When the boiler is running (especially in heating rather than hot water mode) watch the pressure gauge: does it rise almost all the way to maximum? And do you then get a sudden discharge of hot water through the pressure relief discharge (PRD) valve and pipe to the outside of the building? (If the boiler is mounted on an external wall the PRD pipe usually goes directly through the wall behind the boiler.)
If so this is where you're losing water (and hence pressure). It indicates that the expansion vessel (probably built-into the boiler) is faulty. It may just have lost pressure (there's a Schraeder(sp?) car-tyre-style valve you can use to top it up) but could have failed entirely (e.g. if water comes out of the air valve!). For expansion vessels built into boilers it's often more trouble and expense than it's worth to replace the fitted one and quite acceptable to fit a vessel externally.
The uk.d-i-y usenet newsgroup is generally a good place to discuss this sort of problem. There is also a wiki/faq at http://wiki.diyfaq.org.uk/
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Your boiler was likely installed in a space that does not provide sufficient combustion air.
The better and long-term solution is to provide a proper vent to provide the needed air. You seemed to have figured this out on your own.
The next logical question is where to put the vent and what size is needed.
I have always felt the answer to this question is not found in the law or 'code,' but what is safe.
The major concern to me is carbon monoxide production by the boiler. CO is made when there is not enough air to completely burn the gas the gas that is firing the flames.
The code seems a good place to find a 'safe' number for the amount of air needed.
The answer to what does the code say, is not easy to determine. There are a number of boilers in the US Boiler, Burnham Series 2 (Model B) product line. The difference is BTU input rating.
The install manual does not provide venting information, instead it points NFPA 54. NFPA states that JHA makes the final determination.
The NFPA answer requires knowledge of the furnace, and the amount of air in the house that can be used for combustion.
The amount of air, found through a formula based on measured or calculation of air provided by the house.
The issue of proper venting should also be addressed, this requires more, and different ventilation.
I know this did not give you a satisfying answer to your question. The CO issue is enough that you should have the needed changes made.
Best Answer
Water cannot compress, only gasses like air and steam can. So if there is no steam an no air in the system, the expansion of the hot water as it pushes against the cooler pipes can be problematic. So most systems have an "expansion tank" that holds an amount of trapped air to allow for that change in pressure so that the pipes don't burst. Once everything is at the same temperature, no pressure shows. But the hot water should be flowing in the pipes, that's why you get the heat. When it turns off, the water cools faster than the pipes, so you get that negative pressure again for a while. In other words that part all sounds normal.
Heat given off into the room is based on the flow of hot water. So there is usually a circulating pump in the hot water piping system, often right next to the boiler. Do you hear that running? If not, it should be and that's your problem. Hot water could still be circulating a little bit due to natural convection, but you need the pump to make it work well. Even if it is running the pump may be bad, especially if it is old. The impellers are usually made of bronze and can corrode if your water chemistry is bad.