The water is not moving too fast!

hot_rod

Repeat after me, higher flow rates, all things being equal = higher awt average water temperature and as a result higher output

See the fin tube examples at 35, 12, and 6 degree delta, notice the output difference.

This same info is in the output charts from the heat emitter manufacturers.

Radiant loops running a 10 delta output more than loops running a 20 delta, it’s in all the radiant design manuals.

It true in fin tube, true in fan coils, true in radiant loop

Yes I know it cost a bit more pump power, and obviously you want to keep velocity reasonable, below 5 fps feet per second.

This  journal is free for the taking, beginners and pros alike can learn about heat transfer

slowing flow in your fin tube will not, cannot increase heat output, assuming SWT stays constant

EdTheHeaterMan

higher flow rates, all things being equal = higher awt average water temperature and as a result higher output

higher flow rates, all things being equal = higher awt average water temperature and as a result higher output

higher flow rates, all things being equal = higher awt average water temperature and as a result higher output


higher flow rates, all things being equal = higher awt average water temperature and as a result higher output

did I get that right Bob?

It's too late, I'm going to bed

bburd

I think the concern about high flow rates leading to poor heating refers to cast-iron radiators with both connections on the bottom. A slower flow rate allows the warmer water to drift up to the top of the radiator and slowly drop down as it cools. @DanHolohan has mentioned this IIRC. If the flow rate is too high, the water can short-circuit across the bottom of the radiator instead of heating the whole thing.

hot_rod

Happy Hydronic dreams

EdTheHeaterMan said:

higher flow rates, all things being equal = higher awt average water temperature and as a result higher output

higher flow rates, all things being equal = higher awt average water temperature and as a result higher output

higher flow rates, all things being equal = higher awt average water temperature and as a result higher output


higher flow rates, all things being equal = higher awt average water temperature and as a result higher output

did I get that right Bob?

It's too late, I'm going to bed

hot_rod

I think the concern about high flow rates leading to poor heating refers to cast-iron radiators with both connections on the bottom. A slower flow rate allows
the warmer water to drift up to the top of the radiator and slowly drop down as it cools.

@DanHolohan has mentioned this IIRC. If the flow rate is too high, the water can short-circuit across the bottom of the radiator instead of heating the whole thing.

bburd said:

I think the concern about high flow rates leading to poor heating refers to cast-iron radiators with both connections on the bottom. A slower flow rate allows the warmer water to drift up to the top of the radiator and slowly drop down as it cools. @DanHolohan has mentioned this IIRC. If the flow rate is too high, the water can short-circuit across the bottom of the radiator instead of heating the whole thing.

I don’t believe I mentioned cast radiators in the example?

But riddle me this, I have a cast boiler and I want t to move 80,000 btu to a radiator at a 20 delta, what gpm pump would you recommend?

If I want to move 120,000 gpm at a 20 delta what gpm pump would you sell me?

If I run 1 gpm through the boiler at a 20 delta, how much energy could I expect to move?

Could I get 80,000 btu out of a boiler to a radiator with no flow? If so, why install a pump?

Could I flow 0 gpm and expect heat from the boiler to get to the radiator?

So the hydronic formula is selective to the type of heat emitter?

I believe non pumped gravity systems, the water moved?? Didn’t the higher radiators need to be balanced down as water raced to the top bypassing lower floors?

I ran a cast radiator in my shop at 1 gpm and 5 gpm, filmed it with an IR camera. If there was a difference in heat output neither I or the camera could see it

If a radiator was designed for low or no flow, how does the 80,000 btu get out of the boiler?

I’ll try that demo again with my camera and a new certified BTU meter attached

bburd

@hot_rod Your example was fin tube. Your title did not specify type of radiation. The only time I have heard about excessive water flow rates causing underheating was with cast-iron radiators and pumps selected without calculations, though I expect the same effect could occur in panel radiators.

hot_rod

bburd said:

@hot_rod Your example was fin tube. Your title did not specify type of radiation. The only time I have heard about excessive water flow rates causing underheating was with cast-iron radiators and pumps selected without calculations, though I expect the same effect could occur in panel radiators.

I think there are 3 posts going on here regarding under performing fin tube systems, and I see advise to slow the flow to increase output, so I though maybe this would help clear up the misconception as it pertains to fin tube, or radiant, or fan coils.

The unanswered question I have is how you size the pump, select the flow rate for those cast radiator systems? Would a 200,000 boiler use the same circulator as a 50,000 boiler? At some point you need adequate flow to move the btus from the boiler or it short cycles. Ho do you find the balance. A formula? A rule of thumb? Trial and error? How slow is slow enough to make the radiators perform best?

Back in the day, I recall Series 100 circulators were one speed, about the lowest gpm circ money could buy. So a balance valve is used to dial them down?


Panel radiator output sheets would answer the question about flow rates and heat output. Probably that small diameter tube inside many panel rads would be the limiting factor for flow rate?



At days end, it is much easier, almost perfectly linear to adjust heat output by varying SWT temperature, based on outdoor temperature changes.
As opposed to small flow rate changes and relatively small; difference in emitter output.

The post title was left vague to get folks to click on it It does however relate to the 3 different types of emitters mentioned.

bburd

@hot_rod wrote:  “The unanswered question I have is how you size the pump, select the flow rate for those cast radiator systems? Would a 200,000 boiler use the same circulator as a 50,000 boiler? At some point you need adequate flow to move the btus from the boiler or it short cycles. Ho do you find the balance. A formula? A rule of thumb? Trial and error? How slow is slow enough to make the radiators perform best?”

That is an excellent question. Others  probably have the experience to answer it. Somewhere I have a copy of Dan’s book “How Come?” which discusses converting gravity hot water systems with cast iron radiation to forced circulation. I’ll see if I can find it.

In my experience cast-iron radiators work fine with reasonable flow rates typically used in the industry. The concern was with systems that were ridiculously over pumped.

mattmia2

bburd said:

In my experience cast-iron radiators work fine with reasonable flow rates typically used in the industry. The concern was with systems that were ridiculously over pumped.

I think that came from people sizing the circulator based on gravity sized pipes.

Maybe low flow in a converted gravity system doesn't short cycle the boiler, maybe the delta t is enough to move the heat out of the boiler. Gravity systems certainly had a much higher delta t when they relied on gravity circulation. Maybe when the flow is low and the delta t is high you can keep the boiler form condensing with a lower than normally recommended return water temp because it is returning slowly enough that the boiler can get it above condensing temps quickly.

hot_rod

bburd said:

@hot_rod wrote:  “The unanswered question I have is how you size the pump, select the flow rate for those cast radiator systems? Would a 200,000 boiler use the same circulator as a 50,000 boiler? At some point you need adequate flow to move the btus from the boiler or it short cycles. Ho do you find the balance. A formula? A rule of thumb? Trial and error? How slow is slow enough to make the radiators perform best?”

That is an excellent question. Others  probably have the experience to answer it. Somewhere I have a copy of Dan’s book “How Come?” which discusses converting gravity hot water systems with cast iron radiation to forced circulation. I’ll see if I can find it.

In my experience cast-iron radiators work fine with reasonable flow rates typically used in the industry. The concern was with systems that were ridiculously over pumped.

I guess I'm looking for a definition of an "over-pumped" system. I understand flow velocity is the common number to watch. The Copper Development Association suggests 4 fps for hydronics running 140F and up, I think.
Residential DHW can run up to 8 fps assuming it is not a 24/7 flow.

Pump size and operating cost are another "over pumped" thought. Not many residential system that can't be pumped up to maybe 12 gpm with a 40W ECM, maybe even 27W! So electric costs become a minor issue.
Even ECM high head circs are way below 100W. One 100W incandescent light bulb switch to LED in the home will offset the pump operating cost .

But obviously 4 fps in a 1-1/2" would be 20- 22 gpm. I doubt that is an acceptable flow for cast radiators if the flow path is straight across.

Flow velocity in pipe 2" and up is usually calculated on pressure drop, not fps.

So how did the gravity designers size the pipe? Pressure drop, flow velocity, radiator flow spec?