23 November 2017
When you’re designing a multi-zone heating system you need to make a decision about how to supply heating water to the different zones.
There’s two basic choices. Either we have a single primary circulating pump, and a motorised isolating valve the opens or closes the flow to each zone and is controlled by a thermostat.
It seems intuitive to have a single pump because it uses less electricity. In practice, there’s plenty of good reasons to have multiple parallel pumps rather than the valves. Here’s why.
One of the main issues is that most pumps come in standard sizes, even across manufacturers. Valves generally don’t. So if you have to replace a pump, it’s usually a simple job to remove the pump motor from the base, and swap in any new pump.
With a valve, you’ll usually find that the new valve doesn’t fit in the place of the old one. Since manufacturers regularly change their product lines, this means you’ve got a bit of extra plumbing to do to get your new valve installed, which will cost you money.
The next problem you’ll have is balancing the pump load. Let’s say you have three zones on your system – two zones of heating and a HWC. Your boiler has one circulating pump for the system, and a ball valve on each zone.
When all zones are open and the pump is running, there’s little resistance on the pump. But if two zones close, the pump is now running at the same speed with much greater resistance. This is going to cause extra wear on your pump, and it will wear out faster. Almost no domestic heating systems are installed with variable speed pumps because of the extra cost.
If you eliminate the valves and just have a pump for each zone, then the thermostats can switch on their respective pumps whenever they like and each pump can be sized for the appropriate resistance of the zone. That will reduce pump maintenance.
Motorised valves have proliferated greatly in modern home heating systems. But in most places these could be replaced with dedicated zone pumps which would actually lower maintenance without impacting system performance.