Flow Rates

Kevin A Gerrity_2

> install TRVs on every rad, where does the <BR>
> separate "zone issue" come-in?_BR__BR_Dave <BR>
<BR>
The customer. They want to control each level individually and I am have a problem explaining to them tahat zoning is redundent.(See previous post #4)

Kevin A Gerrity_2

Flow rates for radiators

How can I find or calculate flow rates for old cast iron Radiators?

Unknown

Do you mean,

the flow rate for the pipe size main "supplying" CI rads? Was this an old gravity system conversion, or?? We need more info to better help.

Dave

Kevin A Gerrity_2

What I am trying to do is install a new heating system in a Post & Beam home. The homeowner whats to use old cast iron radiator instead of baseboard. I suggested Radiant panels, but they what old cast iron radiators. I am installing a Munchkin Contender MC80 with outdoor reset. I am goning to install a monoflow style system with TRV valves on each radiator. There is goning to be 3 zones of heat. I need to figure out what flow rate and pressure drop each radiator will have to size the mains and circulators. I would like to design system with a max. temp of 140 degrees, if possible.

Steamhead (in transit)

That should be easy

size your radiators for the heat loads at design temp with maximum water temperature of 140 degrees. At this water temp, they will emit 110 BTU per hour per square foot of radiation. Rads toward the end of the mains should be a bit larger, as the incoming water will be cooler.

A supply and return branch of 1/2-inch pipe will handle any radiator up to about 90 square feet. Larger units would use 3/4-inch pipe.

With the radiator branch sizes known, you can size your mains to deliver the needed amount of flow to each branch.

Your TRVs should have minimal resistance to flow when open, and should bypass around the rad instead of closing completely. You must maintain flow in all parts of the system for it to work properly.

To Learn More About This Professional, Click Here to Visit Their Ad in "Find A Professional"

Unknown

If your going to

install TRVs on every rad, where does the separate "zone issue" come-in?

Dave

Kevin A Gerrity_2

A few questions

#1 How do I determine the square feet of the radiators.

#2 When you say bypass is a mono-flow system O.K.

#3 What about sizing circulators and head loss of radiators

#4 How can I get my mouth to stop getting me in these situatuions? (I love learning, its just my mouth get's it faster the my brain.)

Unknown

Slow down Kevin,

take it easy,,,do you have any room HL calcs to begin with?

Dave

Kevin A Gerrity_2

Put simply

The customer. They want to control each level individually and I am having a problem explaining to them that zoning is redundent.(See previous post #4)

Kevin A Gerrity_2

Yes

But, the customer has already bought the radiators and had them painted. So I am trying to size backwards.

Unknown

OK, first off,

ALWAYS give the customer what they want, it`s up to you to tell them how to go about doing-it! TRV`s on every rad?, then start-off by sizing the rads properly(super-important), then size one circ for them all.

Dave

Unknown

I gotta say,

this was not the "brightest thing" your customer should have done! Kinda like "Build A House Around This" scenario!

Dave

Unknown

Well if they,

> ALWAYS give the customer what they want, it`s up

> to you to tell them how to go about doing-it!

> TRV`s on every rad?, then start-off by sizing the

> rads properly(super-important), then size one

> circ for them all. _BR__BR_Dave





Dave

Kevin A Gerrity_2

Okay

But what do I do if they have already pruchased the radiators and they are sitting in the house. This all started with a ballpark price and next thing I know a check is in my mail box and the customer is ready to go. I have talked to them about the issue of these radiators and that they may not work properly. So now I am trying to see if I can get their radiantors to work as best as I can. They understand that they may have moved to fast and are willing to correct the problem even if it cost them more money. So you can see that I want to try and make this work with the radiators they have choosen.

Unknown

Well if they,

randomly bought the rads, you will have no choice but to use TRVs and each floor(zone) thermostats, I can see no-way around this, may as well throw your HL calcs "out the window". You may also ask Scotty to beam-you-up, as no 1 water temp will satisfy them all!,,too bad they did this. Next question?

Dave

Kevin A Gerrity_2

Yep!!!!!

I said the same thing. So now I have to find an anwser. It may turn out to be the radiators have to go back. I will call the radiator company and see what they will do on Monday. Ya know I went in to business so I could schedule my time. Was going to go to Sox's game (Patiots Day at Fenway) with brothers now I may have to either make call from park or Ark. Please be sunny!!!!!!!!!!!

Unknown

But that`s OK,,

you can still be the "Hero" here, just make sure the rads that they bought are at least closely matched to the rooms HL(not undersized), and the TRVs should do the rest. Remember though you will have to run a bypass on each rad to continue-on down the line.

Dave

Brad White

Overview

Not to repeat the other good advice given here (and not to repeat the obvious) but rather to consolidate an approach, this is what I would do.

1) Calculate room-by-room heat losses.

2) Size the radiators by EDR. One hopes that they have not yet been installed in specific rooms, but regardless, match the EDR to the appropriate rooms.

3) Triangulate radiation to heat loss both room-by-room and to the house as a whole. This will determine your maximum required water temperature.

Your boiler will still be sized to the heat loss. Size the circulator for, absent better ideas, a 20 degree drop. If you have a 60 MBH heat loss, 6 GPM will do nicely. Flow is not as critical as many think. You can be off by 50% and still get 90% of your output.

TRV's on the radiators will take care of many variables, balancing among them.

I agree, it is a backwards way to design a system but assuming the radiators well exceed the heat loss and consistently so (the surplus percentage is similar room to room), you should be OK.

Now, the electrician has a challenge- If this design procedure holds and it is time to screw in a lightbulb, how many electricians will it take to turn the house?

Uni R_3

Monoflos plus TRVs

Isn't there a risk with having too many TRVs on a monoflo system? If too many branches are closed, the overall head for the main loop goes way up. When the head goes up, the velocity goes down. Once the velocity drops enough the venturi tees no longer influence the flow of water down the open branches. The heating system becomes ineffective because the flow through the open branches is random.

I would think that 3 way TRVs would be the only way to do this, but I have only seen them once - and that was here.

If the piping is still accessible, is there any chance you could change the piping? Monoflo is a very inefficient piping method for M-C boilers. The branches all return temps far lower than the return of the main pipe.

My system is monoflo and I'd bet that my water temps could probably be at least 6° less, and my ΔT would be double if everything were piped through a manifold.

If I measure the temps of mains and branches, here's what I'd typical find on a medium day.

1.25" Main 120° Supply 114° Return - 6°ΔT

Mid-Branch 118° Supply 108° Return - 10°ΔT

Monoflos are a much better match for conventional boilers than for M-C boilers.

Uni R_3

Brad. don't forget...

"Size the circulator for, absent better ideas, a 20 degree drop."

If it gets piped as a monoflo system like the poster states, he'll have to size the circ for sufficient velocity. A circ sized for a 20°ΔT could be extremely bad in this case.

hot_rod

Or a homerun system

it may be possible, depending on the room loads to use a 1/2" pex or PAP supply and return to each radiator location. Like you would with a panel radiator system, or radiant loops.

hot rod

Brad White

Extremely bad?

Not sure what you mean, Uni. Nothing inherently wrong with my advice, the premise being that the piping is also sized accordingly and properly in the first place. All things being equal, the velocity will be within acceptable limits (meaning to move air along and nothing else). To your other point about TRV's closing, that is not a bad thing either. It means that the building is coming up to temperature and if flow is restricted, it is not needed many places either.

Velocity is only part of the equation; pressure differential is the motive force. Even at low velocities, the differential pressure drop between the tees is still sufficient to pass water up and through a radiator if properly designed. I submit that a poor installation (meaning with excessive branch lengths and resistance) is compromised regardless of main velocity. All that is needed is to assure greater initial PD in the main than the branch. After that, flows and pressures seek a balance point.


To size a circulator for some particular velocity without regard for temperature drop serves to move more water than needed for a given heat loss and extraction at the radiators. I see no benefit to doing that unless I own stock in the electric company.

Uni R_3

Yup... unless randomly heated circuits are good.

Brad, these tees need a minimum flow to create the venturi affect. TRVs closing branches along the main will all serve to reduce the total flow, exacerbating the ability of the open branches to get heat. If I'm wrong, I'll happily supply the Sam Adams! ;-)

"To size a circulator for some particular velocity serves to move more water than needed for a given heat loss and extraction at the radiators. I see no benefit to doing that unless I own stock in the electric company."

I totally agree, but for a monoflo system you must. Like I said and hot rod reiterated, the OP should homerun the system to a manifold instead of using a monoflo system. Then you can actually have the higher ΔT and maximize the potential for the m-c boiler, while lowering the electrical demands for pumping.

Mark Hunt

Mono-flo and TRV's


This would be a problem when the TRV's start to close. The flow through the system will be restricted to what makes it through the diverter tee. Unless you use TRV's with a built-in bypass and even then it may be a bear to balance.

We have done systems just as you described but we piped it in a two pipe configuration with a PDV at the end of the supply and return mains. The circ comes on when the OD temp drops to 60° and the TRV's control room temp. The boiler reset controls loop temp.

HR mentioned a home-run system. That would work just as well.

Mark H

To Learn More About This Professional, Click Here to Visit Their Ad in "Find A Professional"

Brad White

I agree that

a homerun system is a far better choice, Uni but for monoflow systems, of which I have designed and had installed several, properly done they work fine under a variety of conditions. Randomly heated circuits? Only if the system is poorly designed to begin with. Adjustment of flow up or down within reason is very forgiving.

Remember too, as stated, if TRV's close, there is no heat loss going on so the system has not a whole lot of work to do at that time.

The venturi effect is well understood here but in the real world, closing down the occasional branch is not catastrophic. In fact the effect may well be undetectable to an occupant. Besides, TRV's do not necessarily close to shut-off; there is more often at least some flow hence less pronounced effect on the mains.

In a poorly designed venturi tee system all bets are off. I am speaking of a system properly designed in the first place. No big deal, just academic differences, that's all.

Brad White

I especially agree with your point

that Monoflow systems are more applicable to conventional boilers. P/S in any of it's forms passes the unused hot water right back to the return, often defeating condensing. A Monoflow system, especially if over-pumped, does this wholesale. It is all about proper application regardless.

Kevin A Gerrity_2

Thanks to all

I have been reading up on my design possiblities and want to thank all for their input. It is nice to run idears by some one else to help clear the head. Again thanks to all, will keep you posted.

Wayco Wayne_2

Suggestion

Is it possible to run the radiators in a home run from a manifold down by the boiler, instead of a monoflow tee set up? Then each radiator could be on a TRV and you could have a differential bypass down by the boiler. Makes it work better and easier to control IMHO. Sounds like a house I did 2 years ago. Radiators were bought before I got there. I did like Brad suggeted above. Sized them as close as possible to the heat loss and then did a home run with TRV's. All attached to a Munchkin 80M. Worked out well. Good luck.

To Learn More About This Professional, Click Here to Visit Their Ad in "Find A Professional"

Mad Dog_2

I would do a home-run system with PEX

If you can. Depends on their budget. Yes, I woulod do some careful calculations before you touch a wrench. The HOs have put the cart before the horse, but if you can make 'em work, it could be a very interesting project. Question: What kind of old rads? Are they old or new? Burnham Classic? Mad Dog

To Learn More About This Professional, Click Here to Visit Their Ad in "Find A Professional"