# Can you plumb two units in series?

Discussion in 'General Discussions' started by turkeyboy, Sep 23, 2009.

1. ### turkeyboyMember

Could two water to air units be plumbed in series so that when either of the units is on, water flows through both units? If so, what would be the advantage/disadvantage over plumbing in parallel?

Piping in series would have water leaving one unit and entering the other. That would be too much temperature rise/drop before returning to loop, always pipe parallel for multi unit installation.

3. ### turkeyboyMember

So, if I had a standing column, and I run say 4 gpm through one unit, changing the temp of the water by say 10 degrees, then run in through another unit changing it another 10 degrees, would this be different than running 8 gpm and changing the temp by only 10 degrees? I'm not an engineer or anything, but seems like it might be a wash? The advantage, though, might be the need to pump less water from the well.

4. ### engineerWell-Known MemberIndustry ProfessionalForum Leader

The second unit would run less efficiently owing to less favorable water conditions.

5. ### turkeyboyMember

Thanks. So, it sounds like the benefit would be pumping less water (so presumably less electricity). The cost would be a less efficient second unit (so more electricity). I wonder if there is a way to estimate if the benefits outweigh the costs before I have my system set up?

What you are not considering is pumping cost is figured by GPM and ft/hd (resistance to flow). Two units in series is 2 times the ft/hd. You will not save pumping cost by piping in series.

7. ### engineerWell-Known MemberIndustry ProfessionalForum Leader

Parallel is the standard way to connect multiple units to a single loop field

I believe two in series is actually more like 8 times the pressure drop --
assuming that you also double the GPM to maintain the same flow rate
through the loop and similar water temps entering/leaving the loop.

IIRC, pumping loss (dynamic head) goes as GPM squared -- so, doubling
the GPM will quadruple the pressure-drop across each heat exchanger.

9. ### turkeyboyMember

Interesting. Thanks! That's the info I was looking for.

10. ### Mark CustisNot soon.Industry ProfessionalForum Leader

I would consider

Primary/secondary configuration. The head loss issue goes away.

11. ### turkeyboyMember

Re: I would consider

Not sure what this means. Can you explain?

12. ### Mark CustisNot soon.Industry ProfessionalForum Leader

The best explanation

I have seen is by Dan Holohan, who runs heatinghelp.com., you can find his book on the site.

The basic idea is to create a Primary loop with its own pump, and then use secondary loops to add or subtract heat.

A project we are building will use this design. The primary loop will be feed hot or cold water by a pair of water to water heat pumps each with their own small pump. The building will then be feed hot or cold water to each of four systems depending on demand. The advantage of being able to bring on one or both of the units will save on operating costs.