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[Gen Patton]

I'll start with saying thanks for the forum, i have studied for hours on here,
these are probably basic questions that have been covered, but anyway
background
1. wanting to build an OWB + solar system to heat 1939 built 2 story 1700square foot house and 40 x 70 x 16 foot side wall garage, wall and ceiling total insulation value average around r10-12, 2inch foam around slab, no insulation under slab,
2500 foot of 5/8 pex in 6 inch slab on 1 foot centers
no plumbing in house as of yet, plan to lay sleepers for 1/2 pex under eng. laminate hardwood, to that end i want the system to run at an operating temp around 90-110f, will have a 1000 gal insulated tank in the garage for storage, goal is to let solar do most of the work and only build fire as needed but i've read that low boiler temps cause corrosion??? is this an issue,
would like to build a fire in the morning or noon to bring the tank up to 130-140 only burning one wheel barrow load a day, unless super cold
have seen a few homemade systems running like this with no issues but want to see if anyone on here has a similar setup
will run a heat exchanger to the tank anyway, i could run the boiler at a higher temp for less time, but dont want to if not needed
here are a couple pics if i can load them
thanks for any pros or cons

[willieG]

2500 feet of 5/8 pex in the slab and no insulation under the slab. Normally (with an insulated slab) you would be told to use about 7 to 10 loops, with no insulation under the slab I would shorten the loops and do 15 or 16 loops
 radiant heat is a slow recovery heat and you must have your btu able to keep up with your rooms heat loss. you need to know what that is so you can plane for how close your floor loops need to be. you may find you need 2 runs in each space between your floor joists (upstairs) to achieve the btu per square foot you require

the only way to increase btu in a radiant system is 1-increase the flow or 2- increase the temp or 3-increase the feet of pex in the system (add more loops of the same length)

[Gen Patton]

sorry, left that out, everyone says 1/2 pex 300 foot max, so went with 10 loops of 250 foot each 5/8 pex,
i thought i was overkilling it but may still need more
i have looked all over the net
is there any place that gives pex/ slab btu output at given temps?

[Wood Nutt]

Here is a post from another site about the solar/OWB combo.  I was considering it too, but this post makes a lot of sense so I talked myself out of it!  But, something for you to consider.


For those who are considering adding solar heating panels to their OWB furnace, I tried it and here is what I learned. Now, I don't want to rain on anyone's party, I'm just offering my experience and observations for your consideration.

After installing my first OWB, a Shaver 160 Pro, I thought it would be a great idea to add solar heating panels to provide heat for DHW during the summer and to reduce the amount of wood burned in the fall and spring. I had run the OWB for about 1 1/2 years at that point and had a baseline for how much wood I would expect to burn. After extensive internet research I settled on plans to build the solar panels at a cost of about $375 each plus the differential controller. Total system cost was around $1000 including the pex lines and insulation. The panels worked great! But after running them for about 2 years it was painfully obvious that pairing the panels with an OWB proved to be a big mistake. It seems that there are good reasons that no major OWB manufacturer offers solar panels as an option to their systems.

The mistake lies in the way an OWB operates and the inherent heat loss that is necessary for an OWB to be controllable. In order for the temperature of the OWB to be controlled it MUST be able to shed a little more heat than is generated by the smoldering coals in it while it is idling. If it did not lose enough heat you would soon begin to boil off the water and would have a ruined unit. Those smoldering coals produce a significant amount of heat that has to be lost to maintain the desired water temperature. In my system with no fire burning in it the solar panels collected enough BTU's during a good sunny day to heat the OWB to almost 150 degrees. That is just enough to provide DHW in the summer. In order to minimize heat loss I made a box out of 1 inch foam board and covered both the loading and the ash clean out doors since they acted like a giant heat sink to remove heat from the system. I also would put a foam insulation plug in the chimney to prevent convective heat loss out the top. Unfortunately, I found that the system would lose 30 degrees or more heat overnight and would be too cool for making DHW by morning shower time.

A second consideration is that the efficiency of BTU collection in the solar panels is greatly affected by the temperature of the water being supplied to the solar panels. During the winter when there is a fire burning in the OWB and it is up to operating temperature the water is already too hot for the solar panels to be of much use. Once the supply water gets to 180 degrees the amount of BTUs that are absorbed from the solar heat is very small. There has to be a fairly large delta T between the supply water and the solar panel temperature for there to be much heat transfer. I'm sure a much more expensive panel would have improved on this a bit, but all heat transfer systems have this attribute.

In summary, during the summer with no fire in the OWB the system could not retain the collected heat well enough to provide the required DHW and during the heating season the fire would keep the water too hot for the solar panels to be efficient enough to result in a noticeable reduction in wood usage.

If I were to try solar water heating again I would have it heat a separate extremely well insulated water tank and run the cold water supply to the DHW tank through that tank first and then during the heating season it would run through the OWB heat exchanger second to bring it fully up to temp.

I hope someone finds this information useful!