Boiler pressure variation

[stevecase51]

I’d appreciate advice from somebody with experience in combi central heating systems. My system was installed just over 12 months ago. The combi boiler is a Vaillant ecotec plus 831. My house is an average 3-bedroom semi, with six radiators plus towel rail. The combined output of these is quote modest - around 10kW. During the first 12months of operation the boiler pressure has reduced very slowly, suggesting a tiny leak (though none is visible anywhere). Since the middle of January to thepresent (7 months later) the cold pressure has fallen from 1.3b to 0.8b (i.e.less than 0.1b per month). In the colder months when the heating was on, the pressure drop was a bit quicker – perhaps 0.1bevery 3 weeks. I am not particularly perturbed by this, but I am a little intrigued by two other things that I havenoticed:-(1) During the summer months (when the heating is not used) the pressure can vary by as much as 0.3b depending on the ambient temperature. For example about 3 weeks ago when we had a cool spell, the pressure was 0.8. Over the last couple of days, when theweather has been hot, the pressure has been 1.0b. (2) Irrespective of what the pressure is when cold (i.e. 0.8bto 1.3b), it climbs to about 2.2b when the heating circuit reaches its target temperature of 70 degrees. I know that the expansion vessel is designed to control the pressure, and stop it rising to 3b or more. But I wondered if the above variations in pressure are normal for what is a modest size of system, or does it suggest that the expansion vessel may need topping up? I have a Vaillant engineer calling in a few days time to carry out the first annual service, and I wondered whether to mention this to him. Any advice appreciated.

 

[kris]

Your pressure gauge that your reading off the boiler is the pressure in your heating circuit and nothing to with the cold water pressure , hot water expands when heated so when heatings on you will have pressure increase and decrease, if your having hot water pressure issues then this is totally seperate issue , let the vaillant engineer re charge etc and give it a once over and youl be good to go, as a home owner the only thing you can do is keep an eye on your heating pressure and top up when neccassary and let the engineer know of your issues and he'll get u sorted mate , you could feel round your radiator pipes to make sure no weeps,

 

[stevecase51]

When I said "the cold pressure" I didn't mean the cold water (mains) pressure, but the pressure of the heating circuit when completely cold. I suppose the question I am asking (in a roundabout way) is this: If I have an expansion vessel that is very slowly losing gas but, at present, still retains some charge, what symptoms wouild this produce?

 

[chris watkins]

The swings in pressure that you are experiencing !

The vessel charge should be the roughly the same as the system cold fill pressure, so that as the system pressure increases due to the heating the additional water is accommodated with in the vessel.

The slow loss of pressure over time will be a micro leak look very closely at the compression joints on the rads etc for any signs of staining which would indicate a weep & nip them up, most of the time this is the problem.

 

[jonnyswamp]

I'd say your boiler pressure is pretty normal ( although you may have a slight weep on a comp fitting that dries before it has chance to form a drip)
The changing pressure you mentioned when cold (heating off) would depend on if the hot water had just been used or not, as the central heating water indirectly heats the hot water via a heat exchanger and this could affect the boiler pressure slightly
If you're topping up the pressure every 7 months you've not got much to worry about
Normal use will cause evaporation within the central heating system over time and air can find it's way back to the boiler and out through the auto air vent causing a pressure drop
Also if you manually vent any radiators this will do the same

 

[stevecase51]

The changing pressure you mentioned when cold (heating off) would depend on if the hot water had just been used or not, as the central heating water indirectly heats the hot water via a heat exchanger and this could affect the boiler pressure slightly

Thanks for your comments. The pressure values I mentioned were all noted first thing in the morning before any hot water was used. If the HW is run for, say, a shower then the rise in temperature of the water in the internal boiler circuit causes the pressure to increase slightly - usually by 0.1b.

 

[chris watkins]

That is fine & to be expected, there must be a small raise in pressure for the expanded water to flow into the expansion vessel.

If the vessel had lost some of its charge the pressure increase would be much greater.