Garn from forced air HX to radiators, does it work ?

[rombi]

I currently have an older Aquatherm and every year around this time I am shocked by the amount of wood I am putting into this thing. My friend is getting his Garn this spring and I have boiler envy.
My question is with my set up. I would be going to the house which has forced air but I would also like to heat my basement which has no ductwork in and not enough space to run new as we would like to finish the basement. Can I run the water after it leaves the forced air HX to the radiators in the basement then back to the Garn? From what I gather the radiators can be run on cooler temps than the HX.
I am also assuming that installing the Garn would not be that bad because I have all the pipes and pumps already running. Then again if I add more to the run I may need a bigger pump going to the house which would not be that big of a deal.

 

[jebatty]

The answer is yes you can circulate the forced air return through the basement radiators and yes you may need a bigger circulator to accomplish this, these answers are not dependent on whether you have a Garn or another wood gasification or other hot water boiler, but ....

1) I assume your Aquatherm is an unpressurized system and the Garn also will be. I also assume you will be using the same house boiler supply/return lines as you do now. What is your boiler supply temp to the house and what is the return water temp to the boiler now? Do you know approximately what the gpm flow rate now is? What size is the boiler/house piping? Does the Aquatherm maintain a relatively constant supply temp through its firing and idling cycles? What is that temp?

2) The Garn is designed to be used in batch burn operations, heating the Garn, for example, to 180-190F, fire out, then drawing down the Garn to a minimum usable temperature, and then refiring. With its integral storage and batch burn operation, the Garn is well suited to systems that can operate at water temperatures down to about 140F and less, the lower the better. Will this work for you? If not, a Garn still may be a usable system but you may want to also think about other options.

3) Assuming a Garn would work well for you, IMO I think the most simple way to feed your new radiators from the forced air HX return may be a closely spaced T supply to the radiators and return from the radiators as a separate zone with its own circulator properly sized and on/off controlled. But this would supply hot water to the radiators only when there was flow through the forced air HX. In essence you would have a primary/secondary system, with the primary loop being the forced air HX and the secondary being the radiators. Depending on the temp of the return water from the HX you may or may not realize sufficient btu's to add the heat to the basement that you want with your radiators. The heat you will get from your radiators based on supply temp and flow is another calculation you will need to make as you determine whether or not to proceed with your plan.

4) Depending on the sizing of your current house/boiler piping, you may or may not have reasonable ability with a higher head circulator to force more flow through that piping so that both the forced air and radiators could be supplied with hot water. A simple "series" run of piping [boiler to HX to radiators and return to boiler] and a larger circulator may add considerable pump head and with an unpressurized system you may end up with insufficient NPSH which will make the system unworkable. A "bigger pump" could be a very big deal if it won't work.

I recommend a more complete technical analysis of your system and possible options. You have a reasonable goal worth pursuing.

 

[Tennman]

We have some longer duct runs and when the water temp gets much below 150F it's tough to keep some parts of the house warm. Our circulation pump cutoff is set at 155F. The fan blows continuously if it's fairly cold outside. People with radiant boast of staying warm with temps down to 110F or so. Use the search engine. I asked about the Garn and forced air about a year ago and got great answers. An excellent boiler ideal for radiant.

 

[rombi]

The aquatherm is a closed system and it does fairly well heating with the forced air if the water temp stays above 140. Under that and the furnace just keeps running. The way it is set up now the pump runs all the time to the house regardless of demand then back to the boiler. With the garn does the pump only run hen the thermostat demands heat?

 

[jebatty]

With the closed system Aquatherm, what is the psi of the system? A typical pressurized closed system runs 12 psi minimum to about 20-25 psi maximum, with pressure relief at 30 psi. If a low psi, you may have the problem I mentioned in 4) above.

You can run a Garn circulator all the time, and that typically is done in a primary/secondary system. Look at the sticky note on that topic. But this also may defeat an advantage of a Garn (or other wood gasification boiler with storage, depending on the design), and that is stratification of the hot water in the storage rather than mixing. Stratification provides the hottest water available, which usually provides for the best operation. But that too is system dependent, and if you don't need the hottest water and you can use lower temperature, then lack of stratification is of little concern.

 

[rombi]

Looks like I need to get somebody like jebatty out to take a look at what I have so I don't waste time and money.
Thanks for the advice.

 

[maple1]

You didn't mention anything about it - but temps will also greatly affect DHW heating, if you'll be heating DHW also, and especially if you'll want to be heating DHW with wood in the off-heating season. Just something else to consider.

 

[heaterman]

Get in touch with Kenny Oaks, the Garn rep for Wisconsin. You can get his phone number from the guys at Garn or off their website at www.garn.com

 

[bpirger]

I'm one of those Garn guys with radiant.....love it. But if I HAD to have 140 degree water to heat my house....I'd have to estimate my time between fires would go from about 24 hours to about 8...maybe 12. I'd be bumming. Now, if your HX was oversized you might be able to use lower temps....same with additional baseboard....etc.

With the goal of firing the Garn to 190 (sensor in front well) and not beyond, this gives me a supply temp of about 180 tops, probably closer to 175. When my supply has dropped to 140, my panel temp will say about 160.

So if you fired to a higher temp, i.e. more stored heat with a bigger differential (say 190-140 vs. 175-140), the Garn would be boiling....not desired for water chemistry longevity.

With radiant, when my supply is 115, I'm warm. Can't make DHW very well, but the floors are fine......

My experience says that 175-140 drop happens a whole lot faster than the 140-115 drop, despite the reduced amount of heat. But the bottom of the Garn isn't anywhere near 190....losses are higher at the higher differential when hot.....etc.

But indeed, before you allow that Garn even to grow to much.....learn some more and wait and see. How does your neighbor heat? Will be interesting to see if he used baseboard or air HX how things go.....Help him out and send him here. Hopefully he can oversize his HX.

 

[bailey farm]

I'm one of those Garn guys with radiant.....love it. But if I HAD to have 140 degree water to heat my house....I'd have to estimate my time between fires would go from about 24 hours to about 8...maybe 12. I'd be bumming. Now, if your HX was oversized you might be able to use lower temps....same with additional baseboard....etc.

With the goal of firing the Garn to 190 (sensor in front well) and not beyond, this gives me a supply temp of about 180 tops, probably closer to 175. When my supply has dropped to 140, my panel temp will say about 160.

So if you fired to a higher temp, i.e. more stored heat with a bigger differential (say 190-140 vs. 175-140), the Garn would be boiling....not desired for water chemistry longevity.

With radiant, when my supply is 115, I'm warm. Can't make DHW very well, but the floors are fine......

My experience says that 175-140 drop happens a whole lot faster than the 140-115 drop, despite the reduced amount of heat. But the bottom of the Garn isn't anywhere near 190....losses are higher at the higher differential when hot.....etc.

But indeed, before you allow that Garn even to grow to much.....learn some more and wait and see. How does your neighbor heat? Will be interesting to see if he used baseboard or air HX how things go.....Help him out and send him here. Hopefully he can oversize his HX.

Bpirger, I'm researching a new boiler and garn is one of the boilers in my top 3. I'm wondering how your boiler fared last winter with the long, cold temps we had. I'm in Michigan but I think we have a similar climate so I'm interested to know if it changed your 24 hour burn cycle and to hear more about your radiant setup. If I go with the garn I will more than likely also install radiant under floor and radiators in the upstairs rooms.

 

[eauzonedan]

Rombi
Give Karl a call at Northwind. 1 (715) 630-6451 They're in Stevens Point and within striking distance of you. I just wrapped up my Garn and had him do both the detail design as well as the brainey part of the install. Great to get along with, very knowledgable...and now a Garn dealer. Can't say enough good things about the process. I will get a photo post out when I find some time. Short story is remote purpose built Garn building. Mix of in-floor and radiators along with Dhw in an existing house and shop. Dual temp mixed feeds depending on what calls for heat along with variable speed pumping at the tank which is yielding 25 to 30 degree stratification and thus optimized long run times between firings Throw in an x300 8-lead temperature monitor the system after the fact - and call me a happy camper

Bottom line is Don't be afraid to invest a few bucks up front and get a real design. My experience in the civil engineering biz tought me that $1 spent there will save $10 later on. When it comes to this stuff one size does not fit all

Open for tours if you get up this way and good luck

Dan

 

[jebatty]

Heating with low temp radiant vs high(er) temp forced air are two very different applications. And if your system is pressurized and you are using an unpressurized boiler and/or storage, normally you can expect to have unavailable about 10F of hot boiler water with a "normal" heat exchanger sizing. Example: unpressurized boiler/storage hot water temp of XXXF, maximum delivered from the heat exchanger to the system would be XXX-10F. You can achieve a closer approach temp but the expense in heat exchanger to accomplish this can be considerable.

Next, if you need 140F water, your available stored btu's are only from the high temp of storage (assume 185F) and the needed 140F. Then if unpressurized to pressurized, take 10F off the high temp. And this too is complicated, because the storage may not be fully mixed to the high temp, and the average high temp may much less than the high temp -- thus, even less stored btu's actually available.

The simple point is that if you need high temp water, you will be burning often to maintain a usable water temp that meets your heat demand. This also is true with a pressurized boiler/system, but the pressurized system may provide an additional 10F of stored btu's. With a 1000 gallon tank, that's an extra 83,400 btu's.

A Garn with sufficient tank size and a gasification boiler with sufficient storage can both excel with low temp radiant. But if 140+F hot water is needed and a pressurized system, IMO the scales tip to a gasification boiler with storage, mainly because of the extra 10F of hot water that normally would be available with a pressurized boiler/storage. This assumes that the btu rating of the Garn and the pressurized boiler are the same and the storage amount is the same.

My direct experience in these area is with a Garn WHS3200 (unpressurized) and a Wood Gun E500 (pressurized), both serving the same pressurized system.

 

[jebatty]

Dan gives excellent advice on using a pro to design a system that meets your needs and expectations. Don't shortcut the design without expecting to pay a big price down the road.

 

[wardk]

I'm one of those Garn guys with radiant.....love it. But if I HAD to have 140 degree water to heat my house....I'd have to estimate my time between fires would go from about 24 hours to about 8...maybe 12. I'd be bumming. Now, if your HX was oversized you might be able to use lower temps....same with additional baseboard....etc.

With the goal of firing the Garn to 190 (sensor in front well) and not beyond, this gives me a supply temp of about 180 tops, probably closer to 175. When my supply has dropped to 140, my panel temp will say about 160.

So if you fired to a higher temp, i.e. more stored heat with a bigger differential (say 190-140 vs. 175-140), the Garn would be boiling....not desired for water chemistry longevity.

With radiant, when my supply is 115, I'm warm. Can't make DHW very well, but the floors are fine......

My experience says that 175-140 drop happens a whole lot faster than the 140-115 drop, despite the reduced amount of heat. But the bottom of the Garn isn't anywhere near 190....losses are higher at the higher differential when hot.....etc.

But indeed, before you allow that Garn even to grow to much.....learn some more and wait and see. How does your neighbor heat? Will be interesting to see if he used baseboard or air HX how things go.....Help him out and send him here. Hopefully he can oversize his HX.

I am also a garn owner and go through a HX to pressurized secondary, the storage circulates continuously so I think the temp should the same though out. I have radiant and WTAHX about 160,000 btu it has an electric valve on it's loop so that when the thermostat calls for heat the valve opens and the fan starts just like normal. I rarely take the Garn up to 180 , usually run 150 to110 and the WTAHX is still effective at those temps,