Recirculating hot and warm water plumbing system design

With all that has been said about Legionella growth in recirculating hot and warm water piped systems, I find that some hydraulic consultants and engineers are apprehensive designing these systems. There are many considerations to take into account - more than in most systems we design. For example, designers often oversize or undersize pipework, they may also select by specification (or lack of specification) or pipe fittings that are ill-suited for the system. I’ll outline below what we should be considering when designing these systems.

What is a recirculating hot or warm water system?

These systems basically consist of hot water heaters (plant), a piped ring main from the hot water heaters to the building(s) serviced and back again, and a circulating pump(s) installed at the end of the ring main. The piped ring main has branches (dead-legs) that service each water outlet. Water is tempered near the outlets or at the plant (in the case of warm water ring mains). The pump pushes water through the ring main and back into the hot water plant to reheat the water. These systems are generally designed when you have one hot water plant location and hot/warm water outlets that are not necessarily close by.

Availability of hot/warm water at outlets

The hot/warm water system should be designed so that the user does not wait longer than 30 seconds for hot/warm water once the tap is opened. For these types of systems, that means limiting dead-legs to say 12m for up to 20mm ID pipework, 7.5m for 25mm pipework and 3m for pipework over 25mm in length.

Pipe size

Pipe sizes have a direct relationship with the velocity of water in the pipe as well as pressure lost to friction. The larger the pipe size, the lower the velocity and the lower pressure loss through the pipe. These are generally desired traits for a piped system, lower velocity means less chance of issues such as water hammer or pipe scouring and lower pressure loss means more pressure will be available to the user of water. However, larger pipe sizes also mean greater surface area of the pipe, which increases the heat loss of the pipe. The idea of these systems is to limit heat loss to approx. no more than 5 degrees Celsius. The larger the pipe, the higher your pump duty will be to compensate for the greater heat losses.

Pipe Insulation

The ring main should be insulated to avoid excessive heat loss due to ambient temperatures. Insulation manufacturers often give thermal conductivity figures to help select the appropriate thickness. Consideration should be given seasonal temperatures (winter). Consideration should also be given to location of pipework (if installed in frozen ground or in direct sunlight for example). UV light can damage insulation and reduce its effectiveness. Wrapping the insulation or using foil facing insulation should be considered where insulation is exposed to direct sunlight.

Pipe fittings

Pipe bends in a ring main should have a large radius. Tight radius bends tend to wear-out more quickly in a pumped system, especially when carrying hot water.

Recirculating Pump sizing

Hot water ring main pump(s) should be sized to account for heat emission. Simply put, the duty flow of the pump(s) will be directly proportional to the heat emission of the ring main per hour. The pump(s) should be installed in weatherproof, acoustic reduction housings where installed outside.

These are just some of the considerations and is not a complete list. Get in contact with us to learn more.