Retrofitting Mod Con on Gravity System

Steamhead (in transit)

This old gravity system will pretty much circulate itself. The only thing the system ("building") circ needs to do is move the water in and out of the system loop from the primary loop. For this, you don't need much pumping power. The primary loop and circ decouples the boiler circulation from the system circulation, as it should. The boiler loop circ should be sized however Lochinvar says to.

For a guide to sizing circs on gravity systems, look here:

http://www.heatinghelp.com/newsletter.cfm?Id=125

It pays to wander off the Wall, as Dan would say.

"Steamhead"


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Magnehelic

Retrofitting Mod/Con on gravity system

Hi Guys,

I just got the news from a new customer that she has chosen us for her boiler installation. We are a Knight VIP contractor, and have dozens of awesome installs since switching to the Knight. This will be our first retrofit where the existing system is gravity type. Picture in your head an old gas fired circular boiler with his arms up at 45 degree angles (supply piping taking off from both sides at the top of the boiler, and progressively dropping in size the further they get from the boiler) and his legs sticking up at 45 degree angles as well but from further down on the boiler (return piping doing the same as the supply) Of course once the piping meets with the ceiling it is no longer at the 45 degree angles. This is a really neat old system and works like a dream, just very expensive to run (4-6 hundred a month last winter on average) There is not an open tank in the attic as I have run into before so we have no worries there. I have an idea or two about how we're going to pipe in the new boiler (absolutely 100% primary secondary at the least, using closely spaced tees) but I wanted to get your guys input on how you would or have done the near boiler piping on jobs like this. We will be using a Knight 105 for the job (approx 1000ft main floor and 800 upstairs, cast iron radiators throughout). I am planning on installing a new pump on the "building side" and matching it exactly to the "boiler side" pump. Ideas? Thoughts? Pictures? Warnings?

Don

John Ketterman

Gravity sends more water to upper floors; circs tend to send more water to lower floors. You're going to get uneven heat. In addition to balancing and/or flow restrictors at downstairs radiators, TRVs are very helpful. The circulator should not be sized to match the boiler circulator, but sized to get enough heat upstairs. With a gravity system you can use a pretty small circulator. Many former gravity systems are over-circulated, which wastes energy.

I have found that Steamhead's article (link in another message) overestimates the flow needed. But it is a great starting point. I recommend using a multi-speed circ and turning it down as far as can be done without causing problems.

Steamhead (in transit)

I found

when researching and trying out the concepts stated therein, that if I went much below the recommended GPM levels the hot water didn't always get to the ends of the system. The circulation in an old gravity system was not always constant; it varied based on the boiler temperature. With a circ, you're trying to duplicate the GPM the system would see at full fire in the old gravity boiler.

But if you have a system where lower flows will heat the ends of the system reliably, by all means use them.

"Steamhead"

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Magnehelic

Thanks Guys!!

Guys,

Thank you for the link and your quick replies. This now makes very good sense to us. In past installations, we have sized our pumps for the knights at a 20 deg delta t. (it seemed to maximize turndown of the boilers.) Do you see any advantages to running a higher delt for the gravity system?

Don

Steamhead (in transit)

The greater system delta-T

would help you stay in the condensing range. It also helps you avoid over-pumping the system. There is so little resistance to flow in a gravity system that if you pump too fast, the water can short-circuit thru the radiators. This obviously reduces their ability to shed BTUs.

"Steamhead"

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Mike T., Swampeast MO

Your primary (boiler side) circulator should be as recommended by the mfgr (typically sized for 20F dt @ full output).

On the secondary (emitter) side however, things get a bit more complicated with gravity conversions. Remember that such systems have effectively zero head loss even at ridiculously high flow rates. This is why simple circulators like the B&G 100 or Taco 007 are so commonly used--they are designed for high flow at low head. If you managed to find a suitable (unless things have changed I don't believe that the small wet rotor, low-flow, high head circulators are appropriate) circulator of about the same flow as the primary circulator @ zero head, you would likely find extremely poor flow balance in the system--almost certainly if it serves multiple floors. The problem with gravity system design is that upper floors intentionally have higher head losses. 2nd floor rads will typically have 2x the head loss as the ground floor; 3rd floor rads typically 3x the head loss. While the absolute amount of head loss is miniscule, the relative differences are extreme. Secondary delta-t (using suitable circulators) will typically be extremely low as the flow rate will usually be far higher than required for say a 20F delta-t.

Two-pipe original gravity systems are EXTREMELY well suited to TRVs (thermostatic radiator valves). Not only do they give independent, room-by-room control of temperature, but combined with a well adjusted reset curve will result in secondary flow rates very similar to what occured naturally via gravity.

If you install TRVs on ALL radiators, in most cases converted gravity systems offer a rather unique opportunity to use a single circulator for the entire system. Contrary to popular belief, modern mod-cons do not necessarily require primary/secondary. You must do some calculations and verify with the mfgr, but nearly all mod-cons can be used. Not only does this eliminate added circulator(s), but it gives you COMPLETE proportional flow throughout the entire system--just like happened with a solid-fueled boiler under gravity flow. Considering that most gravity systems have greatly oversized radiators, this allows a mod-con to utterly thrive despite changing weather and other conditions. Do NOT though consider using one of the new variable speed circulators in such an application as such a circulator must be under direct boiler control. To my knowledge, the Vitodens 6-24 and 8-32 (they have a built-in variable speed circulator under its own control) are the only boilers available in the U.S. with this ability.

J.C.A._3

Hey Don,

Something that no one has yet mentioned but is VERY important, is a good Wye Strainer.

The water passages in the Knight are quite small, and pumping on the system side WILL get some pipe goo moving to it in short order. I suggest making the strainer as easy as possible to service, as you will want to watch it closely for the first few weeks.

We've used the Caleffi separators with good success, and they also have a drain right on the bottom to help remove debris. Chris

Charlie from wmass

Just a thought here

What are the thoughts of using a heat exchanger here as the old gravity system is going to have lots of dirt and rust that is going to start moving now that the flow velocities are up due to the pumps. I have not installed any knights but the buderus and weil mclains would require heat exchangers so the do not get plugged up. WYe strainer is the least you will need. Filtration or isolation maybe needed.

Mike T., Swampeast MO

THANK YOU J.C.A! Yes, you certainly want to include a wye-strainer at a minimum and you [might] want to include a side-arm filter as well. Side-arm filters receive only a small portion of the total flow but remove FAR smaller particles--they may require VERY frequent cleaning (early on) but at least when they utterly clog will have no significant affect on the total flow.

In all honesty, I would suggest that you remove the boiler, install a high-volume circulator and wye strainer and run it CONSTANTLY while you're working on the near-boiler and venting. If the wye strainer does not clog, [likely] no problem, but if it does, repeat until it's utterly clean for at least 24 hours of circulation. I would not however suggest any sort of additive that attempts to "clean" the pipes and DEFINITELY not if you have not installed a side-arm filter!

tim smith

pri/sec piping w/ modcon boiler w/ old gravity????

I have found no reason for this with old gravity system conversions that we have ran into. I also agree that if needed or wanted, thermostatics are very good in this application except you need to inform client that you will very likely still get some thermal rise of water up the return on ones that are off if system calling for heat. Just a footnote. Tim