Hot Water Boiler
Low water protection isn’t just for steam boilers. Hot water boilers face the same perils of overheating damage if the water line drops too low. Many people don’t think of this as often as they should because hot water boilers serve “closed” systems. They have pressure reducing valves that are supposed to feed water automatically should a leak develop.
The truth, however, is that a pressure reducing valve is no substitute for a low water cut-off. Pressure reducing, or “feed” valves, often clog with sediment and wind up not feeding at all. A buried pipe can corrode and spring a leak that flows faster than a “feed” valve can satisfy. Relief valves can pop and, while dumping water at a great rate, actually prevent the feed valve from operating. Let’s take a closer look at how we can protect these boilers.
Hot Water Systems (Large image with links)
Hot Water Systems Fig. Z
As we said, the things that affect steam boilers also affect hot water boilers. If you run them with too much water the relief valve will open. If you run them with too little water they’ll overheat and suffer damage. A low water cut-off is the only sure way of protecting a hot water boiler from sudden loss of water. The ASME boiler code recognizes this by requiring all hot water boilers of 400,000 BTU/HR or more input to have low water fuel cut-off devices. ASME doesn’t call for low water cut-offs on smaller, residential boilers, but we think all hot water boilers, regardless of their size, must have protection. However, the International Mechanical Code requires low water cut-offs on ALL hot water and steam boilers. ITT McDonnell & Miller make several devices, both float and probe type, that protect and meet the needs of any boiler whether it’s cast iron, steel, or copper construction (Series 67 Float Type Low Water Cut-Off, Series PS-851 Probe Type Low Water Cut-Off, Series RB-24 Probe Type Low Water Cut-Off). Hot water systems regularly lose water through faulty air vents, loose valve stem packing, cracked boiler sections, loose nipples, corroded pipes, broken or loose pump seals, leaking gaskets, dripping relief valves, to name just a few places. Most installers depend on their pressure reducing or feed valve, to replace the lost water automatically. But feed valves often clog with sediment, especially in hard water areas. And it’s very easy to close the supply valve to a feed valve and forget to open it again. On systems with buried pipes (say, a radiant heating system) a feed valve will open if a pipe breaks. It will feed fresh water continuously until it either clogs (and stops feeding) or destroys the ferrous components of the system with oxygen corrosion. A simple feed valve can wind up costing a lot more than its purchase price. This is why major suppliers of feed valves, such as ITT Bell &Gossett, recommend you close the feed valve once you’ve established your initial fill pressure.
This is also why we strongly recommend you use a low water cut-off on every hot water boiler. Feed valves are not a substitute for low water cut-offs. They can’t protect your boilers from a low water condition. Feed valves are fine for filling the system initially, and for helping you vent air from the radiators. But once the system is up and running, you shouldn’t look to them for protection. Read more (pdf file: 206KB).
• Understanding Residential Electronic Boiler Controls – Technical Guide (pdf file: 905KB).
• Hot Water Boilers System Selection Chart (pdf file: 23KB).
• How To Select Boiler Controls (pdf file: 75KB).