Pump Speed that creates the largest Delta T

Le John

Hello All,
As a rule of thumb using a 3 speed circulator (grundfos ups15-58frc) - at what speed is the greatest amount of heat transferred using hot water baseboard (1/2 pipe). Is it at Speed 1 Speed 2 Speed 3 etc?

hot_rod

The circulator ideally should be sized to the piping and heat emitter connected to it. Adjusting the pump speed, and sometimes adding a balance valve will dial in the exact requirement.

Technically speaking the higher flow rate, the higher the average temperature across the fin tube, and the highest output as a result.

Excessive flow could cause noise and wear to tube and fittings, 4- 5 FPS feet per second for hydronic piping, about 3- 3.5 gpm in 1/2 copper tube max.



https://www.hpacmag.com/features/a-relationship-with-benefits/

Snowmelt

faster the pump you will get more heat. here is the theory in my experience........
had a tank-less with high head pressure doing a heating coil.
had a 007 and the coil will start at140 degree and come out barely 90 degree's
I installed a 009 or a groundfos 26 - 99 and then the return water was 120 degree

SuperJ

Snowmelt said:

faster the pump you will get more heat. here is the theory in my experience........
had a tank-less with high head pressure doing a heating coil.
had a 007 and the coil will start at140 degree and come out barely 90 degree's
I installed a 009 or a groundfos 26 - 99 and then the return water was 120 degree

Within reason, the chart from the link @Hotrod posted, shows there are limits the additional heat you can gain. In your case you dropped your delta from 50 to 30. So your average coil temp might have gone from 115F to 125F, giving you a bump in capacity. But since you entering temperature doesn't change with flow, you will only be adding capacity as long as you raise the emitter leaving water temperature.

If the system is underpumped it will operate near the left edge of the chart, and will gain substantial capacity by increasing flow, but once your past 1GPM the capacity benefits of additional flow rapidly drop off.

Rich_49

Nobody has answered the question asked . The largest Delta will be acheived using the lowest speed .

When Desired Delta narrows we would like to slow a circ , when Delta widens past desired point we would like a circ to speed up .

Henry

Too high a speed with a D T of 10F, the heat transfer is poor. I have encountered this over 40 years on several sites where the installers thought that more is better. it is with explosives! Get a B & G System Syzer Calculator. It will give you the physical facts of DT, pipe size and velocities to use.

hot_rod

When it comes to heat delivery from any heat emitter, fin tube, radiant loop or forced convector, output increases with increased flow.
Here is an example of a section of copper fin tube supplied with 180° A flow rate of
1 gpm = 16,980 BTU/hr
4 gpm = 22,060
8 gpm= 23,030

The fin tube with the highest gpm, highest average temperature has the highest output.

Two question in the OP, as I see it

Pump speed that creates largest ∆ T= slowest

What speed is the greatest amount of heat transferred in baseboard? = highest speed, within acceptable velocities

Jamie Hall

I think the confusion is between the original question -- which was what flow creates the largest delta T -- and what flow creates the most heat delivery.

The largest delta T is pretty simple -- reduce the pump speed and flow and eventually, at some low flow, the return temperature will approach the space temperature (can never quite get there). That flow will vary with input temperature, of course, but since the output is as cool as it can get, that will give you the largest delta T.

I'm not just sure why the OP -- @Le John -- wanted that information, since it has very little to do with heat delivery, but... that was the question.

Le John

Thanks @Jamie Hall @Rich @Henry @hot rod @SuperJ I appreciate all the comments.

As I understand it a mod con only condenses when the return water temperature is below 140 degrees - yielding the highest efficiency. I am playing with the outdoor reset on my Lochinvar Noble Combi to get the coolest return water temperature for the highest efficiency.

In this application does the pump speed matter?

Leon82

If there is a display for return temp you can use that and switch the pump speed and monitor it to see the best result

There is a point which thermal transfer stops for instance on an engine if you remove the thermostat there is so much flow that the coolant can't absorb the heat from the heads and the engine can overheat

Le John

that's a good idea. There is a display for inlet and outlet temperatures. I will take temperature readings at each speed setting.

hot_rod

just know, the installation manual shows the max delta T that is allowed, around 40 maybe in yours?

So be aware of that when you slow flow. The control may have a lockout function is the acceptable delta is exceeded

Jamie Hall

Ah. Now perhaps I see what you may be driving at. There is an important point to remember here: the efficiency gain for a condensing boiler -- modulating or no -- comes from condensing the water vapour in the stack gas, thus releasing the latent heat (which, incidentally, is how heat is transferred in a steam system -- but that's another story). Condensation will occur anytime the stack gas temperature drops below its dewpoint, which is, as I recall, somewhere around 140 for oil and 130 for gas.

The important point is this: to get that gain, you need to have return temperatures that low -- but there is no further gain in efficiency by going lower. So the objective of the exercise is to adjust the pump speed so that the return temperature is right around 140 -- and adjust, at the same time, the inlet temperature of the system so that the combination of flow and the resulting delta T give you the heat output from the system that the space needs. That is the function of the outdoor reset. If your boiler also modulates -- and most do, hence the casual term "mod/con" -- its controls will modulate the firing rate to get the boiler outlet temperature to whatever the control system wants for system inlet temperature.

It really isn't rocket science -- but it is absolutely necessary to consider the system as a whole, and not focus on any one particular aspect of it.

Gordy

You must refer to the manual for minimum, and maximum flow rates across the heat exchanger. There should be a page in the manual covering this.


@Jamie Hall that is incorrect that no more efficiency is gained by lower than 130 return temps on a mod/con.

Jamie Hall

Thank yo @Gordy -- always learning something...

Gordy

This is what you need to pay close attention to as @hotrod noted.

hot_rod

The above graph also proves that a 20°∆ is not required to provide best boiler efficiency, or any heat emitter for that reason.

A mod con, or non con will both be more efficient with cooler returns. But non condensers require protection for return temperature as they are not designed for extended condensing operation.

Boilers seeing really cold returns like a snowmelt boiler will condense like crazy, gallons per hour. You cannot go too low on condensers are far as efficiency is concerned.

mikeg2015

As mentioned, limitation to a Mod-con is minimum flow rate and maximum delta T depending on the design.

Not mentioned s that the efficiency is increased further at lower front rates. The heat exchanger s effectively oversized so approach temp s lower between EWT and stack temp. This s not true on a atmospheric boiler because you get more excess air and end up wasting more heat up the stack. Actually, adjusting the draft on a atmospheric boiler is probably overlooked.

A good trick to achieve higher efficiencies on Mod cons Is to pipe loads in series, or seres-parallel loops. So indirect is first, then fan coils, fin tube or cast Ron, then ideally some in floor radiant.