gravity versus forced

computerwhizyeah

Having done maybe 10% of my work throughout my life in wet heating, I am not sure how to answer a customers question satisfactorily. He wants to know what savings he would get by converting a gravity hot water system to forced circulation. He wants to keep his present boiler. I know there are piping considerations that may have to be checked, but for the sake of his question, we could say they are not an issue. Leaving all other issues aside, what might be the fuel savings from moving the water faster through the system. It has the standard standing radiation. Thanks

Steamhead

The big advantage of forced

is that circulation speed is no longer tied to water temperature. So if some parts of the system don't heat well in mild weather, going forced can cure this.

That said, many such conversions have wildly oversized circulators. These move the water so fast that it doesn't pick up enough heat in the boiler or shed it in the rads.

If the system heats OK in mild weather, I'd leave it gravity. But do a heat-loss on the house to see if the firing rate might be too high.

Ken_8

Steamhead is of course

correct.

But the notion that the speed or velocity of a gravity vs. pumped system has anything to do with mild weather heating system shortcomings vs. cooler weather dynamics being "better" is questionable.

It is not the temperature of the water required to meet the load/demand that assures circulation. It is the relationship of return - to supply water temperature that is critical. If the return water temp on a mild day is 65°, which would be about what it would be on a 65° indoor temperature/day; the outgoing water temperature might be 20° higher. The flow would result from that 20° dT - and the boyancy differential of warmer water being "lighter" than the cooler and more "dense" return. On the coldest day of the year, assuming that might be zero outdoors, the supply water temps may be 160° and the returns 140°, still having a 20° dT. So regardless of water temps, gravity systems will have equal flow in both extremes IMHO.

The flow rate is based on the dT of S vs. R water temps. This is a linear response dynamic, not exponential. As long as water is not in the 32-38F° range (where water remarkably does a reverse action to what logic suggests), it's density/boyancy characteristics are linear, meaning the flow rate of a dT of 20° at any range typical in home heating (ambient 65° to 212°F) does not change the flow characteristics enough to impact actual the flow rate substantially - nor the resulting notion of balnce or comfort.

Or so I was led to believe.

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

Steamhead

Ken, that's true

when the system is at steady state. But it takes a bit to get going from a cold start, and in the usual direct-return system this means the heat may not reach the last radiators before the burner shuts off on a mild day.

This seems to be more the case in buildings with a very wide footprint, which necessitates much large main piping to reach the various rads.

It will help if the supply piping is insulated but the return piping is not. This will keep the delta-T as wide as possible.

Bill Jirik,

Adding a small circulator does help in some situations, but draining the system and adding a circulator also does a great job of disturbing any mud dirt and sediment that has been peacfully resting for years, remeber bigger is not always better when it comes to circulators

Ken_8

Apparently,

I have never had a footprint wide enough to have the phenomenon you witnessed, but I've only seen and/or worked on maybe 20 of these things in 40+ years, so...

However, I have witnessed the complete destruction of all balance and comfort by installing a circulater. That includes thos with reduced pipe sizing as per Dan's suggestion in one of the books dealing with that very topic. The fundamental principals of gravity systems can never be improved - or equalled, IMHO by installing any circulator in any configuration imaginable.

Gravity was and still is the ultimate hydronic system - especially when installed by the dead men we now revere so much - thanks to Dan re-telling their tales and engineering principals.

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

Steamhead

Like anything else

it has to be done right. Reduced near-boiler piping is sort of the right idea but why choke down a big pump when we now have the right sizes available? We want to mimic the gravity flow rates when the system is running at its maximum circulation rate. And the only thing the circ needs to do is move the water thru the boiler. The system will add its own "delta-t motive head" if it hasn't been butchered.

Next time you run into one of these, try sizing your circ from this table:

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

You might wind up with something that looks ridiculously small, but you'll love the way it works.

Ken_8

Thanks Frank.

Since I have not seen a gravity system in over five years, I fear what I learned from your research will not be useable in a timeframe that will allow me to recall who wrote (what you already have) or the source it is located in.

It would not surprise me not ever see a somewhat pristine and restorable gravity system for the rest of my lifetime. But how long I live is not the issue, is it (:-o)

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

CarlR

I have a similar question


I recently acquired half a house (1359 sq. ft, built 1911) in Boston that features a gravity system with huge old radiators hooked up to a rusty circa 1967 Pennco gas boiler (3024-WA, input 144,000; output 113,200).

The previous owner's gas bills, including heat, hot water (c.1992 tank), and cooking averaged out to around $130/month, which seems high, given the size of the unit and the fact that there are two heated storeys and an attic above most of the space. The windows were replaced a couple of years ago, and I am my co-owners are planning to seal and insulate, which I expect will help a lot, but what, if anything, should I do about the boiler? Could anyone estimate the efficiency of this old system? I may live it with it for a year, but I'd like to get a sense of what will have to be done eventually, and perhaps sooner rather than later.

One contractor has suggested replacing the boiler with a Burnham Revolution of about the same size (RV-5) for around $6400. Does this sizing make sense? What other options might I investigate? I gather that condensing boilers and TRVs might work, but this may be beyond my budget.

Thank you.

Carl

Unknown

As Steamhead stated

These systems do perform well with substantially small circulators due to the large system pipe sizes, but the big concern is the effects that prolonged periods of cold flow at startup will have on the particular boiler. The RV (Revolution) is an excellent candidate for this type of application due to its ability to modulate the flow through the boiler at any given return temperature with its built-in variable speed injection and bypass piping. With most conventional or even condensing boilers this type of technology would have to be added. Hope this helps.

Glenn Stanton

Manager of Training

Burnham Hydronics

www.burnham.com

Mike T., Swampeast MO

Savings if keeping the present boiler? Likely none to negative because of the electricity required for the circulator.

Of course that's if the system is still working properly under gravity. As mentioned, there might be a problem in mild weather if the system is far-flung with very long horizontal mains. The more likely problem is corrosion that gradually slows gravity circulation. As corrosion adds resistance you need more delta-t for the same amount of flow. Still not too much problem until it slows to the point that during a heat call the boiler cycles frequently on high limit in very cold weather. When and if that situation occurs, you'll likely gain a bit of efficiency by converting to forced flow.

If they want real energy savings, use a condensing/modulating boiler and TRVs. Without the TRVs you have to move much more water (at a lower delta-t) to achieve full circulation. Why? Because gravity systems are sized so that the rads lowest and closest to the boiler have the least restrictive flow path--the exact opposite of a system desiged for forced flow.

Add TRVs and they automatically compensate for that piping problem and allow a MUCH, MUCH smaller circulator to be used. My system with 1,049 sq.ft. EDR needs no more than 4 gpm and the circulator is only capable of a touch over 6 at zero head loss in the piping. It's still VERY responsive when you crank even a number of TRVs.