Primary Secondary Pumping

[MagicJS]

Hi, I'm designing heating for a 12 story residential building with PTAC units @ 400 GPM & 180F. The developer does not want to use primary-secondary pumping. I know the benefits in control and design by using primary-secondary, but is it absolutely required. Will not using it a cause problems in a system of this size.

Thank you.

 

[lilliput1]

It should not be a problem if you can incorporate an automatic HWS temperature reset schedule based on outdoor air temperature and that all heating loads varies in proportion to the outdoor air temperature (no reheat water system, only perimeter heating systems).

 

[MagicJS]

I am using three boilers for perimeter heating only, and the PTACs will have two way motorized valves. Can partial load conditions cause inadequate flow through the boilers or is a pressure differential valve be enough to maintain minimum flow?

 

[cme]

Varying the flow thru a boiler is a no-no, the same can be said to shocking via high delta T's. I would install some kind of 3-way valve or pressure bypass circuit to keep the flow relatively constant (avoid these conditions) since you have 2-way valve control.

Your developer has typical logic for your project. Primary boiler pumps are typically single phase and around 10' head. They, along with VFD's are relatively inexpensive.

I gather that the developer has a fixed fee for heat charged to the tenants, included in the rent ..... Right?

 

[walkes]

You can vary the flow through the boiler depending on the type of boiler you are using. If you are using a condensing boiler like Fulton, Aerco or Viessmann you can get away with the varying flow without thermal shock (they can also tolerate high delta Ts). You can use a DeDietrich Cast Iron boiler with varying flow rates as well. Therefore it is possible to have only a primary loop on your heating system.

I have designed primary systems with two-way valves and flow control devices without adverse affects on the boilers used. Boiler selection would be critical in this case.

The pressure on the boiler may be an issue depending on where the boiler room is situated. If it is a penthouse boiler room there won't be a problem, if it is at a lower level you may have issues depending on the boiler type eg. Viessman doesn't like to exceed 28 psig through the boiler. You would then have to isolate the boiler via a heat exchanger.