Flow in a Proportional System

Mike T., Swampeast MO

NM

Paul Rohrs_2

Wow

Talk about a micro-zoned house! Are you using zone valves with the proportional mixing because of the miniscule flow rates? What kind of boiler are you going to use? It seems the use of a buffer tank is a must to prevent short cycling. Nice program, what format is that?

Regards,

PR

Mike T., Swampeast MO

I = B = R rating of boiler: 70,000

TRVs on all radiators.

Maximum head loss in piping system:

0.08 gpm through 40' of 3/8" copper + 22' of ½" copper + 15' of 1¼" black iron + 8' of 2½" black iron + 5' of 3" black iron .

(I'm too lazy to calculate this LOW number) so will high-ball and say ½')

At least I think this is the highest head loss in the system--next candidate is 0.34 gpm through 70' of 1" + 19' of 1¼" + 4' of 1½" + 18' of 2" + 8' of 2½" + 5' of 3" (all black iron).

At ½' head loss single system circulator will move about 5.9 gpm.

System circulator is capable of moving 4.7 gallons of water against 5.7' of head loss. This gives a "headroom" of 5.2'.

Maximum required flow through any given TRV in the system is less than 1 gpm. Danfoss TRVs 3/4" and larger.

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If ANYTHING here looks amiss or if you have questions, PLEASE comment/ask!

This is my "proof" that I can use a Vitodens 6-24 on this system with no low-loss header, no primary-secondary pumping and ONLY the built-in circulator.

Mike T., Swampeast MO

Program

Heat loss came from HVAC-calc.

Surface temperature came from prediction that has been VERIFIED by actual surface temperature when system is maintaining setpoint.

Zone delta-t came from supply temp - ((supply temp - radiator temp) * 2) OR 68° (supply temp - ambient) whichever is lower. And YES, return temperature from an oversized TRVd cast iron radiator CAN be ambient! I have numerous rads in this house that prove it.

Flow comes from Zone BTU output / (500 * zone delta-t)

Return temperature coefficient comes from the formula for computing combined return temperature.

The "checksum" IS different. Why? Rounding and because I have used simple formulas that don't correct for that little "problem" when answering, "How much does a gallon of water weigh?"

Current reset curve is set around 148° at 8° outside, but it's a digital boiler so part of that supply temperature becomes "lost" in the burner cycle. With that supply temperature total flow through the system works out to 2.85 gpm and system delta-t a whopping 48° or so!

Used the 20° delta-t for the least oversized zone to play "how low can I go" with supply and also for utter highest maintenance flow. Actual system delta-t in the current system (with higher mechanical reset curve) has NEVER been below 30° and is generally around 40° or higher.

Duncan_2

How does the boiler handle low flow ?

What kind of boiler (gravity conversion?) and how long are the burn times?

Sounds like the system I want to put in my place.

Duncan_2

How does the boiler handle low flow ?

What Paul asked...

What kind of boiler (gravity conversion?) and ballpark, how long are the burn times?

Sounds like the system I want to put in my place.

Duncan_2

How does the boiler handle low flow ?

Mike,

What kind of boiler (gravity conversion?) and how long are the burn times?

Sounds like the system I want to put in my place.

Mike T., Swampeast MO

"How does the boiler handle low flow?"

Do you mean "essentially zero flow" with all of the TRVs closed down? If so, you STILL use a differential pressure bypass valve and the burner will stop firing if the lowest modulation rate still produces a supply temperature higher than required by the reset curve.

Current boiler is a W-M CGM. Wildly oversized. Severe short-cycling (burn time about 2 minutes 40 seconds with only the interval varying with outdoor temperature).

hr

Hey Duncan

nic to hear from you again.

Did you ever read DVW post about running a condensing boiler/ buffer with 100 degree delta T? With a low temperature emmiter system i.e. plates! You might buffer an MZ , for example, and design around that.

I have that article around, and with Davids permission could forward it to you.

hot rod

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Mike T., Swampeast MO

Flow with System Nearing Cutoff

Note that low-end burner modulation rate of the Vitodens is 22 mbh. How lucky can I get considering this is my calculated loss 5° away from current WWSD?

Viessmann does not give a "minimum flow" for the Vitodens, but it does recommend a low-loss header if flow is below 1.3 gpm. Of course when you approach the point where heat is no longer needed in any TRVd system, flow will reduce to near zero.

Dare I say that the flue gas temperature will probably be around 75° and that the boiler will be at its most efficient at the time the current boiler it at its least?

Mike T., Swampeast MO

Minimum Maintenance Flow

Here I've upped the supply temperature by 10° thus making forcing all of the radiation into maximum possible delta-t (supply temp - ambient temp).

This produces the absolute minimum flow possible. Raising the supply temperature any further will not result in: increased delta-t, decreased flow, or decreased return temperature. If the burner keeps firing the supply temperature will "run away".

I suspect that the Vitodens uses this sort of logic to determine when to shut down the burner and wait until outside temperature falls.

Note that flow is low, 1.13 GPM--below that one "minimum recommendation."

Mike T., Swampeast MO

The \"Perfect\" Reset Curve

Does it really exist? No.

In a proportional system, you are at best defining instants with any calculation. The weather, occupancy loading and the setting of TRVs can all change things significantly.

Say I have a big party in 50° yuccky weather. Occupancy load will rise significantly. People won't be going in and out a lot because it's not very nice outside. What's going to happen? The boiler is going to hit that point where supply temperature tries to "run away" and it will stop firing REGARDLESS of the outside temperature!

Now say I want to keep the entire house significantly cooler than design. Required surface temperature of the radiator will drop significantly. Delta-t across each will increase as far as possible. Both system flow and system delta-t will decrease--possibly to the point where the supply temperature again tries to "run away". The burner will be FORCED to start cycling off and on. NOT a good situation for efficiency. That's EXACTLY why the Vitodens has those user-adjustable dials that make a parallel shift in the reset curve. If you're planning on maintaining a lower temperature than design in the space you just TELL the boiler that you're doing it so IT can compensate for you and retain the maximum amount of modulation.

Remember that the Vitodens as a WEATHER RESPONSIVE variable speed pump. The warmer it gets the slower it spins. It's essentially assuming a reset curve quite similar to the one produced by the "Flow at Design" and "Flow near Shutdown" tables.

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BUT for this gravity system I DO want to try something a bit different. As you know, this system contains LOTS of water in the mains. It's essentially a giant buffer if you use it properly. How do you use it properly?

By making its delta-t as consistent as possible regardless of outdoor conditions. Why is this desirable? Because it lets the boiler produce a near constant out of chaos. If you think of delta-t as the speed of a flywheel, it's going to try to keep that flywheel spinning at the same speed all of the time.

I finally put that table into a spreadsheet to make this easier for me.

I decided to find the supply temperature that would produce 1.3 gpm of flow near shutdown temperature. This produced a 36° delta-t. I then adjusted supply temperature at outside design to achieve the same delta-t.

So now, supply temp at design is 141°; near shutdown it is 107°. Reset ratio 0.8. Delta-t 36° period! The boiler's brain should LOVE this ratio!

Weezbo

er uhh Say..while you are at it.....

could a fellow low temp oil type guy take apeek at the designe stradgey? We have natural gas here and from time to time we have spun a bit in..to date i know of no gravity conversions 99.99 fine have all been new instalations..still,sometimes i tend to see applications where it may have remained elusive :)on previous perusal:)

Mike T., Swampeast MO

If you want to play...

The spreadsheets (Excel) are attached:

To use: Click. Choose to save, then change the name INCLUDING the .xls extension!

To play with the existing, ONLY modify supply temperature. If you want to "monkey with the balance" (or is that non-balance in this system) the output and surface temperature are INTERDEPENDENT and you will have to modify them BOTH as explained below.

To play with your own:

Use for standing iron and steel panels. IF the radiation proves to be extremely well-balanced (e.g. surface temp near the same for all areas) you can get by without TRVs--IF you have a well-controlled variable-speed pump

1) Determine heat loss at design and near your warm-weather shutdown point for each space.

2) Adjust heat loss downwards by some fixed factor. I used 15% for HVAC-Calc in an old but thermally improved house with original windows and storms. All I can say is make an "educated guess" from there depending on the structure.

3) Determine surface temperature of radiation. For panels/iron rads use this formula: Output of ONE square foot EDR = (Surface Temp - Ambient Setpoint) * 1.5. So, if you need 7,000 btu and have 250 square feet of radiation:

7,000 / 250 = 28

Total BTU / Total EDR = Output per Sq.Ft. EDR

28 / 1.5 = 18.67

Output per Sq.Ft. EDR / Output per degree of temp difference = Required degrees of difference.

70 + 18.67 = 78.67

Ambient Temp + Required degrees of difference = Surface temperature.

FOR BIG PANELS LIKE FLOORS/CEILINGS OPERATING AT EXTREMELY LOW SURFACE TEMPERATURE USE 2.0° FOR THE OUTPUT PER SQUARE FOOT OF THE HEATED PANEL SURFACE. ALSO USE SURFACE TEMP TO ROUNDED TO 10TH OF A DEGREE INSTEAD OF ROUNDED SINGLE DEGREE. This will ONLY work IF you can determine the supply temperature required for the desired surface temperature at outdoor design. You will add the difference between this and what is computed to your "base" reset curve.

3) Insert your outside temp, total heat loss, supply temperature, room temperature, zone outputs, zone surface temps and everything else calculates for you.

4) In the "Flow Near Shutdown" spreadsheet, adjust supply temperature until you achieve the minimum desired flow rate and/or maximum achievable delta-t.

6) In the "Flow at Design" spreadsheet, adjust supply temperature until delta-t is the same as delta-t in the "Flow Near Shutdown" table.

Divide change in supply temperture by change in outside temperature for your "perfect" reset ratio.

Mike T., Swampeast MO

Self-Promotion

Look in the Community Bulletin board for offer to purchase the spreadsheets, including an "improved" version of the tables provided here with an r-value input for use with radiant panels.