Refurbishing heating systems — the golden rules
When it comes to refurbishing heating systems in schools, offices, municipal facilities or even churches, building-services engineers are faced with many choices. If budgets are tight, tough decisions have to be made, especially when the cost of replacing entire heating systems is prohibitive. Peter Gammon of MHS Boilers, explains the potential problems of new boilers on old systems, and examines the simple and cost-effective solutions available.
On the face of it, replacing a commercial boiler shouldn’t be a problem — especially when modern appliances are generally smaller, more powerful and far more efficient than their ageing cast-iron counterparts. However, when the boiler plant is nearing the end of its life cycle and needs replacing — but the distribution system and heat emitters still have some life left in them — there are a few issues that need careful consideration.
The first one, which is often a problem in multi-level buildings, is the pressure rating of the existing distribution system compared to the maximum operating pressure of the new boiler. If the original system works on an operating pressure of 5 bar, but the replacement boiler has a maximum of 3 bar, there is an obvious mismatch. And surprisingly, this is not an uncommon scenario, especially as more and more antiquated systems are replaced with new, high-efficiency condensing boilers.
The second is the problem of dirt transfer from the (less-than-clean) existing system connecting into new boiler plant and pumps. This is particularly problematic in low-velocity sections of older systems where a lot of magnetite has accumulated over the years and, as soon as the system is drained, the silt and magnetite dries out — turning it into a fine dust. This isn’t too much of an issue with the older boilers as they usually have large waterways with low velocity, which spread the silt thinly. However, with modern counterparts, there can be major problems.
When the system is refilled or flushed, this fine powder gets distributed and is free to flow throughout the length and breadth of the new equipment — causing no end of headaches. The silt can also combine with the salts in the refill water and bake hard in the waterways of the new equipment. Being magnetic, the silt can clog around the armatures of canned-rotor pumps, causing them to seize. Not only that, but the deposits act as an insulator causing the metal of the heat exchanger to retain heat instead of transferring it through to the surrounding water. So despite having invested in new high-performance, high-efficiency boiler plant, it might actually be causing all sorts of damage.
But that’s enough of the problems. What of the solutions to these issues?
Well, thankfully there is a relatively simple and cost effective way of fitting a new high-efficiency boiler to a tired old heating system.
The main requirement is to separate the old system from the new boiler plant, and one of the best way of doing this is to use a plate type heat exchanger. This creates a barrier between the old and the new and allows the creation of a brand new self-contained environment on the primary side of the heat exchanger. This also acts as a pressure break between the two sides, so even if there is a significant variance between the two operating pressures, both can continue working at their optimum levels without compromise. Furthermore, it enables consultants to specify the boiler that is best suited for the project, rather than having to potentially sacrifice performance with a lesser unit, or replace an entire heating system.
Additionally, separating the new boiler plant from the old system allows the water to be treated on the boiler side, which will not only extend the life of the boiler, but also enable the use of a modern high-efficiency sealed and pressurised model. The old system can then remain open vented and as dirty (and leaky maybe) as it was!
The separation also allows the boiler to be easily cleaned before commissioning, without the associated problems of cleaning an old system.
For specifiers, using a plate heat exchanger is not only best practice, but can easily be achieved as long as there is a temperature difference between the primary and secondary sides. If there isn’t a temperature difference, the boiler will simply need to be run at a slightly higher temperature than the system.
The separation of the old system combined with a new high-performance, high-efficiency boiler, is a real win-win situation. Not only is it sensible engineering, it also acts as a life insurance policy for the boiler plant — allowing it to operate efficiently for its whole life cycle. Plus, with a high proportion of boiler sales these days coming from the replacement sector, we think it makes great business sense too.
Peter Gammon is technical manager with MHS Boilers