What BG50 is
BG50 is BSRIA's guidance on water treatment for closed heating and cooling systems. It picks up where pre-commission cleaning leaves off: once a system has been cleaned and filled, BG50 sets out how to keep the water in good condition for the life of the system.
It's aimed at everyone responsible for a closed system — building operators, FM teams, water-treatment contractors and commissioning engineers — and it's become the reference point for what 'good' looks like in closed-system water quality.
Why closed systems fail
A closed system is sealed, so in theory the water never changes and treatment lasts. In practice, three things go wrong. Corrosion — oxygen ingress through automatic air vents, expansion vessels and make-up water drives rust and the build-up of magnetite, the black sludge that blocks pipework and heat exchangers.
Microbiological growth — bacteria, including sulphate-reducing bacteria, colonise stagnant or poorly-treated systems, causing under-deposit corrosion and fouling. Scale and fouling — from hard make-up water or debris left after poor pre-commission cleaning.
The insidious thing is that all three develop silently. By the time symptoms show — cold radiators, a fouled plate heat exchanger, a chiller losing efficiency — significant damage has usually already happened. That's exactly what BG50's monitoring regime is designed to catch first.
The BG50 sampling regime
BG50 recommends regular sampling and analysis to track the condition of the system water — typically a baseline at handover, then routine sampling (commonly quarterly, risk-adjusted) thereafter.
Samples are analysed in a laboratory against a set of parameters, and the results are trended over time. A single result tells you the state today; the trend tells you whether the system is stable, degrading, or recovering after intervention. Trending is the heart of a good BG50 programme — one-off snapshots miss the story.
Sampling should be done from representative points, properly labelled and promptly analysed, with results interpreted by someone who understands what 'normal' looks like for that system.
The parameters that matter
A BG50 analysis typically covers: appearance and suspended solids (clarity, evidence of corrosion debris); pH and conductivity (the chemistry balance of the water); dissolved iron (a direct marker of ongoing corrosion); inhibitor level (is there enough treatment chemical present to protect the metal?); and microbiological counts including TVCs and sulphate-reducing bacteria.
No single number tells the whole story — it's the picture across all parameters, trended over time, that reveals whether the system is healthy. Rising dissolved iron with a falling inhibitor level, for instance, is a clear early warning that corrosion control is being lost.
The link to pre-commission cleaning (BG29)
BG50 and BG29 are two halves of the same story. BG29 governs the pre-commission cleaning of a closed system — flushing, chemical cleaning and the initial dose of inhibitor that gets a brand-new system into a known-good state before handover.
BG50 then keeps it there. A system that was cleaned properly to BG29 and is then monitored to BG50 should give decades of reliable service. Skip either — a rushed clean, or no ongoing monitoring — and corrosion quietly takes hold. Treating them as a continuum, not two unrelated jobs, is the key insight.
Building an ongoing programme
A practical BG50 programme is straightforward: establish a baseline at handover; sample on a risk-based schedule (quarterly is common); trend every parameter; and act when the trend — not just a single result — moves the wrong way. Side-stream filtration, dosing pots and proper air elimination all help keep the water in spec between samples.
Done well, this is cheap insurance. The cost of routine sampling and a modest dosing regime is trivial against the cost of replacing a corroded heat exchanger, descaling a fouled chiller, or chasing cold spots through a building in mid-winter.