10536 Central heating system water circuit top-up

I have no idea how much interest this might be to anyone (other than possibly confirming to some that I'm a bit of a nana), but I thought it might possibly be useful to someone to hear of the problems we had recently with our gas/wood fired central heating system and how they were very easily resolved.

We had a wood-burning stove installed in the summer of 2007. The camino has a water-filled compartment surrounding the fire-box and coils submerged in the water. The coils are plumbed into the central heating system and, when the fire has heated up the water to a temperature set by us on a control panel, water from the central heating circuit is heated by being pumped through the coils in the stove's hot water bath, passed through the house's radiators and then returned to the stove's coils. There were already radiators in the house connected to a gas-fired boiler, so the hot water supply from the stove and it's return line was simply plumbed into the existing central heating circuit using "T" joints. The new stove was installed in the house's kitchen on the first floor, while the Vaillant gas combi boiler is in a utility room on the ground floor.

This worked great last winter: When there was a fire in the stove and the water had heated up above whatever temperature we had selected, a pump switched on and the radiators all got hot. As long as the fire was burning high enough to keep the stove's water-bath above the set point in spite of the heat being drawn off by the central heating circuit, the pump just kept circulating the water. If the fire wasn't big enough to compensate for the heat lost in the radiators and the stove's water reservoir cooled, the pump switched off until the water had heated up again.

The gas-fired CH system also worked perfectly and it could be switched on manually or controlled by a thermostat.

Unfortunately, when we built a fire in the grate a few weeks ago, all that happened was that the stove's reservoir got so hot that it was actually boiling. The control panel indicated that the pump was on and the pump could be heard running, but the radiators were not getting warm at all. The gas central heating system, however, worked fine.

When we got the man who installed the fireplace in to look at it (next day service, by the way), it took him all of five minutes to spot the problem.

On the gas combi-boiler there are two gauges, neither of which I'd ever paid a lot of attention to. One is for water temperature, which is understandable enough. The other is a pressure gauge with a red line at the 2.5 mark. As much as I had thought about this, I'd assumed it was something to do with incoming water pressure. However, it turned out that this is actually the pressure in the hot water circuit. The pressure showing on the gauge was exactly zero, and this explained the problem with the fire.

While the gas boiler, being downstairs, was able to pump water around the central heating circuit, the stove's pump was at the highest point in the circuit and so there was only air in the pipes on either side of it. Central heating pumps are not designed to pump air and they are incapable of "sucking" on an air-filled pipe and raising water up to the level of the pump. They must have water on the inlet side in order for them to work at all. So although the stove's pump was running and there was nothing wrong with it, no water was moving through the circuit.

The solution to the problem took all of 30 seconds: open a little valve hidden away on the underside of the combi boiler and let water into the system until the gauge read about 1.5.

This was a surprise to me since we actually have two Vaillant combi boilers here and the one that's had the most use has never required manual topping-up. I'm pretty sure it's newer than the one connected to the camino and, while it does have a pressure gauge, it has always read something less than 2.0. Since there is no obvious manually operated water inlet valve, I assume that an automatic valve tops up the system if water and pressure is lost from the central heating circuit. Also, while such systems may exist in Britain, every central heating system I'd ever encountered there had the normal header-tank with float-valve system which automatically filled up the CH circuit if water was lost due to leaks or bleeding off air.

The stove worked okay last year after the system was filled with water by the installation team, but clearly enough water was lost during bleeding and perhaps due to minor leaks that the stove's pump was left high and dry when we switched in on again.

As I said at the start, information that's probably either of no use at all or makes people go "Duuuuh! Doesn't everyone know that?" :bigergrin:

Al

Category
Cost of living - Utility Services

Submitted by Anonymous (not verified) on Tue, 09/30/2008 - 07:09

Thanks for taking the time Al, I was shown the little valve under the boiler by the plumber and he explained what to do, of course this went right out of my head when the system didnt work due to the bleeding of rads,as you say solution in 30secs flat, and the plumber was charming enough to not make me feel anymore silly than I already did.
A

Submitted by Anonymous (not verified) on Tue, 09/30/2008 - 07:36

You should be able to get automatic bleed valves set at the highest point of the system to help alleviate this problem. For sure it is best to keep pressure set at circa 1.5 Bar, this is what we have and just check the guage and if needed open a valve to boost.

Submitted by Anonymous (not verified) on Tue, 10/07/2008 - 10:57

[quote=AllanMason;98139].......As I said at the start, information that's probably either of no use at all or makes people go "Duuuuh! Doesn't everyone know that?" :bigergrin:

Al[/quote]

Allan, thanks for this; we had experienced a similar problem; we hadn't used our stufa/CH for a long time after it was installed as we visit only on and off, and also rarley need the heating on. Initially our 'thingy' valve kicked in and dumped a load of water out of an outlet on our terrace; quite a sight on a cold winter night...the steaming water scared the (ahem) fur off our local feral cats!

Having similarly forgotten the lesson taught by our installer, we fired it up again and kept super-heating our water, until one of our T joints decided "basta!", gave off a very loud bang, and started leaking water over the upstairs.........and then into the downstairs.....how we laughed :nah:

All credit to our builder, Avio (dh'arling man!) who was called and was with us within 30 minutes, with installer, to remind us what he told us 12 months ago. All free of charge......we love that man.

Suffice it to say, we make sure we check out water levels everytime we fire up the stuffa/boiler now :wideeyed:

Submitted by Anonymous (not verified) on Sun, 10/12/2008 - 18:05

Yes that little valve is v important! and don't forget to check if upstairs radiators need any air bleeding from time to time. Downstairs ones seem to look after themselves.

Submitted by Anonymous (not verified) on Mon, 10/13/2008 - 03:46

Also - remember to check the pressure gauge after bleeding air out of radiators - as taking the air out of the rads lowers the pressure in the system.

.

Submitted by Anonymous (not verified) on Mon, 10/13/2008 - 05:45

Another point is that if you have a Grundfoss style of pump, then you can loosen the centre screw to bleed the air out of it.
Most of these type of pumps are really meant to be running at fairly regular intervals, as it helps to prevent build up of debris or calcification in the pump body itself.
Again as Alan says, always keep pressure in the system and recharge as required to the recommended level.

Submitted by Anonymous (not verified) on Mon, 10/13/2008 - 11:39

Quite right Allan , I had forgot that system pressure drops after bleeding radiators.