One circulator pump with 2 zone valves, or two circulator pumps?

dbarck2000

I’m converting my Duplex from a 2 boiler gravity system (1930s vintage), to a cast iron 1 boiler 2 zone system.

The Duplex has two floors, with one apartment on each floor. Each apartment has cast iron radiators with it’s own gravity boiler (1930s vintage).

Should the new system use one circulator pump with 2 zone valves? Or, two circulator pumps?

Will there be a balancing problem with the one circulator 2 zone valve design? For example, if both zone valves are open, won’t the water all go to the first floor apartment?

EdTheHeaterMan

You will be just fine if the installer does the near boiler piping properly. Actually the second floor will probably get the heat first by design, Gravity will take the hotter lighter water higher. But with the proper balancing valves in place, you can overcome any balancing problem. Have you decided on using a modulating Condensing boiler or an economy cast iron natural draft exhaust boiler?

Big Ed_4

I would use two 3 speed circulators ... Start off on low speed

EdTheHeaterMan

The condensing boiler will work well with the large water capacity radiator system. There will be lots of low return temperature water to maximize the efficiency of the boiler. If you go with an economy job, then you need to be sure the installer does the near boiler piping so the return water gets above 135° within 5 minutes or so, in order to prevent condensation of flue gas, that will shorten the boiler life! Low cost boiler with more expensive piping job or higher cost boiler with lower operating cost. Since you will be paying the fuel bill and passing that cost off to the tenant(s) in rent, then I might go with the low cost job. There will be less service issues and the low cost boiler will last longer.

dbarck2000

I'll be using a cast iron natural draft exhaust boiler (Burnham Series 3 84% Natural Gas).

How does the HVAC installer balance a system like mine with 1 circulator and 2 zone valves? Are there pressure valves that can be adjusted? It is a dynamic system; If the system is optimized for both zones being open at the same time, then how can the system be optimized if just one of the zones is open?

How does the installer know when the system is balanced? It is just by good design and experience? Are there instruments the installer can use to measure the system balance?

I'm trying to educate myself a little because I got conflicting information from the different HVAC companies that bid on my job. This made me skeptical about trusting the HVAC company proposals without some basic understanding.


P.S. I'm not an HVAC professional, just a curious home owner

hot_rod

A delta P type ECM would be a good match for a single pump zone valve system. On auto adapt it will learn and adjust output based on one or two zones. A balance valve could be added if you want to fine tune the system. If you know the exact flow rate you could get an pressure independent balance valve.
A Quicksetter manual balance valve would allow you to play around with flow rate, observe it.

dbarck2000

Thanks for all your input!

hot_rod

Ideally you would know what gpm each zone requires. Add those together and that becomes the pump spec. 6 gpm at 7’ of head for example

Dial in the pump speed to get as close to that gpm as possible 

assume it is a delta P type ECM, it provides 6 gpm when both zone valves are open, simple enough

when either zone shuts down, the pump detects the pressure differential difference and slows the pump

If one zone was 5 gpm and the other 1 gpm if the 1 gpm closes the pump tries to find the 5 gpm speed. If the 5 gpm load drops off the pump revs down to a 1 gpm flow rate

not so easy to find a 1 gpm circulator for a zone pump.  So you could buy the closest, smallest circ to a 1 gpm flow, put a balance valve on it and choke it to 1 gpm, If you have a fixed speed  zone circulator. 

That is the purpose of balance valves, to make that final, accurate adjustment to flow

Not often  do residential contractors dial in pump flows. Or do they even take the time to define what flow they need.

when you see 15 zone pumps on a wall ask the installer what flow rate each is providing😄
Common  to see the same exact pump on a 10 gpm or a 1 gpm zone one running off the bottom of its curve, the other run off the top

Commercial Hydronics always have balance valves if an engineered design was followed
A third party contractor comes in to balance, confirm and sign off on a balance report
Same with the air flow side of a hVAC system

They system designer wants the system to run efficiently, comfortable, and accurately 

Idronics 16 goes into more detail on how the various electronic circulator logics work