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a heating boiler cad cell relay switchHeating Zone Valve Flow Rates
Effect of zone valve pipe diameter on heat delivery

This article describes the flow rates and gpm delivery capacity of hot water by heating system zone valves.

Page top photo: a bank of six heating zone valves each controlling a separate heating zone in a New York home. Usually these valves and heating zones are piped in 3/4" copper.

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Heating Zone Valve Size Impact on Heat Delivery

Honeywell 1/2" zone valve with sweat fitting and CV of 3.5 at InspectApedia.comQuestion: the effect on heat delivery if we install a smaller-diameter zone valve on a hot water or hydronic heating system.

ac godbout

We live in a condo, our zone valve was changed recently and reduced from 3/4 to 1/2, what effect will that have on heating in colder temperature like -35degrees Celcius - 2021/11/10

This Q&A were posted originally at HEATING SYSTEMS - home

Photo at left: Honeywell (TM) 1/2" diameter sweat-type zone valve.

[Click to enlarge any image]

Moderator reply: 0.2% reduction in typical zone valve gpm flow rate

Thanks that's a helpful question: the effect on heat delivery if we install a smaller-diameter zone valve on a hot water or hydronic heating system.

Bottom line: there will be no noticeable difference between the heating water flow rate through a 3/4" zone valve and a 1/2" zone valve, if we simply take as an example the Flow Capacity (Cv) ratings given for each of those products (illustrated above).

Both the Honeywell 1/2" sweat-fitting zone valve (#V8043E1004) and the Honeywell 3/4" sweat-fitting zone valve (#V8043F1036) have identical Flow Capacity (Cv) ratings of 3.5 according to the manufacturer's literature.

FYI Flow Capacity or Cv is defined as

the volume of water at 60°F (in US gallons) that will flow through a valve per minute with a pressure drop of 1 psi across the valve.

Below in a fancier calculation I set out to demonstrate why the two valves can deliver essentially the same heat rate in gpm or BTUs.

The Flow Characteristic Cv Charts shown below give data for three different zone valve settings or capacities, at Cv's of 1, 3.5 and 8, excerpted from the Honeywell zone valve installation instructions cited at the end of this article. 3.5 Cv is the standard rating for these zone valves.

[Click to enlarge the Cv chart shown below.]

Honeywell zone valve flow rates or characteristics Cv rating for the V8043 series - at InspectApedia.com

Details: how does pipe diameter affect the flow of heating water or heat delivery?

You will want to take a look at

WATER SUPPLY PIPE DIAMETER vs FLOW

where we argue that changing pipe diameter in any water flow system does have some effect, potentially significant, on flow rate.

On that page we give some data

But if ONLY the zone valve diameter was reduced from 3/4" to 1/2" - that's just about 3-4 linear inches of piping, in our OPINION you won't be able to detect the reduction in heating water flow nor BTU delivery to the occupied space .

That's because the impact on flow rate is small until the linear feet of change in pipe diameter is quite a bit more than 3-4 linear inches.

Honeywell heating zone valve, 3/4" sweat fitting, CV 3.5 at Inspectapedia.com

Calculate the Flow Restriction When Changing from a 3/4" to 1/2" Zone Valve

Here's an example using PEX tubing: (I know yours is probably copper but this is just data that we already had and that allows a clear example)

So while the pressure drop is significant when the diameter of piping gets smaller, notice that it's per 100 ft. of run.

Going from 3/4" to 1/2" gives us a drop of 1.36 gpm over 100 ft.

Now if we calculate the pressure drop over 4 inches: I make that to be

(4/1200) x 1.36 = 0.0045 gpm reduction in flow rate - or really really tiny.

A typical hydronic heating circulator pump like a Taco 007-F5, at 1 ft. of head, pumps at about 22 gpm.

So your reduction in gpm would be

0.0045 / 22 = 0.002 or about 0.2% reduction in the gpm delivery rate of hot water through your heating baseboards or radiators - not much, right?

Ask your heating service company if they have a different way to calculate the effect we're describing.

Watch out: Reducing large lengths of heating pipe in diameter can make a significant reduction in the rate of heat delivery in the building.

sketch of water pipe diameter effects

Illustration courtesy of Carson Dunlop Associates, a Toronto home inspection, education, and report writing tool firm.

Heating Zone Valve Capacity References & Research


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