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[juddspaintballs]

It could well be leaking air into the system all the time and the pumps might just be overcoming the water rushing back down the hill in the return lines via gravity during pumping.  In a way, that would actually be reducing the head pressure and allowing the pumps to work with a little less effort. 

Halfpress, you could go with one large shell and tub exchanger rated for 200k btu's (same website I linked earlier) and just Tee both 1.25" lines into it.  You would keep your existing pumps to pump both 1.25" PEX lines and rather than connecting the supply and return pex lines to your Heatmor, connect them to the exchanger.  Run the other side of the exchanger to the Heatmor and use a small pump to circulate that water (Taco 007 is cheap at around $60).  The only reason I didn't recommend a flat plate heat exchanger is because you're moving such a large volume of water and I don't believe the flat plates will handle that much water flowing through them all the time.

[yoderheating]

 I have a hard time believing a air leak would cause drain down only when the pumps are running and the system is under a little pressure. If you had a air leak I would think the problem would continue after the pumps were stopped. Why would you have a sudden air leak and resulting water loss only when the pump shuts off but at no other time? That being said I have never installed a Heatmor and don't understand the bladder system completely.  I would love to see what others think on this.

[willieG]

after thinking long on this problem i thnk (yonders suggestion) of isolating the  old house system form the OWB is the best route for you to go. first i would isolate the two systems and do a pressure check on your house sytsem (i take it that it was a closed pressurized system in the past) if this system holds pressure you have no leaks and in fact the water loss was bleed out do to siphoning at the furnace when pumps were off

if this is also true than to seperate the systems with an exchanger is what needs to be done

as for the size of your exhanger measure all your rads  into square feet and multiply x 150 (this is how many btu per hour your rads can produce with 170 degree water) if this was a steam system in the past then multiply by 170 to give you the maximum amount of btus your old system could produce.

if your pump is sending water to your exchanger at the rate of 10 gpm and your water is 170 you will be delivering about 100000 btu per hour to the home with a delta of about 20 degrees, as long as your total square footage of the rads does not excede this you should be fine (you did say earlier that the home did heat well)

[halfpress]

 Willie, Judd and Yoder:

  Many thanks for all the feedback. I am definitely leaning toward doing the exchanger. I've already asked my dealer to price out the parts with that design in mind.

  I know my pumps are Grundfos XL models with three speeds. I'm currently running them on low. They are natively 1.25" inlets and outlets to match our PEX supplies and returns.

  So I'd have these two pumps running the supplies from the furnace into the house to the exchanger(s). I'd have another new pump or two on the house side after the exchanger to circulate through the radiators.

  Are there any other key parts? Pressure tanks of any sort? It's supposedly a closed system, so I don't guess I need any kind of automatic low-water feeder like our propane steam boiler used.

  The old steam furnace we had was rated at 171,000 BTU and could heat you out of the house. The 200CSS is supposedly a 200,000 BTU unit (under, I suppose, optimal conditions). So I figure if we get into that 170,000 - 200,000 BTU range on the exchanger(s) we'll be good to go.

  Is there any significant loss of efficiency in the exchanger approach vs. our original attempts here for direct, whole-house circulation? Possibly any increases in efficiency? I want to know if introducing this intermediate element into the system is costing us any real loss of heat transfer from furnace to house in the grand scheme of things.

  Finally, we'd be mounting this system in our, uninhabitable, hellish dungeon of a basement where the furnace lines come into the house. Does the exchanger get mounted in any kind of special insulated housing? If not, does it make sense to build a housing for it full of R13 fiberglass insulation or some such thing? I'm planning to foam-wrap all of the accessible under-house radiator lines, too, to keep as much heat in the system as possible and out of the basement where nothing would benefit but critters.

Thanks again!
 - Aaron

[juddspaintballs]

  So I'd have these two pumps running the supplies from the furnace into the house to the exchanger(s). I'd have another new pump or two on the house side after the exchanger to circulate through the radiators.

  Are there any other key parts? Pressure tanks of any sort? It's supposedly a closed system, so I don't guess I need any kind of automatic low-water feeder like our propane steam boiler used.

If you're doing the heat exchanger in the basement, I'd imagine your pumps you currently have might be big enough to put one on one side of the exchanger (moving water from the furnace) and the other on the other side of the exchanger moving water through the house.  You'd have to look up Grunfos's pump tables and calculate your head for each side, but if two pumps can pump the entire circuit, one pump is probably big enough to pump half the circuit and the other pump big enough to pump the other half. 

I think you will need a small expansion tank on the closed circuit side (house).  I'm not familiar enough with closed systems to tell you whether you need an automatic water feeder...but I don't see how one could hurt either. 

[yoderheating]

 For that many radiators I would recommend a 35 or 40 gal expansion tank, that may be a little over sized but for something like this you don't want to under size it. Make sure you get one that is rated for hot water. A automatic water fill would be nice. 

[juddspaintballs]

And yes, if you insulate the heat exchanger, that would be ideal.  There would be little to no loss using the exchanger route. 

[halfpress]

Quick followup to, again, thank everyone and to say that we're likely going to pursue the exchanger design.

I'm also going to give the pipe network feeding the radiators a good hard look again and make sure there are no loops or other oddities in relation to how our two pumps and the associated feeds and returns interact. I have indications the two pumps aren't serving two isolated loops of radiators (water into one feed with a hose dumps water out of both returns once the system is full)... so I just want to rule out that causing headaches.

HOWEVER, for some freakin' reason, things are working very well right now. I had valved off two radiators that are not bleedable (no bleeder valves and not feasible to install them) and then topped off the furnace sometime last week. I witnessed it blowing water out the top when the pumps stop (as described in my earlier posts) and the bladder ultimately going flat. Nothing had changed, basically.

A few days later, I topped it off again just for good measure. Since that time, it's been working beautifully by all indications. I don't know what changed. The main indicator, though, is that the bladder has stopped going flat for the first time... it gets thinner when the water temp is lower and the pumps are running, but gets quite full (but not rigid) when the water is hot and not circulating. Previously it would be totally flat no matter what within about 24 hours. This has been almost a week now.

The only things that seem different are that those radiators are valved off... but they were between the last two fills and only the most recent time around has seemed to work... and the wood I am burning has changed.

I was initially working from a supply of fireplace-sized, VERY seasoned split oak and some semi-seasoned ash from a tree we had to drop this autumn (been dead a few years). I was going through this wood like crazy, as you'd imagine: very dry, small, split (lots of surface area). So it made hot fires and heated up the furnace fast, but was eating wood like mad.

I got a huge load of enormous, greener oak logs. They, too, are split - but only to the point of making them manageable (and even some of those are a stretch). It's a real workout getting them in the furnace, but it has me down to two loadings a day and I'm no longer seeing overtemp issues. I hope my next load can be unsplit 6-8" logs... working on that.

Again, I don't see how the wood would have a real impact here unless something about the overall heat has an influence.

Keep ya posted!
 - Aaron

[juddspaintballs]

My Heatmor has been acting up lately and boiling over.  I've traced the problem down to a sticking flapper over the blower motor's output.  You might want to take your blower off and clean that flapper.  Maybe follow my thread too:

http://outdoorwoodfurnaceinfo.com/forum/index.php?topic=780.msg5076#msg5076