Maximum Flow Velocity

[Tennman]

I asked a question about flow velocity on Woodsmaster's post about PEX insulation. It was taking that topic pretty far afield so I thot it better to start a new post. It's no secret I've designed my system basically from inputs at this web site. I'm pleased with all my above ground decisions. Because I now have the data that indicates my PEX is immersed in water at some point in the transit from my barn to the cellar I'm going to dig up my lines and polyurethane my 1 1/4" PEX probably in the next few weeks. Winter's about over for us here. Because in all my previous research last Fall I never heard of anyone needing 1 1/2" PEX and all the debate was focused on 1 vs 1 1/4. I thot my decision to go 1 1/4 and not consider 1 1/2" was adequate. Now that I'm going to dig up my lines for the LAST TIME of my life we're gonna do it right. The following is my assumptions and a question about when does the flow velocity get excessive. If I need to add a 1" line to my 1 1/4" to reduce the flow velocity, now's the time. Thanks in advance as usual. I don't remember maximum flow velocity (feet/sec) ever discussed here before, its always focused on flow rate (gpm). The following was asked of Heaterman in the insulation post but any input is appreciated:

Heaterman, Your reply made me feel somewhat better about my 1 1/4” piping system. Per the Taco TD10 data sheet I knew I’d be exceeding the TD10 maximum 1 1/4” recommended flow rate of 11.2 gpm. With my computed head loss of a little more than 22 feet and assuming my 210kbtu boiler could consistently put out about 170kbtu I computed a required flow rate of 17 gpm (20F deltaT). Those two numbers flow rate and head put me right on the Taco 0013 curve. I had not read of anyone here using more than 1 1/4” PEX so I presumed others were routinely exceeding the recommended flow rate on 1 1/4” PEX. The existing 115kbtu propane furnace was adequate to keep our downstairs adequately warm during our short heating season, so I didn’t attempt to do a heat loss. Since I’m about to dig up my PEX to foam insulate, from your experience am I way over pushing the flow rate in that 1 1/4”? Until recently I don’t recall ever hearing discussions about 1 1/2” PEX.

The TD10 document implies the gpm limits are to keep flow noise at acceptable limits. But they also say keep flow velocity between 2-4 fps. At 17 gpm and 1 1/4” I’m at 6.25 fps! Now I’m wondering if I need to add a 1” line to my existing 1 1/4”. Geez what a learning experience related to the underground part of this system.

 

[ewdudley]

...

With my computed head loss of a little more than 22 feet and assuming my 210kbtu boiler could consistently put out about 170kbtu I computed a required flow rate of 17 gpm (20F deltaT). Those two numbers flow rate and head put me right on the Taco 0013 curve. I had not read of anyone here using more than 1 1/4” PEX so I presumed others were routinely exceeding the recommended flow rate on 1 1/4” PEX. The existing 115kbtu propane furnace was adequate to keep our downstairs adequately warm during our short heating season, so I didn’t attempt to do a heat loss. Since I’m about to dig up my PEX to foam insulate, from your experience am I way over pushing the flow rate in that 1 1/4”? Until recently I don’t recall ever hearing discussions about 1 1/2” PEX.

The TD10 document implies the gpm limits are to keep flow noise at acceptable limits. But they also say keep flow velocity between 2-4 fps. At 17 gpm and 1 1/4” I’m at 6.25 fps! Now I’m wondering if I need to add a 1” line to my existing 1 1/4”. Geez what a learning experience related to the underground part of this system.

The TD10 document lists maximum flow rate of 4 fps to avoid noise problems, particularly for heating pipes running under floors and through wall spaces to and from heat loads. Presumably noise is no big deal running underground from the boiler to the house.

For underground pumping the water fps is limited by the economics of pumping power, and it is limited by wear and tear on the pipes and fittings. I've seen the wear and tear limit specified to be about 8 fps, 10 fps in a pinch.

You can get the same gpm using more power with a smaller pipe, or using less power with a larger pipe, so you need to decide if the extra expense of the electricity over time can offset less capital expense for smaller pipe, or vice versa for a smaller pump and bigger pipe.

But if we can increase the deltaT then things can get a lot easier.

I gather you are pumping to a water-to-air heat exchanger, so there's not much you can do there to increase deltaT because you need nice hot air for a hot-air system.

But if you could install a couple radiant panels or some under-floor radiant strategically then it would be possible to tap in to the return side of the air-to-water heat exchanger and extract more heat and increase your deltaT. If deltaT could be increased to 40F then your flow requirements are of course cut in half from what they would be for 20F deltaT.

--ewd

 

[Como]

I designed a system using the Taco LoadMatch software, that suggested 1 1/2 inch but that was with a 500,000 btu load. The cost factor goes up when you get to these sizes which is probably also a reason to keep the pipe sizes lower.

 

[Tennman]

EWD, Apparently I incorrectly understood that a 20F deltaT was desirable. Yes, one of my upgrades during the next 4-6 months is to add 2-3 radiant heaters strategically under floor where this season we had cold spots. You're correct, when I changed to just a 30F deltaT my gpm dropped to 11.3 gpm. Which... had I understood this better back in the Fall, I could have used a smaller pump if I had planned to leave more btus in the house. Presuming that the boiler has sufficient capacity to keep up with demand, a larger deltaT is preferred. I misunderstood that.

As far as piping making noise that's irrelevent and that's the first I've read of a wear and tear velocity limit. I'm well under that with my 0013. Since my goal is to save thousands on my heating bill, which we did this year, the loss of efficiency for the oversized pump is not significant for me. I'll be feeding the added radiators downstrean of my water-to-air HX.

Good news about my existing 1 1/4" PEX which is in keeping with all the similar systems I read about here. Thanks

David if your 110 YO hotel is anything like my 160 YO drafty home, you must need a bunch of wood.

 

[ewdudley]

Apparently I incorrectly understood that a 20F deltaT was desirable.

From what I gather the 20F deltaT is a rule-of-thumb for typical baseboard systems where the desire is to have comparable heat all along any given loop, plus with lower deltaTs you can get away with less capital expense for baseboard heaters. So from that point of view the standard 20F deltaT is desirable.

But now we have better low-temperature options.

On the chicken side of low-temperature heating there has been the advent of condensing gas boilers, and solid fuel heat storage systems, where there's need for heat emitters that can use lower temperature water effectively.

On the egg side the growing popularity of in-floor hydronic systems and large surface area wall and ceiling panels have made lower temperature water viable.

Both of which have given us nicer alternatives for keeping our houses more comfy.

It's the same old challenge of balancing efficiency against operating and capital expense, so more deltaT is desirable up until the cost of achieving it becomes prohibitive.

And here's one of the references to wear and tear that I was referring to:

http://www.engineeringtoolbox.com/flow-velocity-water-pipes-d_385.html

 

[Tennman]

I get why with radiators the 20 dT would keep the house a more uniform temp. With my hot air system its about dumping the heat. Thanks for the velocity data.

 

[heaterman]

I get why with radiators the 20 dT would keep the house a more uniform temp. With my hot air system its about dumping the heat. Thanks for the velocity data.

In Europe it is common practice to design a radiator system based on a 40* temp drop. Just food for thought. It's really enlightening to talk with a heating mechanic or designer from over there. They are all about large emitters, large (but not oversized) tube or pipe and little dinky circs. Less energy consumed to move the water.