Heat exchange setup question

[klocke]

Hello Folks,

This is my first post here but I’m sure it won’t be my last. I have decided to install a wood gasification boiler with storage in my home next year and I need some help. My current oil fired hot air furnace has a cracked heat exchanger (hx) and I need to replace the unit in the next two weeks. When I have the new unit put in I am going to have the sheet metal done for the hx as well. I currently have a hi-boy furnace and I’m thinking of replacing it with a low-boy so I have room on the supply side for the hx. It is going to be tight even going with a low-boy but it should work.
Here is my question to you all. The plumbing supply house told me that I could put the hx on the return side of the unit if room above is a problem. This sounds good but my concerns are having 160-180 deg. air going across the blower. Is this ok? Am I going to loose efficiency this way? Is this a bad idea? He also told me that this would simplify the thermostat setup and I could use the current one. He also told me that I could adjust the furnace control so when the house calls for heat the blower would come on and the heat from the hx would cause the oil burner not to fire. Is this correct? Any help would be greatly appreciated. Also, I’m looking for books or web sites that will help me with the installation of the boiler, boiler controls and hardware, and water storage sizing.

Thank you,
Ken

 

[Hunderliggur]

Welcome to the Forum! You said you were looking for web sites for help? This is the best one I have ever found. Use the search, look at some of the "sticky notes" (like pressurized storage) and you will find a wealth of information. Don't be afraid to ask questions, most of the folks here are very helpful. I don't have the answer about the return HX and the blower, but in a perfect world I would not do that. However, is perhaps the blower belt driven by an external motor? In that case, the hot air over the fan blades won't matter. If the hot air IS going over the blower motor, then you should check the specifications on the blower motor for max operating temperature. You can set it up that your furnace will not fire if the return temp is high enough. Don't know what kind of control you have, it may require another very simple furnance control - the main house turns on the blower, and the simple control turns on the burner if the return air < set point AND the blower is on. Oh yeah, the first thing someone is going to ask "Do you have a heat loss calc so you know what size boiler you need?" Residential furnances are notoriously ovesized (the supplier doen't want to get called to say it is too cold in your house and the furnanc can't keep up). In your setup, a gasser that is size appropriately will work great with the inline backup for those really really cold days (unless you are trying to go 100% renewable). Planning a year ahead is not a bad dea!

 

[klocke]

Hunderliggurm,

Thanks for the reply. I had a heat loss done and it came back at ~93.5k btuh. I am planning on buying a Econoburn 150 or maybe a 200 with a STSS 800 gallon storage tank for the basement. My house is 3000 sq/ft on the first two levels and a 500 sq/ft finished basement. I am also planning to add some heat to the garage to keep my shop at ~60F. So maybe the Econoburn 200 would be the better unit for size. I’m going to buy the boiler by the end of the year to get the tax credit and set it up next year. I have been splitting wood on and off all summer and have ~20 cords split and stacked with 5 more to go. The wood will be seasoned anywhere between 2-4 years by the time I’m online. I figure 25 cords will give me a 3 year supply for heating the house and the domestic hot water.
As far as the HX goes I feel better putting it on the supply side. I’ll see if I can move the controls around to accomplish the same thing as putting it on the return side. I was hoping that more people would chime in with different view points. One thing I was planning to do was to setup the HX so it could be removed in the summer time to reduce the flow restriction when I’m running the AC. I have heard people complaining about poor flow after adding the HX and felt this would be an easy way to solve that problem. Any thoughts on doing this?

 

[Hunderliggur]

Of course you will buy the boiler and have it INSTALLED by the end of the year for the tax credit. Pictures of the "installed" until will help (if it is in its shipping crate that would not count). How you handle the pics is up to you.

I would not worry too much about the airflow restriction of the Water/Air HX during cooling with a properly size HX. Way too much work to move it out. You could perhaps have the sheet metal man build a basic bypass around the unit to us seasonally.

You should be able to put the HX on the supply with a differential controller for the boiler or some other setup. There are many ways to do the control (look for control or monitoring threads on this site) including storage tank temp) so I am sure you can make something work. One of my fall projects is to put in some monitoring on my system using Dallas 1-wire devices (OK - I'm a geek).

 

[Medman]

I agree that airflow restriction of the HX is minimal if it is properly designed. Search some of the threads about this, especially pybyr's threads on the custom coil he ordered. Just be wary of the cheap coils available on ebay and elsewhere - you get what you pay for. A custom coil, made to suit your particular needs and application, won't cost that much more than an off-the-shelf coil and will work much better.
I personally would not put the coil in the return due to the damage it could do to the A-coil or the fan motor. I know my A-coil isn't rated for 190*F temps below it.

Control issues seem to be the one area for which we don't have a good set of standard solutions. Everyone seems to do things a little differently, depending on your level of expertise with computers, electronics, and mechanical controls.