I've been learning about primary-secondary boiler systems for my home, and have landed on a two-boiler system (two apartments), very much like Siegenthaler describes in his illuminating book and here in Radiant and Hydronics.

Large diameter rail for supply and return to/from a hydraulic separator

In order to save space I want to introduce a couple of bends into the primary loop, what's the effect? The rails are intact but they don't feed directly into the separator as before. Go/no-go?

Fold-over space-saving primary loop design

In actuality these two boilers are installed along a basement wall with two manifolds extending several feet to the right of the hydraulic separator. Why not use some of the Z-space?


2 Answers 2


That is actually a very good system you are proposing. My only reservation is that the cost will be excessive for only 2 apartments or zones. This is a new twist for me on a primary/secondary piping system. I have installed them before but this device makes it so much easier.The amount of elbows and other fittings should not be a problem as long as the total piping friction loss is included in the selection for sizing the primary pump. The use of 2 smaller boilers instead of 1 large boiler will lend itself to better efficiency by allowing for a lead/lag boiler operation and will be much more dependable than 1 boiler for each apartment.


Keep it simple: why 2 boilers? Get (if needed) a bigger one (if you arn't looking for redundancy). With your design there's no 'independence' between apts and gas usage is common.

If you need a boiler per apartament i think you'll need a second separator(but you'll no longer need the multi-boiler controller).

Why do you need a separator? It's an overkill!
Is the system that big that the only 'primary' pumps cannot do all the work?

I'll have used only boiler pumps and 'actuated zone valves with automatic by-pass' (if multi-zone) or directly controlled by the t.stat.
You have a lot of stuff installed really for nothing if you really have to heat only 2 apt: yours looks like a condo setup!
PS I don't know the law there, but here over 35kW an engeneer is required to work on a heating system so consider hiring a pro.

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