Condensing plant, designed around the return water temperature

A condensing boiler recovers latent heat from the flue gas by condensing water vapour out of it. That only happens when the heat exchanger surface is below the dew point of the combustion products, which in practice means return water below about 130 °F, and efficiency keeps climbing as it falls further.

So the design question is not which boiler, it is what the return water temperature will be across the season. A system built around 180 °F supply and a 20 °F drop returns water at 160 °F and never condenses. The same building on a lower-temperature emitter with a properly configured outdoor reset can spend most of the season below 120 °F.

That usually means larger emitters, or radiant, or both. It is a distribution decision dressed up as a plant decision.

Boilers are sized for a design day that occurs a handful of hours a year. For the rest of the season the plant is at part load, and what determines efficiency there is turndown.

A 10 to 1 boiler can follow the load down to 10 percent before it starts cycling. Two smaller boilers in a lead-lag arrangement extend that further and give you redundancy. Four boilers where two would do is an equipment-supplier answer rather than an engineering one.

We size for the calculated load, check the modulation range against the load profile, and stage the plant so it does not short cycle in October.

In-slab radiant is usually ruled out by the cost of the concrete rather than the cost of the tubing. Where a slab is being poured or replaced anyway, as it was on the Aurora warehouse, the economics change completely.

A slab at 105 °F supply with a 20 °F drop returns water at 85 °F, which keeps a condensing boiler in condensing mode essentially all season. It also solves stratification in tall spaces: the Aurora building went from 14 °F of vertical stratification to under 3 °F at the working floor.

Loop layout, tube spacing and manifold balancing are all design decisions with consequences. We draw the loops, state the spacing, and balance the manifold with flow meters rather than by ear.

Gas equipment installed above 2,000 ft has to be derated and, on most non-condensing and many condensing appliances, fitted with the manufacturer high-altitude kit, which usually means different orifices.

A common rule is roughly 4 percent less input per 1,000 ft above 2,000 ft, but the number that matters is the one in the specific manufacturer listing, so that is the one we work to.

At startup we run a combustion analysis and give you the printout: O2 or CO2, CO, stack temperature and calculated efficiency. It goes in the handover file, and we repeat it at the first annual service so there is a trend rather than a single reading.