call it a heat exchanger, so the combi combines the water heater and calorifier together.
Advantage of combi, the installer can’t get it wrong.
Disadvantage of combi, needs a larger space, because combined you can’t build it around other items.
Comparing a hot water storage system to an instant system has nothing what so ever to do with combining the water heater and calorifier together, and also has nothing to do with modulating and non modulating boilers.
There are advantages with storing the hot water, main advantage is you can combine multi fuel water heaters, like solar, wood burning and oil, with separate calorifier you can with second pump actually store the water it heats, very common on narrow boats where the engine cooling water is also used for domestic hot water and central heating, this is not impossible with a combi but it is harder.
However I would assume the article is limited to house central heating, and does not include narrow boats.
We all tend to live in a bubble, and forget systems which we don’t deal with, I have had two houses, but heated with gas, one circulated air, the other circulated water, the former had electric only domestic hot water, the second at first used a hot coil, then an independent water heater was fitted for domestic hot water.
I thought I knew it all, until faced with my mothers’ house, it was the house not the central heating which was a problem, massive bay windows collected morning and evening heat from the sun, forget glass that does not let heat escape, and we needed glass that did not let heat in.
I tired all sorts to stop one room over heating in morning, and other over heating in the evening, it was sorted in the end, but it was the problems with this house that made me realise control is the main problem.
My first house all rooms were the same temperature, could hardly be anything else, circulating hot air all rooms have to be the same, OK there were dampers, but it was like the old song. The problem was at that time single glazed, so blowing hot air against cold windows was expensive.
Next house open plan, problem here was bedrooms became too hot.
So the main problem was design of the house, not design of the central heating, and this is the main point, one can’t compare methods of heating a house without also considering how the house is built.
We see all these reports on how this system is better, but it all depends on the house, so in a well insulated house moving the air around is likely good, but where insulation is poor we want hot spots where we sit.
So to write a report one how to heat a house would needs books, you could not squeeze it all into a couple of pages, yes I think the combi water heater was good, not because it did something which could not be done with separate units, but because it reduced what the installer could get wrong.
