Circulator Flow Rate Question - Viessmann Vitodens 100-W

Old 09-10-17, 12:39 PM
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Circulator Flow Rate Question - Viessmann Vitodens 100-W

In the process of hooking up my Vitodens 100-W to my 35 gallon indirect tank, in the installation manual for the boiler it states that:

DHW supply and return piping between boiler DHW connections and the Viessmann DHW tank connections, shall be a minimum of 1” nominal pipe diameter (irrespective of the ” DHW connection outlet sizes provided on the boiler and the DHW tank). This will ensure pump head is fully utilized to overcome the resistance of the DHW heat exchanger coil and to provide sufficient water flow to the boiler heat exchanger. In non-Viessmann DHW tank applications, perform, in addition to the above, accurate calculations for DHW tank coil pressure drop versus boiler pump head to ensure sufficient water flow to the boiler heat exchanger. Failure to heed the above instructions may cause boiler short-cycling and inadequate DHW supply.

I am using a non-viessmann DHW tank, the only information I could find for pressure drop for my coil is the following:

Coil Curve - Mine is the red line, 35 gallon top.
Brochure - Flow Rate: 6GPM ||||| Head Loss: 7ft

So my question is, what calculation should I preform to ensure sufficient water flow? And is it necessary to use 1" pipe instead of 3/4"

From Vaughn website:

There is no major restriction of water flow through the Vaughn water heater. Cold water inlet and hot water outlet sizes are available in ” and 1-1/2” sizes to accommodate your application. By far, the ” size is more than adequate for most applications using a Model S or ME water heater. If your flow rate through the water heater is greater than approximately 10 GPM you may want to consider the 1-1/2” connection option (must be specified at time of ordering). As an approximate, pressure drop through any E model water heater with ” connections will be <4 psi with a flow rate of <10 GPM, and with 1-1/2” connections the pressure drop will be <3 psi with a flow rate of <20 GPM.

This mentions Model S and ME water heaters, not TPP indirects, but I figured I would add it... Any other information guidance would be helpful, thank you.

EDIT: The internal circulator in the Viessmann is a Grundfos UPS15-78

Last edited by obg88; 09-10-17 at 12:45 PM. Reason: forgot

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