How do I calculate the capacity of my central heating system (2)

Able_Baker

This follows on from a previous thread in which it was decided that adding a chemical to clear out sludge didn't require an accurate calculation.

My system has 9 large radiators and 3 small; it is driven by a large combi-boiler. I've now had two heat-exchangers in the boiler burst in 5 years. The manufacturer tells me this is unusual (it would do, of course!) and that I should determine the capacity of the system to determine if the expansion capacity of the boiler is sufficient. In an ideal world, it says, there would be little change of pressure at any point of the cycles.

ormus

the water content of normal smallbore (15mm pipe) steel panel radiator systems can be estimated using the following formula,
divide the boiler output rating ( in BTU's ) by 10,000 and multiply by 4 ( e.g. 50,000 BTU boiler system = 20 gallons ).

Able_Baker

That's an interesting formula which might explain my problem since the pipework and radiators are mainly over 30 years old and the boiler is only 6.

The manufacturer's specification for the boiler is:
Max input: 105802 BTUs
Max output: 95563 BTUs
Max heating system capacity: 21.9 gals
Expansion vessel: 2.2 gals

The system was originally installed with a slightly smaller boiler with a capacity, I think, of 85000 BTUs. Applying your formula gives a water capacity of 34 gals.

For boiler figures to make sense, there must have been a trend towards radiators with a smaller water capacity over the last 30 years. In other words, I presume a new system would contain much less water and the formula does not apply.

Canucklehead

Hi

Curious to know what make combi you have.

Corgi Guy.

Able_Baker

The boiler is a Vokera Linear Max. I'd be interested to read your comments on it.

ormus

its obviously a general formula, based on a typical home ch system.
(say a 3 bed semi)
larger radiators will need an allowance as will larger pipe sizes and runs.
so i think possibly your figure is nearer the truth than the normal 20/22 gallons?

Able_Baker

I can go with any rule of thumb having its limitations.

However, since Vokera produce a boiler with an output of over 95000 BTUs and a water capacity of only about 22 gals, they must be expecting engineers to design a heating system for a large house with a relatively small amount of water. Obviously no such constraint applied when my system was installed and thus I suspect this boiler is incompatible with the radiators and pipework.

Vokera told me that they expect the heat exchangers to last for ever and that incorrect gas pressure or excessive water pressure was the most likely causes of failure. The firm that replaced mine yesterday are replacing three more next week. They told me that they regard a hot/cold variation of 0.75 bar as not unusual, whereas Vokera gave a much lower figure. I deduce that I'm not the only one with the problem of too large a water capacity for the boiler.

The solution that Vokera offers is the increase the size of the expansion vessel. Before I embark on finding someone who know how to do it and the consequent expense, I'd like to be sure it is necessary. Another possibility, for example, is to take out some water so that the cold pressure is 0.5 bar and this probably would ensure the hot pressure was not excessive.

At the moment I'm monitoring the water pressure as I don't know what range the system is experiencing. Having a leak, probably for much of the boiler's life, means I have no figures.

ormus

to design a ch system proper, is a long and complicated process, involving the u values, room sizes etc.. pipe sizing and therefore boiler size.
most plumbers know from experience whats required for a typical teraced, semi, detached house. it usually works out ok, so they stick to that.
the problem of course is that sometimes the above is not correct for certain homes.
plumbers are not central heating engineers.
i used to have a tech ch book, (now lost) explaining all the above. it took me many hours to design a system for my 3 bed semi.
thats why few plumbers bother. they wing it and get away with it most times.