Posted this in a different thread, but thought it deserved its own. Haven’t seen too much discussion of GPM rates as applied to hydronics in a slab:
question about the GPM that I should be pumping at. This is a single zone, 3 – 250′ wirsbo loops stapled to the 2″ rigid under the 24 X 29 foot 4″ thick slab. Has a Rehau (?) manifold and a Grunfos “medium head” three speed pump controlled via slab sensor and SP 30 Goldline unit. Heat source is a 50 gallon AO Smith propane WH.
Initial setup has the temp going in at ~100 degrees and coming out at ~ 80 degrees, running at 14 psi. Hardly needs to go on at all to maintain a slab temp of 55 degrees. Actually, the 10 -150 watt bulbs that provide overhead light warm the air temp up 4- 6 degrees in a ‘couple of hours with outside temp at 20 degrees. R40 ceiling, R23 walls, no windows, two insulated garage doors, one man door. Purpose of the system is just to maintain a comfortable baseline temperature to work in, with a side benefit of helping to keep the well pressure tank, BIRM filter, etc, located in an adjacent room, from freezing.
The manifold has little red knobs on the return side that screw in and out, controling the pins, which in turn affect the GPM readings on the little floating meters right above the valves. When wide open, the meters read almost 2 GPM. What should I run them at? Are they intended to provide the right flow so you maximize the amount of heat passed to the slab, or to control the flow to individual circuits? Or both?
Should I run them wide open?
Edited 11/11/2003 9:58:17 AM ET by johnnyd
Replies
you might get answers here, but may I also suggest to check over at http://www.heatinghelp.com? They specialize in hydronic heating.
Norm
Thanks, I'll give it a try.
250 foot loops are pretty long for a 4" slab. Unless it is 3/4". 5/8" maybe okay but 250 feet of 1/2" will limit you to 2.5 gpm or so. And therefore limit the BTU delivery rate. Also, during period of high high input (sudden cold snap), different sections of floor will have very different temps.
But it should work okay because you are only trying to maintain a slab temp of 55 degrees, it is bare concrete, you've got mild outside temps (20F) and a well insulated structure.
I'd run the valve wide open - why make a pump create pressure just to waste it across a valve? Unless there is some balancing you wish to do. Maybe you want the section of floor under the taters cooler to be a better root cellar. Or the cloor under the parked car to be warmer to melt snow off the car and evaporate the drips more quickly.
One might design a house with one zone but play with the valves to keep bathrooms warmer and bedrooms cooler. But only if the tubing was laid out with that in mind.
Love my radiant-floored garage when is -40F outside. (that's -40C for you Canadians).
Hmmmmm....
One poster over at heatinghelp.com said he thought my circulator pump is grossly oversized. That 2GPM was way more than what is necessary to keep the slab at baseline temp.
BTW, it is 1/2" pipe, and it does get -30F on the coldest winter day.
What is the relationship between flow rate and btu transfer?
Anyway, I've got to look at the gauges again tonite, and I'll play around with the three speed pump a bit.
This is a little tough to do during relativly warm weather.
Basic formula:
Btu=gpm x (Tin-Tout) x 8.34
if you want Btu/hr, typically used in heat loss calculations, then multiply by 60
quick number 20 deg delta = 10,000 Btu/hr
If your doing your own design get John Siegenthalers book Modern Hydronic Heating available from Amazon or, I think, on heatinghelp.com
"One poster over at heatinghelp.com said he thought my circulator pump is grossly oversized. That 2GPM was way more than what is necessary to keep the slab at baseline temp."
Not sure which Groundfos pump you are using, but if it is a UP 26 of one flavor or another, yes, that seems like a lot of pump for three 1/2" loops. Its higher pressure would give a touch more flow, but flow is most strongly a function of pipe diameter, not pump pressure.
If it is a UPS 15-42, that's seems like the right size. And the electric consumption will be less with the lower wattage motor.
If you in a mood to play with it a bit, feeling the floor temp in various places can be interesting. A non-contact infrared thermometer (hand-held, point&shoot from several feet away) is really quick but not necessary. Seeing the range of temps in the floor will give you insight about how the tubing layout effects that. If completely uniform temps are important, laying the return next to the outgoing portion of the loop for its whole length works best. If you want some areas warmer than others, take the outgoing tubing there first.
Even in mild weather, you could crank the thermostat to confirm that you can create the design goal of a 85F differential with outside temp. That is, if you don't mind bringing the garage up to 100F. Obviously, outgoing water temps need to be bumped up and that shouldn't be done if you can vinyl flooring products that can discolor.David Thomas Overlooking Cook Inlet in Kenai, Alaska
Turns out I was mis-reading the flow gauges...jumpy little buggers...they have a 2 GPM indicator, but the flow really is just UNDER 1 GPM in each loop. I'll check the pump model # tonite.
By the way, given the fact that I'm not really pumping against anything except friction in the tubes, what pressure should I be running? Indicates ~14 pounds now, and I could adjust that just by hooking up the hose again, or releasing some of the circulating water into a bucket.
Also am getting more consistent in-out temps now than when the slab was first ramping up.
This is more fun than the erector set I got when I was a kid!
Snowing and blowing 35+ mph out of the NW here in SE MN...what's it like where you are in Alaska?
Edited 11/12/2003 2:27:19 PM ET by johnnyd
Just got our first dump of the year. 10 more inches forecast for this week. But at 30-31F, the road are slick. So - Come to Alaska and learn to drive in the snow! Seems to be what everyone else is doing!
The 14 psig pressure on the gauge is probably (almost certainly) total system pressure. Not differential pressure that the pump is creating to circulate the water. Total system pressure need to be above zero or many pumps will cavitate (vaporize water into water vapor at their inlets and the resulting slurry of gas & liquid is very abrasive to the pump impeller. 15 psig is great for system pressure. It will flucuate as the water in loops heats and cools. I assume/hope you have an expansion tank in the loop. That will dampen out the flucuations. But going down to 12 and up to 19 is no worry. A decreasing trend to zero, however, and you should be looking for a leak.
Differential pressure (from pump preformance curves) of around 5 psig = 12 feet head are great. Lots more than that - 30 psig, for instance - will move you out of laminar flow and into turbulant flow without moving much more water. Turbulant flow does offer better heat exchange, but after 250 feet of tubing I guarantee that it is returning very close the slab temperature. i.e. that most all the BTU have been delivered. Systems with small, high-pressure drop valves and/or multiple heat exchangers (to separate gycol from water) may need more differential pressure to push through all those components.David Thomas Overlooking Cook Inlet in Kenai, Alaska
Yep, got an expansion tank, air eliminator too, and you're right, the valve I'm looking at is total system pressure. Sounds like it's running about right. Only problem is, my feet get too warm, even with slab temp of 60F!
I run big nasty snow tires on a SAAB 9000, all four corners. Often don't even notice it's slippery until I see other folks sliding around.
My wife spent quite a bit of time in Alaska before I met her. This was ~ 25 years ago in the Talkeetna area. We periodically plan (dream) to get up there but it hasn't worked out yet. Only state of the 50 I have never been to.
Edited 11/12/2003 3:35:38 PM ET by johnnyd
I can't pronounce it, but I've got Nokian's Hakkapeliitta 1's on all our cars and have been happy with them. The studs flex in enough to greatly reduce stud wear on dry pavement which is what you drive on, most of the time, even in snow country.
When I'd gotten to all 50 states* in 1986, Alaska was the one I was going back to the most often. Took me another 12 years to move up here but I did as much work and play here as I could.
*Now I'm working on getting to all 3,100 counties in the US. Up to 1,850 so far.David Thomas Overlooking Cook Inlet in Kenai, Alaska
I am now hooking up the pumps pressure gauges and piping to the manifold of my new system. When I asked about pressure the designer said that below 10 psi system pressure the pumps lose efficiency and may well stop pumping. He recommended pressurizing the system to municipal pressure levels as long as it was below 120 psi.An ex-boat builder treading water!
"He recommended pressurizing the system to municipal pressure levels as long as it was below 120 psi."
I'd agree. And muncipal would almost never send you 120 psi. If so, P&T's would be blowing off in the lowest spots in town at 2 am when the pressure is highest.
A nice thing about connecting directly to* muncipal is that you can blast the air out the system really nicely. The low flow and pressure of a circ pump takes a while and I always wonder if some air was left behind at a high spot.
* Local or National code may well require a check-valve or some other kind of isolation between a heating system and potable. I use a washing machine hose to temporarily connect valves on the potable and heating system side when I fill or purge the heating system. Then I disconnnect it. I aim to plumb with no leaks so don't feel a need for a permanent one-way make-up water valve.
"below 10 psi system pressure the pumps lose efficiency and may well stop pumping"
For sure, that can happen. That's why minimum pressures are spec'd in the manufactruers literature. Do you need a higher psig at the pump inlet in Leadsville, Colorado since atmospheric pressure is 4 psi less? I believe so.David Thomas Overlooking Cook Inlet in Kenai, Alaska
Dave, I have been reading this thread and appreciate the fine info you have posted. Our next house will have some radiant floor heat, and some forced air. Working on the house design now.
Years ago I converted our den fire place to nat gas. (Still have a wood burner in basement.) Since I didn't want to waste the nat gas heat, I left in a fluid filled grate over the gas burner,(used it with log fire for years) and put the ceramic logs on the grate. The distilled water/antifreeze mix is pumped to a coil in the central air system. We get a 25-30 deg air temp rise across the coil, which lets the fireplace heat the downstairs when fired. It would have made a nice system for my garage/work shop floor, but that had already been poured years before.
It has an air eliminator, expansion tank, pressure gauge, and two 30# safety relief valves. It is a miniture of the system you have been discussing, but all the readings are very similar. It is pressurized to 12# when cold, and goes up to 15-16# when hot. The cold reading is my indicator that I do not have a leak, as the system has a lot of pipe, some outdoors, a lot in basement ceiling and walls.
If our electricity goes out, a gas line solenoid valve closes off the gas, and if for more than 5-6 seconds, the automatic thermo closes on the gas burner. If the pump fails, (never has), then Sierra antifreeze bathes some shrubbery behind my chimney. I check the Ph of the antifreeze mix, and change it about every 8 years.
You live in a truly beautiful area. We drove from Anchorage to Homer(took all day because of the stopping for pictures)to charter a plane to Katmai/Brooks to see the big fuzzys gobble salmon.
Spectacular flight and very exiting to be only a few feet from the bears. Our photos have been the cause of other friends to take the same trip, sorry about that. How many hours of daylight do you have now?
PaulEnergy Consultant and author of Practical Energy Cost Reduction for the Home
"I converted our den fire place to nat gas"
Nice design to include a normally-closed solenoid to shut off NG in a power failure. And then the normal pilot-light so it doesn't come back on when the power is restored?
I converting a couple dozen wood stoves to domestic hot water by putting a SS HX inside. No quick way to stop a wood fire so a special P&T just off the stove was plumbed to blow hot water to a dry well outside (buried bucket of gravel).
"Katmai/Brooks to see the big fuzzys gobble salmon"
When my wife was booking reservations in the campground, the reservation agent said, "You're taking an infant to Brook's Camp? Are you insane?" (Drake was 5 months old at the time). I would have replied sarcastically, "Yeah, and we're going to dress him up in a salmon outfit and leave him on the beach unattended!" He was never off our bodies the whole time and it was never an issue.
However when the airlines (3 flights from Kenai to Katmai) lost our luggage and we were in a bear-infested campground without stove, tent, sleeping bags or DIAPERS! That was bothersome until the luggage arrived the next day.David Thomas Overlooking Cook Inlet in Kenai, Alaska
Dave, yes to your question, the solenoid doesn't kill the flame for just a lightening flash outage, but in 5-6 seconds the flame failure takes care of the auto pilot safety. Having a coil in the fireplace, I wanted no chance for a steam explosion. I did experience a safety release once.
Had just fired fireplace and was on my way to the basement to turn on the pump (I know, wrong order to do things), when young girl in neighborhood had icy road accident into fallen tree limb in front of house. When I got back to the house, I heard the spray of hot mix, then it stopped as the cool fluid pulled the temp down for a minute or two, and then another release of steam and antifreeze. The good news, the system worked perfectly.
There is one caution, especially using wood. The creosote builds up faster because of the heat captured by the water. Not a big problem with nat gas, but you might want to tell your customers to check the flues often.
Re Brooks: Did you stay in those windowless wooden cabins close to the main meeting hall/resturant? After meeting two bears on the trail to the falls, I can not imagine heading for the restroom building at night! Heard recently about the guy who thought he was part bear.
Too bad they had to destroy two bears because he wouldn't respect their territory.
I would go back in a heartbeat, one of the most beautiful/exciting days ever. PaulEnergy Consultant and author of Practical Energy Cost Reduction for the Home
How about wiring the solenoid through a pressure switch that detects pump operation? It addresses power failures (as now) but also liquid loss, pump failure, and that most common one, operator error :-)
David Thomas Overlooking Cook Inlet in Kenai, Alaska
Love my radiant-floored garage when is -40F outside. (that's -40C for you Canadians).
Good one Dave!An ex-boat builder treading water!
I was always under the impression that a heated garage was a bad idea, in areas where they salt the road; corrosion rates are faster when temp is higher. But, as a friend said yesterday: screw the car, i want comfort!!