Secondary Hot Water Circulators

As a matter of pure curiosity ....

I somehow got myself onto BES's e-mail list a long time ago, so I frequently get stuff from them. I've noticed recently quite a few adverts for "Secondary Hot Water Circulators". In the pictures, they look like CH circulators/pumps (in fact, most look more like the CH pumps of yesteryear than do modern 'CH circulators'!) but they generally seem appreciably more expensive than CH circulators.

So I'm wondering what exactly these are used for, how they differ from CH circulators and why they are apparently more expensive.

Kind Regards, John

These are used for larger systems where there are long runs of pipework far away from the hot water tank. The purpose is to avoid large quantites of water becoming cool in the pipework which would then need to be drawn off before hot water reaches the outlets. There will be 'secondary return' pipework where the water is pumped from the extermeties of the system back to the tank to maintain circulation. The photo attached might help to explain it a bit better. Apart from the nuisance factor of having to wait for hot water to come through, there is also a risk of legionella where water is left stagnant and allowed to cool in the pipework.

The reason the pumps are more expensive is because the pump is dealing with fresh oxygenated water so the internals are usually made of bronze to withstand this, this is unlike a CH systems where the pump will be circulating the same (hopefully treated) water.

On the last two commercial projects I was on, there was no secondary return installed; instead all of the HW pipework was trace heated with temperature maintenance tape to within 1m of the outlet.

HTH

David937 said:

These are used for larger systems where there are long runs of pipework far away from the hot water tank. The purpose is to avoid large quantites of water becoming cool in the pipework which would then need to be drawn off before hot water reaches the outlets. There will be 'secondary return' pipework where the water is pumped from the extermeties of the system back to the tank to maintain circulation. The photo attached might help to explain it a bit better. Apart from the nuiscance factor of having to wait for hot water to come through, there is also a risk of legionella where water is left stagnant and allowed to cool in the pipework.

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Ah, right. Thanks. If I'm not mistaken such a system is very common in hotels and suchlike buildings.

I actually inherited a Victorian version of such a system when I bought my (large) house about 30 years ago. There was no pump involved, merely convection around loops of ~2" iron pipes, which also zig-zagged through a couple of walk-in airing cupboards - all served by a solid fuel boiler (which was an Aga-branded one by the time I inherited it - maybe 'the original'! I suspect they may have had to employ 'staff' to keep the boiler stoked with fuel!

David937 said:

The reason the pumps are more expensive is because the pump is dealing with fresh oxygenated water so the internals are usually made of bronze to withstand this, this is unlike a CH systems where the pump will be circulating the same (hopefully treated) water.

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Right. That makes sense. Thanks again.

Kind Regards, John

They're more likely to be stainless steel nowadays.

Dan Robinson said:

They're more likely to be stainless steel nowadays.

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That also makes sense. Thanks.

I forgot to ask. If these are called "Secondary" hot water circulators then what, if anything, are primary ones? If that term existed and related to the circuit between boiler and an indirect DHW cylinder (or other heat exchanger) then I don't see why they would need to be different from (cheaper) 'CH circulators' and, indeed, in domestic installations would often/usually not be separate from the CH circulator.

Kind Regards, John

Primary circulators would be the heating pump.

Dan Robinson said:

Primary circulators would be the heating pump.

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Fair enough. As I said/implied, that's rather what I suspected.

... so there is no such thing as an (expensive) "primary hot water circulator"?

Kind Regards, John

Rather depends on what you deem expensive. I fitted some recently that were over a grand a pop.

Dan Robinson said:

Rather depends on what you deem expensive. I fitted some recently that were over a grand a pop.

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I meant 'more expensive' than corresponding (similar pumping spec) "CH circulators" (intended for 'closed loop' use).

Kind Regards, John

Some the stainless steel ones we've fitted before can be used for both purposes so it's difficult to compare the costs.