Our (Not-So-) Green Central Heating System

Should we change (upgrade?) our old boiler?

When we needed to get our gas meter changed the nice man from Scottish Gas took one look at the central-heating boiler (a 1978 Ideal E Type CF 80N - see the Installation Manual and the User's Guide) and told my wife we should get a new one straightaway as we were wasting lots of money on this inefficient old thing and in any case it wasn't a very safe system to keep running. We haven't yet though although that was a few years ago. So far as safety goes like any of these older open-flue systems it is perfectly OK so long as its air supply isn't interfered with and it is kept serviced (Scottish Gas sales spiel IMHO). Certainly he has a point regarding efficiency because the E Type is possibly as poor as about 55% although it predates the SEDBUC tables (the installation manual quotes nearer 73% though, so who knows?) whereas a new boiler might be well over 90% if you believe the manufacturer's blurb (you do? - really??).

There are a few downsides to getting a new boiler though:

As a by-the-way, condensing boilers are most efficient when they are run at a much lower temperature than the old ones. Since our radiators were sized taking into account the temperature of our present boiler we would either need to change some or else run the new boiler at a less economic higher temperature.

So it looks as though any fuel savings would be completely wiped out by the costs of installing and maintaining a new boiler. And that's before the dubious pleasure of having installation and maintenance men tramping through the house at regular intervals. To say nothing of the risk of getting a Friday one! So we will wait and see how things go.

Can we make the existing one more efficient?

Our system was semi-gravity which means that whilst the water to the radiators is pumped round, the heating water to the hot-water cylinder just convects under gravity. This scheme works reasonably well in big old houses where the hot water cylinder can be a long way above the boiler, but in our bungalow it isn't so the convection rate is low which means that it can take quite a while for the hot water to heat up. The worse problem though is that while the pump is running there is nearly no convection so in the winter, when the house takes a while to warm up in the mornings, the hot water sometimes wasn't very hot at all.

Another downside was that there was no independent control of the hot water and heating systems. The boiler fired whenever hot water was programmed and the pump ran whenever central heating was wanted. This meant that the boiler would cycle on and off sometimes for hours on end when nothing really needed it and also, when the house was warm, the hot water could sometimes get unpleasantly or even dangerously hot. Our system used the famous old Potterton Mini-Minder Controller (see Installation Manual and User's Guide) still going strong after 30-odd years.

The changes

The boiler itself, a 1978 Ideal E Type CF 80N, is a very reliable old design thought of by many experienced service engineers as one of the best central heating boilers ever developed. Basically it consists of little more than a gas valve, pilot light, thermocouple, main burner and heat exchanger. It's not got a lot to go wrong - the thermocouple burns out at longish intervals, the pilot jet can clog a little due to the dirt in the natural gas and the gas valve could fail (hasn't yet) and all of these are easily attended to. Even after 30-odd years spares are still relatively easy to obtain because of the large number of boilers the parts were used in.

The best solution would be to have a modern fully-pumped system but it is quite a big replumbing exercise to convert our system to that. I did consider instead fitting a second pump in the hot water circuit, but decided that having 2 pumps running wasn't a good way to save money, to say nothing of the extra noise and risks of pumping over etc.. After a bit of research I found the Honeywell 'C' Plan (not permitted for new installations but meeting the regulations for upgrades) which enables thermostatic control of the hot water in a gravity system and decided to install that. This still doesn't get around the problem of slow heating of the hot water when the central heating pump is running and so I decided to help that by installing a full programmer (enables independent control of heating and hot water) which would enable me to start the hot water heating say half-an-hour before the central heating. After a bit of thought I realised that I didn't need the valve in the Honeywell system because when the central heating is on the convection is so poor anyway that there is no need to try to control it and so I used a relay from the junk-box driven from the new hot water cylinder thermostat to simulate the effect of the valve turning on and off. This saved fiddling with the plumbing. The wiring changes took no more than a couple of hours and the new programmer (I used a Danfoss FP715SI - see the Installation and User Instructions) simply clipped in where the Mini-Minder originally was, using the same backplate and wiring (after 30-odd years this is a bit of a victory for standardisation IMHO).

The new control system works fine. With the hot water enabled half-an-hour before the heating it is always hot when needed and when neither heating nor hot water is needed the boiler no longer cycles. I won't know what the savings are for a year or so but I'm satisfied that there will be some economies.

Still to do

It's always irritated me that the centrally heated temperature cycles by at least 1oC and sometimes more. In part this is because room thermostats generally have some hysteresis and in part it's because once the radiators are hot they continue to heat the room for quite a while after the thermostat has tripped. I've fitted a low-hysteresis thermostat but the problem persists. The best solution would be some sort of predictive control but this is one for the future.

Thermostatic radiator valves (TRVs)  can help overcome this to some extent because they have an analogue action (i.e. they turn on and off gradually) rather than the simple on/off of the thermostat but I've never been a great fan of them. They seem to work well enough in closed rooms but if you have all the doors open all the time as we do, natural movement of the air tends to create draughts and reduce their effectiveness. (Also they have a nasty habit of sticking shut after a long warm summer).

Update 25/4/2011

Well it's over a year since I installed the (more-or-less) Honeywell 'C' Plan changes and there are clear savings as a result. Although the recent winter has been longer and colder than last year's (with temperatures down to -16oC) the gas bill for the year was over 15% less than last year's. This is a much greater saving than I expected, and from checking the readings much of the savings came during the middle of the year when the boiler used to cycle repeatedly just to keep itself hot. Quite a success and so I'm unlikely to want to replace this old beast for some time yet. Hurrah!!



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Page last updated by on 13/10/2014