The Hidden Hydronic Problem Nobody Checks: Water Velocity

I would really appreciate it, Ray, if you would develop an ultrasonic flow meter that wholesales for about $500.

Ever try to buy one of those? They're insanely expensive but I'll bet they answer a lot of questions.

HydronicsHandbook from 60+ years ago said 6 fps max as I recall. Decades later plumbing engineers told me that 3 fps is safer. There's a difference between HHW and chilled glycol mix. And between different pipe materials. Ideally one determines system curves (plural intentional) with a variable speed pump and then a fixed speed pump is chosen. Set it and forget it. If you don't want burner to cycle too often then you need some method to maintain minimum flow.

Too much velocity can also be a problem:

https://www.heatinghelp.com/systems-help-center/adjusting-the-flow-rate-for-an-old-gravity-hot-water-system/

I wrote that article.

And it happened just as I wrote.

Mathematicaly,theoretically and pratically it is not possible to increase heat transfer by slowing flow.

All heat emitter output charts indicate this.

I have tested 4 different cast radiators at various flow rates. Confirmed flow with temperature measurements, flow meters and a calibrated BTU meter.

In every case increasing flow provided faster warm up of the radiator and a higher overall temperature of the radiator.

I've used Steamheads piping suggestions, used 2" pipe to the radiator,tried every variable.

More flow= more temperature at the emitter. More surface vs ambient= more heat transfer.

At some point the increase in output slows and the pump size needs to increase. So the juice may not be worth the squeeze.

All the radiators had 1-1/4" connections, I used a pump and 1" pipe capable of 8 gpm as my max. flow.

As for the boiler which is a heat exchanger just as the radiators are. If it runs up to 180° and ∆ of 2, then how does a 178- 180 operating condition cause a boiler to over-heat?

Some exmples

If it is a radiant panel, ceilig, wall or floor slab, the sq footage of the heated part is the sq footage

For cast radiators find the data sheet or here is a generic example.

Cast iron baseboard

Well, bottom line is, it worked. And it's worked other times too. Can't argue with success.

so if you are in front of a class of hydronic newbies and they ask how flow effects a hydronic system heat output. What is your answer and explanation?

As an example , a 100,000 system designed for a 20 delta operation.

Hydronic anomalies might be a topic for upcoming an upcoming detective series 😉

@Steamhead i find this confusing. The article you wrote emphasizes that gravity conversion systems need more flow than modern systems (for the same sized boiler) right? That’s what I’m inferring from the table in your article, anyway. Is your point that the system in @RayWohlfarth’s original post has gone too far in that direction?

by “very low flow rates” do you mean low rates relative to the piping size, which tend to be quite large in gravity conversion systems?

At days end if you need to move X amount of BTU out of boiler at a decided ∆, you need X amount of flow.

So 100,000 BTU needs 10 gpm at 20 ∆

Regardless of the pipe size or type of emitters you need to circulate adequate flow to move the btus., or the space will not warm.

Maybe a system requiring 10 gpm that had a 70 gpm circ may have some unique operating conditions. Probably would not need a boiler with the motor heat and friction developed by a circ that size :)

Too low of a flow, somewhere below 2 fps the fluid goes laminar, doesn't transfer heat or move air along with the water.

The Reynolds Number determines if flow is laminar or turbulent. Reynolds below 2300 will be laminar.

Engineers design long, 100- 100 ft runs between buildings for example at laminar conditions to reduce heat loss through the pipe wall.

I think sometimes heat transport and heat dissipation get clouded. You can move heat through the connecting piping at low flow, but the heat emitter itself needs flow to get the entire radiator warmed. If flow is too low I guess the cast mass heat from conduction?

B&G originally

called the cics a booster, intended to fix or make gravity systems with long horizontal runs work. So the gravity induced systems had limitations, and the math was critical

This 1930 pre ASHRAE book, into gravity system design, where's my slide rule, again :) It would feel sacrilegious to do the math with a calculator or AI.

@hot_rod I have that Guide.

@DCContrarian it's actually a lot simpler than that. The shortest distance between two points is………… a straight line. The circulator sets up a certain ΔP between the flow and return connections. If the ΔP is too high, the system is over-pumped.

Now picture a typical radiator. If the radiator is connected with flow and return at the bottom on opposite ends, and the system is over-pumped, the water will rocket between the flow and return connections and not much will diffuse in the radiator.

If the pipes are connected with flow at the top and return at the bottom of the same end, and the system is over-pumped, the water will flow downward from the top to the bottom of the first couple sections and not diffuse through the rest of the rad.

In both cases, if the shutoff valve works, you can close it part way and the water will diffuse as it should. Many years ago, one of my customers told me that when she closed the radiator valve part-way in her bedroom, the rad got hotter! This didn't make sense until I realized that system was over-pumped. Installing a smaller circulator solved the problem.

@hot_rod : Thanks for the pictures, this corresponds with what I would expect and what I have experienced.

As the flow gets higher, the temperature drop gets smaller and smaller, and eventually the whole radiator gets very close to the temperature of the incoming water. At that point increasing the flow results in very little additional heat output because there's just not much room to get the radiator any hotter.

What I can't envision is a scenario where the exterior of the radiator starts to get colder as the flow increases even further.

Just know that I am not questioning what you experienced. I would just like to know more about the thermodynamics at play

You mentioned several potential causes which or were all at play? An unsolved hydronic mystery

Excellent synopsis! Mad Dog

@hot_rod , I'd think if water was flashing to steam, the system would bang. None of these systems were banging.

Has anyone ever determined the ΔP of a gravity-designed system that is still circulating by gravity?

isn’t it the density difference that causes the movement?

Density .= mass divided by volume that is how solar Thermo siphon systems work,

Here is one I am building currently.It heats 18 gallons from 55- 130 in about 5 hours, 1/2” washer hose for the trial run. 7 square feet of collector

@hot_rod : "I'm convinced there is a point of diminishing return. The 8 gpm I used is possible with a typical 87 W circ. Would it be worth using a 200W pump to get a small output increase?"

The amount of heat put out by a radiator is determined by its surface temperature. The average surface temperature is going to be between the input and outlet temperatures of the water, and can never be higher than the inlet temperature. At 8 GPM the delta is only 1.6F, even if you got the delta to virtually zero you'd probably raise the surface temperature of the radiator by less than a degree, and the output of the radiator would hardly change at all.

@hot_rod : "BUT, the faster flows made it happen faster, bottom line in my testing anyway."

I've been thinking about what the theoretical justification would be for having flows be 50% higher in converted gravity systems than in modern systems, and I think you've hit upon it. It's about responsiveness. You want the heat to come on when the thermostat calls and to go off when the thermostat is satisfied.

One of the reasons gravity systems were converted is they were famously unresponsive.

What I can't pin down is gpm or flow velocity in the gravity systems. Just because it is gravity driven does't necessarily mean the flow and velocity is super slow.

The stories Dan tell is with tall gravity buildings, the top radiators and floors would get more of the flow and need to be balanced down. So that density difference as well as the weight of the water in taller buildings comes into play.

With a basis two story home, do the calculations change to make it work? Also the temperature difference matters, the density of water at a supply of 180 vs the return at ?? temperature and density. If that ∆ is too wide the final rads don't have as much output. Were the radiators on long horizontals sized larger?

Were the 6th story rads smaller than the first floor. Did pipe sizing matter for the balance?

I suspect not unlike a monoflo piping if changes are made to an original design, removing radiators, reconfiguring piping, it throws off the dynamics?

Were "booster" pump was maybe applied to fix butchered gravity design.

@hot_rod : here's a thought experiment to help visualize the relationship between output and flow.

Imagine you have a radiator that is rated to produce 10,000 BTU/hr when it's provided with 1 GPM of water at 180F in a room that is at 70F. The leaving water temperature is 160F. Further imagine that the average surface temperature of that radiator can be approximated by taking the average of the entering water temperature and the leaving water temperature, and that the heat output of the radiator is directly proportional to the difference between the air temperature and the average surface temperature of the radiator.

What happens if you increase the flow to where the leaving water temperature is 170F? The average temperature is now 175F. The difference between the room and the average temperature is 105F instead of 100F, so the output increases by 5%, to 10,500 BTU/hr. The delta is now 10F, to get 10,500 BTU/hr with a 10F delta the flow needs to be 2.1 GPM. So you've more than doubled the flow but only increased the output by 5%.

Here's a chart of what happens if you take the flow up to ridiculous levels:

There's drasticly diminishing returns. Even at 2000 GPM you're only seeing slightly less than 10% increase over 1 GPM!

OK, what happens if you slow the flow instead? You get this chart:

If you reduce the flow to 0.1 GPM, you reduce the heat output by half. But at that point the outlet of the radiator is at room temperature, reducing the flow even further doesn't reduce the exit temperature or the temperature delta. But it does reduce the heat output, because output is delta times flow.

These are microscopic flows, as a practical matter it's impossible to get this level of precision and if you want part-load out of a radiator you're better off putting on a valve that turns it on and off rather than trying to modulate down so low.

Putting all of that in a chart, you get this:

Basically, there's a small area between about 80% to 105% of rated output where the radiator is sensitive to flow. Above that the chart is essentially a horizontal line, and below that it's essentially a vertical line.