Why feed and vent?

[Toughtools]

I'm new to the form, so Hi everyone.
Can somebody explain why, with a modern boiler in an open-vent system, both the feed and vent are necessary. Surely the feed would cope with any expansion.

 

[dontknowitall]

I'm afraid it wouldn't. Steam molecules are 1600 times the size of water molecules. The system is designed to overflow into the cistern and if this is filled too much, then itself overflows outside the house.

 

[justlead1]

Hi. As dontkall says steam can be produced. How ever during normal working of a system, on heating and cooling the volume of water within the system varies. Say 25 litres when heated would reduce to 24 on cooling (4%). This expansion / contraction is catered for in the cistern and via the cold feed. (thats the reason you may find the ballvalve arm bent, to provide the room for expansion with out overflowing. The vent allows air to be purged from the system on filling and in when draining. In the event of access heat produced by energy source and not controlled (boiler) the steam generated can escape to the atmosphere and replaced by cooler water entering the system, offering a modicum of safety to both the system and occupants of building. Having said all that there was a period when it was approved to tee the cold feed into the vent pipe on heating systems. Now engineering is employed to control new systems, more than conventional water physics (it cheaper) Good Luck

 

[Plouasne]

Any one remember the boiler explosions 1946/7 winter ??

Mostly caused by the boiler boiling dry, then water entering a red hot boiler, flashing to steam with rather more than a pop !!

This was put down to an exceptionally cold winter, bad insulation in the roof, all steel piping, a direct hot water system, with a steel hot water tank/cylinder
The boiler was lit in the morning, the cold feed to the cylinder was frozen solid, water in the hot water tank/cylinder was drawn off, and the boiler commenced to boil the rest of the water, turning it to steam, which exited via the expansion pipe, this caused the frozen water to melt, and release water to a bone dry and red hot boiler, the water flashed to steam

BYE BYE boiler, and anybody who was near it

I have seen (and done it myself) the result of trying to get bad coke to light, in a horse shoe boiler, by pouring paraffin over the hot coke, first of all the liquid turns to fumes, then ignites blowing off the loading lid, the ash pan draught regulating door off its slide with a tongue of flame 6 feet long coming out of the ash pan, that's why you should never stand in front of a boiler if you are contemplating this, people have been burnt to death because of doing such a thing

Solid fuel boiler central heating, needs to be designed by a person who knows the risks involved with solid fuel boilers, otherwise what goes round will come around again, and people will get killed, it is not a DIY job

 

[Hugh]

Using the cole feed to the heating as you expansion is not a good idea because if the system needs to vent over it can not take in water which then starts to creat stem in the boiler and will eventually if all goes well should blow off at the saftey valve on the boiler.

 

[Toughtools]

Thanks guys for your replies. Perhaps I'll stick to convention.

 

[Torsionbar]

In our fits house there was a late 1960's Servowarm master radiator heating system with manual hotwater/heat changeover valve just had a 1/2 pipe running up to small corner tank about window height in the bathroom. This held about a gallon of water whch had to be topped up each month in winter and fully in autumn after disuse as all the water evaporated. almost all the pipework was done in stainless as there was a copper shortage but special steel capillary fitings were used which after 15 year all had pulled and leaked through corrosion which seemed to hav ebeen caused by the flux and all had to be replaced with compression fittings and in some case new copper pipe.