how can you assume enough nitrogen oxides are being regenerated
In a domestic pressure jet burner there is an execess of combustion air so regeneration of the nitrogen oxides is not essential to continue the reactons ( unlike a coke oven where the air ( oxygen ) is restricted to prevent the coke ( carbon ) burning.
the sulphuric acid attack - ie only the bottom inch of copper condensate pipe,
because it runs down the inside of the pipe, surface tension holds it to the end of th pipe.
if you heat a sealed system, what happens to the pressure
when heated the fluid in the system will try to expand its volume.
If the volume cannot expand then :-
(1) if the fluid is a gas then the gas will be compressed and pressure of that gas will increase.
(2) if the fluid is a liquid which cannot be compressed then the containment will be forced to expand by deforming the vessel possibly to the point of rupture.
Hence when a liquid is intended to be heated in a sealed system an expansion vessel is provided where the expanding liquid can expand into the vessel and thus reduce the volume of a gas ( most often air ) held in the vessel. It is the reduction in the volume of the gas that increase the pressure in the system.
The DHW system is not sealed when a tap is opened, it is not heated until a tap is opened.
The part of the sealed system that forms the CH ( central heating ) system is likely to be heated when the boiler fires up for DHW. The expansion of water in the sealed CH system is allowed for by the use of an expansion vessel.
If that expansion vessel has become full of water then expansion is not possible and the expansion of the water will create mild deforming of the components in the system. ( the diaphram in the expansion vessel should be the only component that deforms ) It will also increase the pressure in the pressure release valve which will ( should ) open and allow water to leave the system. The amount of water that leaves will be the extra volume created by the expansion. When the temperature of the sealed system has reached it's operating temperature expansion will stop and any release through the pressure relief valve will cease. It will not be a continuous release.
When the system has cooled down the water will have contracted to its original volume. This will leave some void in the sealed system without water. This void might contain a vacuum with saturated water vapour or it will contain air that has bled back through the pressure relief valve. The next time the system heats this void of vacuum and vapour ( or air ) will become filled with water due to the expansion of the water. In effect it is a pseudo expansion vessel
If it is air then the pressure in the system will increase as the air is compressed but will not increase as much as it did when there was no air in the system. Some water and / or air may be let out via the pressure relief valve but it will be less than was released in the previous heating cycle. After several heat and cool cycles the amount of water left in the system will be reduced and when this volume of water does expand the pseudo expansion vessel wil absorb the expansion without the pressure rising high enough to open the pressure relief valve.