Hot water with heating

[Theo]

A house I used to live in had an early 1970s cast iron lump [1] for a boiler and a mechanical time clock. One 'feature' of this system was that you always heated the hot water when you turned the heating on. I understand that was common in older systems.

I'm curious as to how this was plumbed. I understand about S and Y plan systems, but I can't see how that would cause this effect. Was this set up with the boiler output always going through the coil first? Or with a single heating on/off valve? Or was it an S plan where the controller just told the hot water valve to open when the heating was on?

Anyone know?

Thanks Theo

[1] Potterton Kingfisher I think it was, although I gather there have been a lot more recent Kingfishers since then

[Graham Nye]

The uk.diy wiki suggests C plan:

Was there an earlier version without a zone valve in the hot water side, so hot water was always heated via gravity?

[Owain Lastname]

Probably, especially for solid fuel where you can't have any valve or obstruction in the circuit to the heat dump.

Owain

[JohnP]

I once adapted such a system by fitting a motorised valve to the gravity feed, conrolled by a tank Thermostat.The switches in the valve head controlled the boiler / or the room stat and pump.

[John Rumm]

It was quite common to have a big cast iron lump HX with high water content (couple of gallons). That would have 4 bosses on it - two outputs and two returns. One set would be used for the feed and return on the pumped circuit through the rads, and the other two would be used for a gravity (i.e. convected circuit) to heat an indirect cylinder.

The convention circuit required that the hot water cylinder would be placed some distance above the boiler and not too far displaced from it. It would also typically be piped in 28mm pipe to better allow convection flow. Hot water from the gravity output would rise up through the pipe, and through the coil in the cylinder, the heavier cooler water would fall and return to the boiler. Basically it sets up a thermosyphon.

Some refinements of the system introduced and anti gravity valve to stop heat being lost out of the cylinder through the boiler when it was off. Some had a kind of thermostatic valve in the gravity loop to limit the maximum temperature of the cylinder DHW. Some would add a motorised valve to make it a fully controlled zone. (aka C plan)

Yup, I know someone with one of those and a gravity circulation loop...

[jon]

For many years in the 70s I had a back boiler behind the living room fire. We would use drift wood to keep the fire burning which supplied nearly boiling water for the kitchen and bathroom. Superb arrangement, but don't know how it was fed with mains water, but it just worked.

[Andy Burns]

shhhh! don't go upsetting my boiler now ....

[Theo]

Ah, so 'gravity' meant there was no pump on the HW side, only for the heating?

That's neat. Horribly inefficient, but neat :)

That sounds about right - cylinder was above the boiler and about 2.5m displaced, so that would probably work.

I never investigated but I imagine there was just on/off control for the boiler (gas valve only, since there was an always-on pilot that had a piezo spark-button to relight it if it went out), and an additional time clock output that was in series with a thermostat to run the pump for the heating.

How was the boiler turned off in this system, when the thermostat said the house was hot enough and so was the water? If the heating called for heat and the hot water was up to temp, surely that would overheat the water?

Theo

[Tim+]

Water temperature would be limited by the boiler stat.

It was never a terribly efficient system but it was simple and reliable with no new-fangled motorised valves to go wrong. ;-)

Tim

[Harry Bloomfield, Esq.]

Theo wrote on 14/05/2021 :

A friend of mine used to have a system installed when the house was built, where he had to select where the heat went via a manual 3-port valve.

[Dave Plowman (News]

Mechanical time clock suggested it was designed to work on thermo circulation only - at least at one time. With likely a mechanical valve to add on the heating - again likely thermo, using larger bore pipes than we're used to these days. With the advantage it worked during a power cut.

Later systems still used thermo circulation for the hot water, with a pump for the heating. Pump not running, hot water only.

[ARW]

Were the common programmers of the time

[John Rumm]

Yup, DHW looked after itself, and the pump only served the rads.

(thermosyphon systems could also run with solid fuel heating boilers, where the inability of a pump fault or electrical failure or power interruption to affect the operation of a system that could not easily be "turned off" was seen as an important benefit)

Inefficient yes, although possibly not that much less than some pumped systems that could only extract say 3 to 5kW of power from the boiler to the cylinder via a short indirect coil. Those would have to short cycle the boiler to load match its much higher output to the input of the cylinder (or run it in parallel with the rads to present enough load - hence the popularity of Y plan systems)

The boiler would have its own internal water temperature stat that would interrupt power to the gas valve when the set point was reached (there was often a knob on the boiler to tweak that set point).

That was one of the limitations - there was no system interlock as such that could shut down everything once the cylinder was up to temperature. Most programmers of the era would usually just run a timed heating cycle for the cylinder. If heating the DHW alone, the boiler would run and cycle. As the cylinder came up to temp, the rate of heat transfer would fall, and the boiler cycle would shorten. However if not turned off with the programmer (or manual override) it would carry on indefinitely.

[ARW]

The upstairs rads (or some of them) would have warmed up (not to full heat) if it was not for the anti gravity valve on the heating flow side.

[John Rumm]

Sometimes part of that was intentional, they might put the bathroom rad in the gravity loop for the cylinder rather than as part of the heating circuit.

[ARW]

I had forgotten about that one.

A few years ago I did a combi as an S plan and the bathroom rad was one of the zones. The flow sensor on the bath hot tap triggered a timer to operate the bathroom 2 port valve

[Theo]

I'd forgotten about that - there was a heated towel rail that always came on when the hot water came on. Made the bathroom nice and warm. In summer we used the immersion 'because it was cheaper' - that might have been a reason why! (I very much doubt it was cheaper)

While the gravity might have meant the system would work without power, the timer was electric so there was no way to operate it during power cuts. I don't remember a pump for the central heating but I imagine there must have been one.

Theo

[Theo]

This was the one:

was just a simple 1 rev per day motorised dial plus some cams to operate switches.

Being on gravity makes a lot of sense - the neighbours moved their cast iron boiler into the garage and 'it never worked properly after'. Which would follow if the tank is suddenly further away from the boiler.

Theo

[Roger Mills]

Yes indeed, and no motorised valves either.

Inefficient, yes because there was no boiler interlock. In other words, the boiler continued to fire just to keep itself warm even when there was no heating demand and the hot water was hot enough.

The boiler had an electrically operated gas valve, so would only run when power was applied - plus a safety interlock which prevented the gas valve from opening unless the pilot was alight and heating its thermocouple.

As others have said, the hot water would eventually get up to boiler temperature - which was too hot for comfort unless the boiler stat was turned down. Then the radiators may not get hot enough. One solution - which I installed in my first house in about 1969 - used a thermostatic valve (a bit like a TRV) in the gravity return from the cylinder coil and stopped the circulation when the hot water was hot enough. It didn't stop the boiler cycling on its own stat, of course.

Such systems would typcially have a two-channel programmer (electro-mechanical rather than digital of course!) to switch the boiler on when hot water and/or heating were required and to switch the pump on (often via a room thermostat) when heating was required.

[Andy Burns]

Tank and room thermostats are both fitted here, with water and heating times set separately.

ok, when the heating is "on" and the room stat is calling for heat, then heat is always going to the hot water tank, up to the limit of the boiler's own stat.