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# Science behind determining flow requirements

Member Posts: 1,405
Just so I understand the true meaning of the Delta-T, is that by definion the difference between the supply and return at design conditions (coldest day?). Does that still apply to a mod-con configuration?

If my design temp is 140F, the 750 EDR will emit 67,500BTH at design conditions. Are we saying that for a 20 degree drop that the flow should be only 6.75 GPM?

Unless, I'm missing something, this seems to imply that the amount of water in the system is immaterial to flow requirements and that doesn't make much sense to me. Wouldn't you need more flow if you had larger pipes (ie gravity conversion)?

• Member Posts: 1,405
science behind determining flow requirements

Can someone more knowledgeable enlighten me on the science behind the flow requirement for cast iron radiators (or any kind of radiation for that matter?). Radiator specs always list the expected BTH output based on average water temperature but they say nothing about flow?

If you have a house with 750 EDR of a mix of cast-iron rads and a few stelrads, how do you determine what the system flow should be? Everyone seems to have their rule of thumb but no one seems to have any science to back it up.

I'd like to know what happens to performance if flow is less or more than optimal. I assume that it would take a significant change from optimal to have any noticelable impact. Assuming a circulator can produce enough head to reach all radiators, wouldn't flow just impact how much time it takes for the heat to be distributed to the rads?
• Member Posts: 187

> I'd like to know what happens to

> performance if flow is less or more than optimal.

> I assume that it would take a significant change

> from optimal to have any noticelable impact.

> Assuming a circulator can produce enough head to

> reach all radiators, wouldn't flow just impact

> how much time it takes for the heat to be

Right, and that is why rule of thumb is OK. People do calculations based on getting a 20F deltaT, but if they don't do it perfectly right, it doesn't matter. Nothing magical about 20F.

Of course flow rate impacts more than just the time to get hot water to the radiators. If you have twice the flow, the water temperature entering the radiator is roughly the same, but the water temperature leaving the radiator is now higher (deltaT is halved). So the heat output is a little higher. Only a little, though.
• Member Posts: 2,440
Flow is like, Totally Rad, Dude.

Martin-

Flow rate gets down to lbs. of water per hour and degrees F. One lb. of water changing temperature by one degree F. constitutes a British Thermal Unit or BTU, by definition (and you knew that).

So if I have a heat loss of 10,000 BTUs per Hour (BTUH) and my water temperature drops 20 degrees, that is one GPM.

(One GPM at 8.33 lbs. x 60 minutes is 499.8 call it 500 lbs. per hour. Times 20 degrees gets you to 10,000 BTUH.)

Your radiator output is based on the average water temperature as you note. Half will be warmer, half cooler. So 180F in and 160F out gives you 170 average. Neat. This, against the room temperature, say, 70 degrees, gives you a 100 degree delta-T to the room and it is this delta-T which determines output. Narrow that by lowering your water temperature, and you lessen the output. Raise your water temperature, increase your output. Simple as that.

Now, your 750 EDR of radiation. If that is taken at 170 degree F. average water temperature, it will emit 150 BTUH per SF into a 70 degree room. Your radiators will emit 112,500 BTUH overall. 11.25 GPM if you design for a 20 degree drop (delta-T)

Now, if your heat loss is less than that, great! You can use a lower water temperature on the coldest day.

Don't sweat flow rate too much though. The really nice thing about flow at a given temperature is, if you cut the flow in half you still get about 90 percent of your heat output.

When you run 180-160 supply-return, your average water temperature is 170F, right?

Cut the flow rate in half so you are running 180F in and taking 140F. out. Your average water temperature is only 160F, only ten degrees cooler on average. Your hottest water has not changed. Such a deal.

Hope this helps.

• Member Posts: 6,928

The science behind flow and output is very simple (note however that the 500 constant value is in fact a variable based on the density of water but is "good enough" for most space heating purposes):

Where:

BTUh = BTUs per hour

GPM = gallons per minute

DT = delta-t (temperature drop)

Use these three simple equations:

BTUh = 500 * GPM * DT

GPM = BTUh / (500 * DT)

DT = BTUh / (500 * GPM)

------------------------------------------------

While very simple in theory, it's much more difficult in practice. To calculate one value, you must know the other two and without accurate and often very expensive measuring equipment each of these values is often an estimate--or even worse, an assumption...

Are you trying to do something specific? If so, what? By "stelrad" do you mean a modern panel radiator?
• Member Posts: 187

> Just so I understand the true meaning of the

> Delta-T, is that by definion the difference

> between the supply and return at design

> conditions (coldest day?). Does that still apply

> to a mod-con configuration?

It has nothing to do with mod-con vs cast-iron. It is the difference between supply and return. Of course it is usually calculated at 180F. At 110F it will be much lower (if flow rate is the same).

>

> If my design temp

> is 140F, the 750 EDR will emit 67,500BTH at

> design conditions. Are we saying that for a 20

> degree drop that the flow should be only 6.75

> GPM?

It is common practice to use the flow rate that will give you deltaT=20F at T=180F. But there is nothing sacred about 20F. To save on pump electricity consumption, you should use the lowest flow rate that will heat the house, and that is usually much lower.

>

> Unless, I'm missing something, this seems

> to imply that the amount of water in the system

> is immaterial to flow requirements and that

> doesn't make much sense to me. Wouldn't you need

> more flow if you had larger pipes (ie gravity

> conversion)?

Heat is carried by moving water; the volume of water in the pipes is immaterial. You are thinking of water speed (ft/sec), but that's not the same as flow rate (gal/sec). You can have a lot of water moving slowly (gravity system), or a little water moving fast (3/4" copper); both carry the same heat if the flow rate (gal/sec) is the same.
• Member Posts: 2,440
Going with the Flow

You are correct, Martin, the delta-T is the difference between supply and return and applies to any boiler or radiator for that matter. It is a design value and may never be reached. Some systems never see that (the space is satisfied at lower temperatures than calculated for example). Nothing magic about it. It is a helpful balancing value during initial setup and it is a diagnostic tool once the system is running.

You are also correct that if your heat loss is 67.5 MBH and at the lower water temperature, your flow rate only needs to be 6.75. Truthfully you could run 5.0 and not see a difference on the average day, just so long as the flow to each radiator and the radiators themselves are proportional to the heat loss of the space they are in. (Otherwise warm and cold rooms could result of course.) Isn't having over-sized cast iron radiators the best??

You are (again!) correct that the total water volume is immaterial within reason, especially if you use constant circulation. If you are waiting for a particular particle of water to go out and come back in old gravity piping, bring a good book to pass the time. Key is, the water flow rate does not change, just the velocity along the way. It all is what happens at the radiator. Which reminds me: Are you putting TRV's on the radiators? That is a great investment. And the balancing, with constant circulation, is practically automatic.

I hear you have a Vitodens? Naturally this raises one's expectations of what it is connected to. Excellent choice but you knew that.
• Member Posts: 1,405

I'm not really trying to do something specific other than understanding how flow affects the hydronic system. I'm also interested in determining what the flow should be in my system (the 750 EDR above) but even if I were to calculate that the flow should be around 7GPM, I would still need to determine where on the curve the UP15-42F circulator is operating. If it came out that the current flow is 10GPM instead of 7GPM, is that a problem?
• Member Posts: 187

> If it came out that the current flow is 10GPM instead of

> 7GPM, is that a problem?

As several people have already told you, no, it is not a problem. But you are wasting electricity, and at high enough flow rates you will get flow noise. With a three-speed Grundfos, you can change the speed and see if you notice any failure to heat the house. If not, the lowest speed is good enough.
• Member Posts: 1,405

I considered putting TRVs but since all the rads appear balanced I thought it would likely cause me more problems and wasn't worth the expense.

The thing that confuses me a little, is there's an Hot Tech topic article on sizing circulators for gravity conversions by Steamhead Wilsey on this site and it basically states that a gravity conversion should have a circulator sized 50% larger to compensate for the extra water in the system.

If the amount of water is not a factor, why would we need more flow? About half of the rads are connected to gravity pipes and the rest are home-run on PEX.
• Member Posts: 187

Yes, that article confused me too. With all respect to Steamhead, who is a giant in this field, that article is wrong.

He got his numbers and advice from a decades-old document put out by a boiler manufacturer. At that time they believed you needed a minimum velocity (not gpm but fpm) to keep the insides of the pipes clean. They also didn't know about Pumping Away at the time, although that is irrelevant.

Just use the minimum flow rate that works. If this is your own house, you have the time to experiment with different pump speeds.
• Member Posts: 222
Fluid, Flow and Velocity

Seminar slides from our Fluid, Flow and Velocity presentation.

Its a great topic.
• Member Posts: 6,928

Steamhead's article is correct for a "wide-open" gravity system converted to forced flow.

I seriously doubt it has anything to do with "keeping pipes clean". Instead it has everything to do with the fact that the lowest, closest rads in gravity systems were designed with the least flow restriction. This is the opposite of systems designed for force flow. To achieve some sort semblance of the orginal balance when you convert gravity to forced you have to move more water.

Use the TRVs! They will solve your flow problems in a gravity system and are the only reasonable (residential) way to restore system flow to something near the original design.
• Member Posts: 3

I thought that one MUST put in flow restrictors on first-floor radiators to do a gravity-to-pumped conversion.
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