Plumbing a Garn into a large radiant system

[Father John]

Our monastery is going to install a Garn to heat the 14 zones of our radiant heating system. The people at Garn are recommending that we redo our current system (closed system with oil-fired boilers) so that the Garn is attached directly to the radiant zones (resulting in an open system, with oil-fired backup attached via heat exchangers). They say it is worth doing so we can take full advantage of the water storage of the Garn, and draw more btu's out of the tank than would be possible through a hx.

My worry is our 14 cast iron Grundfos zone pumps, and 2 cast iron Grundfos Miximizers on the injection loops, which I think will not like being in an open system. The Garn people suggest this is not a problem. Am I worrying for no reason? I have always thought cast iron in an open system meant big trouble.

 

[Eric Johnson]

You can treat the water to mitigate the effects of the oxygen that the open system will see. If the boiler vessel won't be compromised (and it won't), then your pumps and black iron piping will be alright as well.

I've always had pressurized systems so I can't comment on open systems, other than to say that they tend to have more problems with foreign materials getting into the system, resulting in things like flat plate heat exchangers needing to be flushed out periodically.

But to answer your question another way, I think the key is in the chemicals and your general maintenance practices.

 

[wdc1160]

As a closed loop guru you would have to do alot of explaining to get me to switch teams. If your set on open looping the system-- I am certain Eric is correct, but there are reasons people decided to go through the trouble of pressurizing.

benedictio

 

[Nofossil]

If I remeber correctly (and I sometimes do), then the Garn has vastly more output than you need for your radiant zones. I think you are getting by with only one of your two oil burners, so your peak demand is way below what the Garn puts out.

Given that, I would think that a little transfer loss with a HX is not going to cause any problems, The heat is not lost - it would have the effect of reducing the load on the Garn. Not a problem - most of the output goes to storage anyway.

 

[Eric Johnson]

I should add that I agree with both ABGWD4U and nofossil, in that I don't think I would convert to non-pressurized. You've got it set up that way and it works, why take the chance when, as nofossil points out, you're way oversized anyway. A little loss at a good flat plate hx isn't going to sink the ship, so to speak.

I've found when making changes in things that the more you change, the more uncertainty you have when something doesn't perform as expected. If you fundamentally change your setup, then where do you begin troubleshooting the inevitable snafus?

But to answer your question directly, corrosion won't be one of your problems. Most OWBs are open loop, and there are millions of them in service around the country, all using iron components and ci pumps.

 

[Father John]

Thanks for the answer to my question about corrosion, and also for predicting my second question about messing with an already working system. I have a real hesitancy to change a layout which works now.

How low can the temperature of the water from the Garn get before it is unable to heat up the flat plate hx?

 

[Eric Johnson]

Pretty low. Flat plates work really well.

My buddy Clyde uses a couple of pretty big ones. You might want to ask him about the specifics, but I think that's the way to go. They come in various configurations, so you can pretty much find one suited to your needs. Bear in mind that the capacities, in btus/hour, are optimum estimates that you and I will probably never see, so I would tend to oversize somewhat. Of course, a wood-fired boiler isn't going to produce its optimum rated output most of the time, either.

 

[TCaldwell]

father john, dectra corp. garn parent co will sell you a nitrite based corrosion inhibitor for the boiler , they will also size a flatplate for your parameters, also flatplate at 717 767 9060 has a tech dept., that will size also. I have a garn am very satisfied, my take on the non pressureized issue is that you could run the garn down to 100 deg and still provide heat to the radiant, this would give you a enormous ammount of stored heat between firings, you can run the boiler 190+deg. I am not sure wich model you have on order, but you can multiply your gal capacity times8.33 lbs/gal=your total water weight,times the high fire temp minus the lowest useable temp.for a 2000gal capacity with a high fire of 190 and a drawdown to 100 that represents approx 1,499,400.00 btu stored or a 62,475.00 btu/hr ave. you should be able to accomplish this in under 4 hrs! possibly the aquastat on the oil boiler would might not let you get this low before firing? , thus you might fire the pressureized boiler before using all the garn storage. to be the advocate, on integrating the two systems, this is how mine is set up,a flatplate between the two systems, a drawdown to 130deg and the oil aquastat set at 135deg. works very well. i had dectra size my flatplate. on the other side as you know martin is very knowledgeable.

 

[heaterman]

Father John:

There's no way I would recommend "opening" your radiant system. Even if you would go to the expense of changing all your circs to non-ferrous material you will probably develop issues with sludge in your radiant lines unles water quality is monitored very closely. I've seen it to many times to think it's an isolated occurrence. In addition to that, you will also increase the chance of air entrainment and loss of circulation in your loops.

It is true that you will lose some ability to utilize all the temp difference allowed in a radiant floor system but to me, the problems you could encounter down the road far out weigh the downside, which basically amounts to filling the Garn a little more often.

Sorry that I have not gotten back with you on piping schematics. The last 3 weeks have all been 80+ hours and it doesn't leave a lot of time. I'll try to get something drawn out for you this coming week.

 

[Father John]

To TCaldwell and heaterman, and indeed all the others who have been a help, thank you much for the opinions and the information. I feel very comfortable knowing the weight of opinion is behind leaving well enough alone, and attaching the Garn to our closed system with a flate plate hx. I actually spoke to the president of GEA Flat Plate, Inc. earlier today, and he agreed to give one to the monastery, so I will let him size it.

I would be curious to know from TCaldwell if water returning to your oil boiler at 135 degrees doesn't pose a problem for condensation? A possibility for us would be, since we are always in residence, to manually switch off our oil backup, so that we could draw the Garn tank temperature down much further, and not cause the oil boilers to kick on. The oil boilers could always be turned back on to provide backup if we anticipated for any reason not being able to build a fire in a timely fashion.

 

[TCaldwell]

father john, I agree with heaterman and the practicality of isolating the 2 loops for the sake of possibly a little more wood and not contaminating the already working radiant system. the manual boiler switch would work although, i bet in todays day and age that somebody could or does make a aquastat with a lower setpoint, johnson controls makes a slab controller with a dry sensor, maybe they have a well available, just a thought. With regards to the 135deg return water, on the oil side of the hx i have 2 closely spaced t's that allow the water to circulate outside of the oil boiler as long as the wood side is supplying heat, when a zone calls the return water heads back to the hx and the supply water then heads through the boiler to the calling zone., there is usally some zone calling, hence hot water running through the boiler, this was done because i installed a indirect fired hw tank a seperate zone, disconnected the dwh coil in the boiler so it would not fire just for hw in the summer, i burn the garn 12 months a year basically in the summer for dwh and a 20,000gal swimming pool.