Push water towards boiler or pull from it ?

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Minister of Fire
Hearth Supporter
Jul 11, 2008
Northern NH
I am starting to do the detail plan to swap out my backup oil boiler on my predominately wood fired system. I have always piped boilers in general to "push" return water through the boiler with a circulator pump moving cooler return water rather than "pulling" hot supply water with circulator pump on the boiler discharge as its "harder" on the pump to deal with high temps (and in rare cases is a cavitation issue). Mty woodboiler is currently a "push through" system (I am using " " marks as these are closed loops so there really is no pushing or pulling but it may make it easier to visualize.

The "new" backup boiler, actually a used system 2000 cold start boiler, is currently piped as a "pull" through with four zone valves. The System 2000 is usually set up as "smart" boiler that maximizes energy efficiency by running a circulator after the thermostats have stopped calling for heat so that the boiler is normally cold when not calling for heat. It has a very low water volume compared to standard boiler and low mass which means a quick startup. It is plumbed as a "pull" through boiler. I marked up their installation sketch to get rid of domestic hot water and added three push through circulators to replace the zone valves. There are actually none return valves on each zone where the zone valves would be located.

So anyone have comments on if going to push through with this boiler (AKA energy converter in the sketch)will be an issue?
The wood boiler uses a Taco heating controller to run the zones and swapping to zone valves would be a major PITA.

Push water towards boiler or pull from it ?

Ours are pushing away, was like that when we bought the house and didn't get changed when we put in a new oil furnace. Have no idea if it's the best way or not but seems to work good. And works pretty good on gravity and when everything is perking return water is pretty hot so wonder how much difference it makes on circulators, have never had to replace one.
I would say wherever/however puts the most positive head on the pump intake, and if that's at the boiler, so be it.
The boiler doesn't care where the pump is, only the pump cares. Older circulators had impellers made of a plastic that could degrade under high temperatures. Nowadays pretty much every circulator you can get is rated for continuous duty at 180f so putting it on the hot side is not a concern. You generally want to place your pump immediately downstream of the expansion tank/feed regulator. If your circulator is working where it is, then changing the boiler shouldn't really affect much unless the new boiler has a much lower head loss through it.
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Guess ours could be both, it pulls it through the boiler and pushes it away. The other way would be pulling it through the system and pushing through the boiler
I think it comes down to that it really doesnt make a difference with modern materials. I think System 2000 offered premium kits which had a lot of the accessories supplied and preassembled and they used pull through so many are plumbed that way.

Thanks all for input
Put it where it would be easiest to service later on.
In that diagram you want it on the supply side so the expansion tank is behind the circulator. Best practice is to make sure your circulators pump away from the expansion tank.
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To elaborate on that. The expansion tank can be considered your point of 0 PSI in a closed system. For water to flow around the loops you need a positive pressure differential. The circulator provides that. I'm just making the numbers up but let's say the circulator adds +5 PSI and as the water flows around your restrictive distribution that positive pressure differential slowly dissipates until it's 0 at the intake of the circulator.

Now let's pretend your static system pressure is 10 psi. That means your circulator boost it to 15 psi. However if your expansion tank is in front of the circulator it absorbs the 5 psi the circulator is adding, however water still flows. To the system it looks more like the circulator is adding suction. So the intake of the circulator is -5psi. Now with your static system pressure of 10 psi The discharge of the circulator stays the same and the intake drops to 5 PSI. When you start pulling lower pressures than your static pressure that is one of the ways to run a greater risk of cavitation.