Water velocity

rugbyrob

What is the negative, in a hot water system, to having a water velocity of less than 2 fps?

Jamie Hall

Oher than...

the expense, both labour and materials, of the big pipes... I can't think of one.

MikeyB

2FPS

I believe you should try to keep the velocity between 2 FPS & 4FPS,  at or above 2 FPS will help keep air entrained in the fluid and help it along to the air separator and get it out of the system , and a Max of 4FPS for velocity noise control & help prevent Erosion/Corrosion. Below 2 FPS might give you a hard time getting rid of the air from the system

DanHolohan

The way I learned it,

air will separate from the water when the velocity drops below six inches per second. No problem above that, but there's not much to be gained by going 2 fps if it means larger pipes. 

HDE

Laminar flow

Low velocity may result in laminar flow, often called streamline flow. Simple terms, the water travels in streams within the pipe, possibly resulting in the hottest water traveling in a stream through the middle of the pipe, while cooler slower moving streams travel on the outside which is in contact with the pipe. Resulting in lousy heat transfer. Of course changes in direction such as 90's will cause turbulence and mix it back up. However real slow circulation and long straight runs with low velocity results in laminar flow.



Laminar flow can also entrain air in the inner streams affecting air elimination on typical air scoops and risers, not so much on a product like a spirovent.

Brad White

But laminar flow

is really only of concern in a heating element. Unless your pipes are the emitters (and that you have good air removal, meaning a good air separator and proper pitch to vents), I would not be concerned about low water velocity. This assumes cost is not an option or that you are dealing with an older gravity HW system.



In an gravity HW system, mine as an example, has 3", 2.5", 2" and 1.25" piping evident in the basement, with 1" the smallest and some radiators. My total flow into the 3" line is 4 gpm.  That is less than 0.18 fps.  Thankfully it is constant circulation or I would still be waiting for my autumn heat, it being spring now.

hot_rod

low flow

I've read that some of the district systems with underground piping often designed around laminar flow to reduce the heat energy loss between buildings.



I think it is fairly unusual to see laminar flow in Modern Hydronics.



hr

HDE

Yes

True but neither I or the OP mention baseboard, emitters or coils for that matter. But then again would we reduce large pipe for emitters and then increase, of course not. If the OP had asked about large distribution piping feeding risers to emitters, we would both agree that is a more defined question that would get different answers.

HDE

But

Laminar flow besides residential that I have to agree is minimal, afterall most circs applied are oversized. However commercially on AHU units with coils, perimeter reheat fan coils and fan convectors it is a common occurance when multiple taps with disregard to flow rates and balancing is installed on central circulation heating and chilled water applications.

Brad White

I think we agree

laminar flow matters on emitters, not on piping.



As for counting on laminar flow in piping to reduce unwanted energy transfer, it is there but marginal and is not a substitute for insulation.



I would point out that in chilled water applications, because delta-T's are so narrow and especially where glycol is used, laminar flow conditions represent a big hit. Especially on fan coil units. This is one area where I "like more pump".

Nick W

John Siegenthaler says

in Modern Hydronic Heating, "All down-flowing piping should be sized to maintain a flow velocity of at least 2 ft/sec to effectively entrain air bubbles." That's 3.25 gpm in 3/4" type M copper.

Brad White

But that presumes

you have no high point venting and are aiming to remove your air bubbles at your air separator, likely near the boiler. The 2 fps rule is really a balance point between bubbles in water, moving the water down faster than the bubbles move up. I do not consider this an effective air transporter but it will take along dissolved air pretty well.



I can reflect on scuba diving, watching my air bubbles rise at a fair clip but not thinking too much else beyond my conditions at the moment