existing underground pipe is gonna make or break the decision

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fgable

New Member
Sep 24, 2011
1
SE Pennsylvania
Hi folks, looking for some feedback.
3400sq ft stone farmhouse with detached garage. Recent energy audit on home rates as average (minimal energy upgrades recommended)
200k BTU oil burner with 2 zones. 1 zone has additional loop underground to detached garage via 1 inch steel pipe - 80 foot run
Spare controller wires available from house to garage (glad I pulled extra wire)
Oil usage, about 1400 gal/year. Summer hookup for electric hot water heater. On cold days, (20F-30F), oil burner may cycle 2X/hour
Wife stay at home now with little ones, cannot turn heat off 8-3 like I used to :-( . Plan heat to run 6 AM to 10 PM <ugh>
I'd like to buy an old used Tarm for $1k from craigslist to experiment, but I don't want to smoke out the neighbors, New Gasification boiler in sites.
Wood supply practically free. I enjoy burning. We figure ROI in about 5 years w/o thermal storage.

1.) Will the 1 inch steel pipe be sufficient? (2 manufacturer feedbacks: pipe can only deliver 107k BTU-hour/1 inch is fine go for it/go bigger boiler to reduce cycling)
2.) Given Pennsylvania heating season approx October-April (4 months of COLD/3 shoulder), will additional $2k investment on pressurized thermal storage payback. I think this is a yes.

Thanks guys, great respect for the knowledge around here.
Frank
 
fgable said:
1.) Will the 1 inch steel pipe be sufficient? (2 manufacturer feedbacks: pipe can only deliver 107k BTU-hour/1 inch is fine go for it/go bigger boiler to reduce cycling)
2.) Given Pennsylvania heating season approx October-April (4 months of COLD/3 shoulder), will additional $2k investment on pressurized thermal storage payback. I think this is a yes.
The biggest factor is the deltaT through your loads. Is this water to air heat exchangers, conventional radiators, staple-up radiant, baseboard, in-floor radiant, panel radiators, ... ?

The fundamental boiler room equation is:
Code:
btu / hour = 
   (gallon / minute) * 
   (60 minute / hour) * 
   (1 btu / (lb * degF_deltaT)) * 
   (8.33333 lb / gallon) * 
   (degF_deltaT)

A.k.a.: btu / hour = gpm * 500 * deltaT.

A 75 watt high-head pump would push maybe 7 gpm cheaply and easily through 160 ft 1 inch pipe round-trip. You could pump maybe 10 gpm with about twice the power. But as you try to get more gpm the power required goes up very steeply, so an upper limit of 10 gpm or 4 feet per second should be reasonable.

Just for feasibility figuring, assume 1200 gallons heating oil burned over 100 days at 80% efficiency, the equivalent average btu per hour delivered through the 1 inch pipe would be 1200 * 140000 * 0.8 / 100 day = 56000 btu / hour. So now what is a generous estimate of peak demand relative to the 100 day average demand? Call it 3X and we have a 170000 btu / hour peak demand.

deltaT = (170000 btu /hour) / (500 * 7 gpm) = 48.6 degF

deltaT = (170000 btu /hour) / (500 * 10 gpm) = 34 degF

So if your heat emitters can pull heat out of the water effectively enough to go from 180 degF down to 130 degF you should able to pull 170000 btu / hour easily through a 160 ft of 1 inch pipe with a small amount of pumping power (e.g., Taco 008).

But if you can only get a 20 degF temperature drop through baseboard or water air HX, then the 107000 btu / hour number is more realistic and you won't be able to keep up on the coldest days, assuming peak load is 3X average, and assuming the average btu / hour estimate is realistic.

To me the other big question is how well insulated is the underground 1 inch pipe?

---ewd
 
fgable said:
.... 1 zone has additional loop underground to detached garage via 1 inch steel pipe - 80 foot run ....
Welcome to the forum. That one jumped out at me, even though I'm not a hydronics guru like EWD. It sounds like you are currently heating the garage via the 1" pipe on zone 1, and are thinking about installing a WB in the garage and converting that line to feed into the house - is that correct? Right after installing my WB, the basement was roasting hot from all of the new steel near-boiler piping. After insulating this (less than 80'), the temp drop was around 15-20 degrees. And if you check the archives you'll come up with a number of horror stories from folks having problems with underground pex insulation jobs. If your pipe is insulated, it would only take a small failure allowing water infiltration to get some significant heat loss going on. And, if it's not insulated, then you have one heck of a heat emitter there - probably makes the worms dance a jig. Unless you're certain the insulation is rock solid, how much of a job would it be to pull it up and replace with the good stuff? Although you say that pipe is making or breaking your decision on the WB, you may get a fairly quick payback by changing it out anyway (or just stop heating the garage). One easy way to check the pipe is to wrap an inexpensive contact thermometer in insulation on each end, and see what the heat loss is. Also, is a garage WB allowed in your area? - lots of places say no to that. If I am misunderstanding you system or your plan, it wouldn't be the first time for that :p .
 
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