Hello everyone, looking for a little help here. I have a slab radiant heat system in my cabin in northern MN. Its a Thermolec electric boiler, 6KW, with about 900 feet of tubing in 4 mostly equal loops. My problem is it just doesn’t get warm. The water temp flowing through it never seems to get above 105-107 degrees. If I shut down 1 loop, it’ll get up to 120, if I shut down 2 loops it will quickly get up to 135 or so.
I’ve talked to the place that sized and sold me the system and they say that the size is right on the edge of being to small (cabin is 864 sq ft on main level) but should be enough. They asked about what pressure I am running in the system. Due to some issues I am having with sealing around the pump, I am only running at about 1-2 PSI. They said that if I can get it up to 15 PSI I should see an improvement due to faster water flow?
Also, they are saying that it’s reccomended to only run water at 120-125 degrees. Is that correct?
The cabin has 2″ insulation below the slab and R38 in ceilings and R21 in walls. There is a 21′ ceiling in the living/kitchen area and then an enclosed bedroom upstairs(about 400 sq ft) that has electric base board heat in it.
This weekend it was 0-5 degrees most of the weekend and the cabin temp never got above 60. It was actually dropping in there on Sunday. Not what I wanted when we decided to build a place up there.
Is the company that sold me my system giving good advice or is my system way undersized?
Thanks for all the help.
Replies
It does take an amazingly long time to "ramp-up" a slab like that...even with insulation below the slab, you're heating mother earth to a certain degree until you reach equilibrium and the heat starts to go where you want it. So that may be part of the problem. Run higher temperatures if you can until you're ramped up. Is the perimeter of the slab insulated as well? If not, that's a potential BIG heat sink...all of outdoors at 0*F>
Certainly have to fix your "pump sealing" problems anyway...why are you having trouble with that? Do you have standard flanges etc? Tried teflon paste on the surfaces? I can't imagine it leaking so bad you can only maintain 1 - 2 psi.
If all this doesn't work, maybe put in a wood stove?
Yep, the perimeter of the slab is insulated also.
Regarding the pump sealing, I've just used the gaskets supplied with the unit and they suck. I think the next step will be to use RTV or some other sort of sealant on there along with the gaskets. Do you think though that higher water pressure will translate into higher temps?
I agree that slabs take a long time to heat up and recovery time will be slow but it seems that after 48 hrs the water temp should rise somewhat. I had the thermostat set at 60 when I got up there and the cabin was at 53. The water temp never really moved after I turned the thermostat up. It only would rise when I would shut a loop or two down.
Thanks.
As long as your return temperature stays more than 10* - 20* less than the supply that heat has to be going somewhere....probably where you don't want it. 10*-20* seems to be the typical accepted delta, some run smaller, and you'll also see the delta decrease as the system reaches the point where the T-stats stop calling for heat and the boiler shuts down.
What brand pump/flanges do you have? I've never had trouble with leaking pump flanges even without extra sealing compound even at test pressures of 50+ psi.
Do you have flow meters on the manifold and what do they indicate? Did you calculate your heat loss and are you sure your 6kw boiler is up to supplying those BTUs?
Well, I don't have actual temp guages on the outflow or inflow. I'm just looking at the water temp guage on the boiler. So I don't know the differences between the two.
I have a grundflo pump. It just seems like the flange is not designed to seal well. It only came with 1 small o ring for a seal and no matter how hard I tighten the bolts, no luck. I'll try to attach a picture so you can see the set up. This was from before I was complete so I realize there is no expansion tank!
And no, no flow meters on the manifolds.
I didn't do the heat loss calculations, the place that supplied the unit did that for me.
Thanks.
Usually, you put isolation valves before and after the pump, in case you ever need to remove/replace the pump. It still should seal and work though without the valves.
It's kind of hard to diagnose faults without knowing your return temps and flow rates. You can get fittings and gauges that go on both the supply and return sides.
Do you have an expansion tank on now? Does it attach underneath the air scoop?
Are you SURE you got all the air out of the loops? Did you try purging each loop separately until no more air bubbles were in the stream?
The pump flange leak buffaloes me....maybe another reader can point out something from the picture. Maybe the wrong flanges for the pump?
It looks like that's a Safelink manifold. I believe you can get and retrofit flow valves to that manifold. They go on the return side, instead of those white caps. Google on "safelink manifold"
Are the guys that did the heat loss and sold you the system available for an on-site visit? How much of this was DIY versus sub-installed? I'm surprised they didn't supply you with a more complete system, knowing that you were DIY.
Edited 12/5/2005 2:45 pm ET by johnnyd
Well, the pump is actually quite easy to remove, just need to disconnect the flanges and out it comes. But no valves around it at all. I guess they would make it nice so you could shut off the water around it and not make a mess.
I'll look at getting fittings and temp guages so I can determine temperature loss over the system. Would that be as simple as just measuring the outside surface temp of the pipe?
Yes, there is an expansion chamber underneath the air scoop now.
Am I sure I got all the air out?? I sure tried but probably not 100% sure and that is something I will look at again next weekend when I go back up. Trying to get the pump sealed up, increase the pressure, and be sure all air is out.
I looked on the safelink info and they do look the same. The mechanism under the white cap on the cirrent manifold is just an on/off valve. Not sure if they can be retrofitted. I will look at them some more also.
The place that sold me the system is probably not available for an onsite evaluation. Haven't asked them to either but the cabin is 200 miles away. The whole set up was DIY (seemed simple enough) and I didn't know enough to ask for a more complete system.
Thanks! I sure appreciate all the help.
Looking at the picture again this morning, I wonder if the flange on the left end of the pump is a 1 1/2" or 1 1/4" flange, while the pump is really designed for 3/4". Could that be the problem?
In my installation, the manifolds are 1" I think, so I reduced down to 3/4" right out of the manifolds and ran 3/4" on both sides of the pump.
Looks like you've got some unusual reductions between the boiler and the pump. Is the copper from the boiler to the pump the same size as the run from the pump to the supply manifold?
In order to pressure flush all the PEX lines, you'll need to supply line pressure....like 30 #s or something, to the whole system, close all the manifold valves, then individually open each loop's valves until the water runs clear, close the valves, open the next ones...etc.
A plumbing supply house will have fittings that Tee into the line, with the look-up part of the Tee having a "well" that fits the thermostat probe. Kind of hard to describe, but if you go to a friendly supply house and explain what you need, they will have it.
You can also get hydronic fittings here:
http://www.radiantec.com/pricingcom.htm
Call them and explain what you need.
Edited 12/6/2005 8:44 am ET by johnnyd
OK, I will try to cover your questions.
1. I am running 1" copper through out the system.
2. The pump fitttings were supplied and they do have isolation valves in them. I just turn a set screw, sorry if I was wrong on this yeserday.
3. I can now supply line pressure to the lines to try to purge any remaining air. Previously I was using a drill powered pump. I will do that weekend.
4. Regarding the pump sealing issues, the pump flange has a small circle machined into it where the oring/gasket sits. Then I put the outer flange over it and tighten. I have it tightened enough that the outer flange is actually bowed trying to seal it up. Am I doing something wrong?
5. Are there temperature gauges that could be plugged into the un used end of the manifolds? That would be the easiest way for me to do this.
I sure appreciate all the help.
3. Will getting the pressure increased by fixing leak by pump and getting any remaining air purged drive up the circulating water temps?
4. Should the system run perfectly quiet? I can hear some gurgling occassionally from it. Is that a sympton of air?
First things first:
Something is "by the numbers" wrong with your pump-to-pipe connection. I would really suggest you take it off, bring it down to a plumbing supply house or really well stocked hardware store, and show what you've got.
Only a little bit of gurgling as some last few suspended bubbles comes out through your air trap. Long term, the only noise you should hear is that of the pump humming. Your drill motor technique probably didn't suply enough pressure to really purge the lines.
Why do you think there is something wrong with the connection? Besides the leak, the connection seems pretty straight forward to me. I used the fittings supplied and they fit into 1" copper. If it's just the leak, I think I can solve that as it is a mechanical joint and they can ll be fixed with enough effort.
And I would guess you are right on the drill pump not supplying enough pressure. Now that I can add water from the house supply, I can get higher pressure in there.
On the connection, exactly....it should be so straight forward, which is why I think you may have recieved mismatched or defective flanges.
Get yourself a wash machine supply hose with two female ends so you can hook one up to your pressure supply and one up to your boiler drain. Then you'll need another length of hose to go from another system drain to a floor drain or bucket or outside.
Good luck. It's supposed to warm up this weekend. At least here in Southern MN.
"I've talked to the place that sized and sold me the system and they say that the size is right on the edge of being to small (cabin is 864 sq ft on main level) but should be enough."
If the cabin was built really airtight and with excellent insulation installation, that boiler might be enough. But it's got the high ceilings and a lot more outside wall area through which to lose heat. Do you have an air exchanger of any type? Do you have a lot of window area (lower R) for views?
6KW or 20,500 BTU's/hour seems on the line to me for regular construction and the size of the place. Many of these boilers have options for more or less KW, can you add another 2-3 KW?
There is a lot of windows in the front wall. Of the front of the cabin there are 150 sq ft of windows out of 336 sq ft of wall. Overall I think the construction was pretty tight. All 2x6 walls, R21 in walls, R38 in vaulted ceiling. We actually don't have the t&g ceiling in yet, just insulation stapled up there now. The sheetrock is all being taped this week finally.
No air exchanger of any type and I don't know if the boiler can be expandable. The people at the place I got it from admitted that they may have been cutting it a little close on size. I think they'd be willing to exchange it for a 9-10 KW boiler but my problem is they all take larger guage wiring and it would be almost impossible to route it at this point.
I would try purging air from the system some more. I had a heck of a time getting the air out of my system when new, but a big difference when finally done. Hooked up to my water pump system and put 40 lbs pressure thru it for a while.
You need a minimum of 5psi for that pump to work, 10 is much better and 15 is wonderful. You need to know the head loss of the boiler, and the system. But at 1psi, I don't think you are moving much water.
Rich,
I must be moving water or else the temp guage would be readin very hot temps. wouldn't it? When I first hooked the system up I flipped the switch and the water heated right up to 200+ degrees because the wiring diagram for the pump was wrong and it wasn't turning on. So wouldn't it stand to reason that if I wasn't moving much water that the temp would be higher?
Thanks for any ideas.
First off, it's basically impossible to diagnose exactly what your system is doing without temperature gauges on both the supply and return lines to your floor tubes.
As others have suggested, it is very important to make sure you have force bled each of the lines individually. You can run water through all of them at once for a long period of time and still may not bleed the air out of an individual line.
Beyond that, it really doesn't matter what the situation is with your cottage, if the boiler will not supply the necessary water temperature to the entire 900' system, it is too small.
I suspect the flanges on the pipes on either side of your pump are dished in or otherwise don't match the flanges on your pump. There is no reason to have a leaky pump flange.
Adding system pressure is not the answer to your problem, although it should be higher. Basically the system pressure allows the water to move vertically into upper floors and you really don't have an issue with that.
Hope this helps!
Most if not all pumps have a minimum inlet pressure. Range depends on model.
Head loss through a device is also pressure drop. Too much pressure drop through a system may result in not enough inlet pressure.
First thing to do is fix the leak. Adding fresh water to a closed loop system is not good.
rich, I agree. I don't mean that the low pressure is not a factor in bad pump performance. I am just saying that his problem of not having enough hot water temperature is not going to be solved by fixing the pump problem as the people who sold him the equipment implied.
All,
This is just some general questions.
1. It sounds like there may be concerns that the boiler is too small. That can be fixed but it'll be a PITA.
2. What should water temps run? I've been told between 120-130 degrees.
3. Will getting the pressure increased by fixing leak by pump and getting any remaining air purged drive up the circulating water temps?
4. Should the system run perfectly quiet? I can hear some gurgling occassionally from it. Is that a sympton of air?
Tubby, Just a couple of other questions,What model grundfos is that and you can check the temp of your tubes going out and coming back with an oven meat probe, tape the end to the pipe when it's running and give it a few minutes. another question is are you in this cabin all the time or do you go there on the weekend and turn the heat on and expect it to get warm quickly?? <!----><!---->
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The pump is a Grundflo 15-42 "brute". Does that make sense? I will try to measure temp differences in the water flow next weekend. i won't be up there this weekend.
And yes, this is a cabin that we go to every 2-3 weekends. I know that radiant has slow recovery so that's not my concern. My concern is that it never warms up. I try to leave it at 60 when we are gone and then turn up when we get there. The thing I see is that even when it is set at 60, it was at 53 last weekend when I got there. Water temp just doesn't increase.
Tubby, yes the 15-42 is gooood for your application, again depending on the length of the tubes. I don't see any check valve in the system although we can't see the other side of the boiler. You do have a water connection from a pressurized source right?? <!----><!---->
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Floorheater,
There are 4 loops, each about 225 feet long. And no, it's a closed loop system and is not connected to an outside water source. I was told I wouldn't need to be.
There should not be gurgling. You will likely be able to hear faint water flow by the pump and fittings, but not gurgling.I'd make sure the loop was free of air and physically moving the water before upgrading the boiler.Temps are often checked with a laser or ir temperature gauge.In case it's instructive, I'll offer our story. First winter, we had a room that never, never warmed properly. Cold like you described. Double checked calcs (done by Wirsbo). Contractor eventually decided that we needed more supply, but the thought of adding a gas boiler didn't sit well with me. I was convinced something was wrong with the loops. So I did as you did and shut off loops. At a certain point the temp shot up and the slab heated in the expected time. Cutting to the chase, we concluded that we had sufficient capacity, but that our supply to the manifold and the manifold was improperly setup despite all assurances. We had too small a supply line or too many loops running off that supply, and there wasn't enough pressure to circulate the water. It's that simple, the water was hot enough, and the loops were fine, and there were no air bubbles, but we simply couldn't push enough water through the line. Solution was to split the one manifold into two, and add a second supply line. Probably coulda taken the supply from 3/4" to 1" and had same result, but a second 3/4" was easier. Result was that the system produced the specified results. Ahhhhhhh. Love it! Everyone still says the original setup shoulda worked (not about to be swayed by reality, you understand), but as long as it's heating the floor with the way we've reconfigured it, they can say what they wish.If you got good temps with fewer loops, maybe you got something similar going on.
"Temps are often checked with a laser or ir temperature gauge."
Tell me about the laser measurement. The infrared non-contact thermometer often has a laser pointer, but it isn't part of the measurement process, just some flash.
Those infrared thermometers are problematic with bare metal like copper pipe. The metal is a reflective in the infrared so you just get an average temp of everything in your utility room. A clearly 130F pipe will read 85F because IR from all the walls, etc are bouncing off the pipe.David Thomas Overlooking Cook Inlet in Kenai, Alaska
Sloppy wording in a rush. Sorry. Shoulda said laser sighted non contact ir thermometer. The guys I watched liked em better than the probes...always fiddling with the probes to make sure they were in contact. Didn't seem to have any accuracy issues with the ir.
Edited 12/6/2005 8:32 pm ET by CloudHidden
Another question Tubby is how long are the actual loops, I under stand that they may be about the same length but if they are 500' loops you may have too much pipe for 1 pump. <!----><!---->
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tubby, You must consider two entirely separate issues:
1) You should have a supply water temperature of around 120 degrees +/- available to your slab. If you get all of the lines properly bled and the pump problem solved, that water temperature should provide enough heat to comfortably heat your insulated cabin without making your floors uncomfortably hot. If your system heater is incapable of providing water at that temperature, it is too small.
2) The other part of the process is designing a system to use that water temperature to provide enough BTU's to heat your cabin. Among the variables considered is the proper sizing of the heater and the layout of the tubing in your slab.
If all of this doesn't jive, you will have a problem.
In general, if you have a concrete slab with heat tubes 12" o/c with 120 water running through them, you should be able to easily heat a newer, well insulated structure.
I just finished a home with 56 windows and 16' ceilings in places . We have both slab and lightweight concrete floors with tubes 12" o/c and it is very cozy in the -5 weather we have had the last week.
The tubing is psaced on 12" intervals so we got that covered. If bleeding the tubes and getting pressure up is all it takes to get the water temp to 120, then I should be fine with the heating. We'll see!
I think I recall operating someone saying supply water temps more like 90-110. 125-130 may be way hot. I've run mine around 95 to just over 100, I think (outside temp is mild 30 degF) and mine is good ... warm floors, space temp 68-70.
For 850 sqft and 20,000 btuh, it GENERALLY sounds not far off, depending on window efficiency and total window area (to name two large items). I've sized systems for 1,800 sqft at 35,000 btuh with double pane glass only (and lots of it).
I've always been told to have 12-40 psi pressure in the radiant system (i.e. w/ pump off). Not sure how that affects pump pressure.
Something is wrong w/ the pump leak. Maybe something isn't in line properly? Maybe you got the O ring in crooked? Maybe you damaged the O ring? Maybe you nicked the flanges when installing or before installation?
Just more food for confusion ... er thought. ;)
I guess I get to ask a dumb question. Are you sure that you have 220 hooked up to your boiler and not just 110?
Yes, that one I am sure of. My wifes uncle did the wiring (retired electrician), not me!
I think that I would have your wifes uncle check how much current the boiler is drawing. One reason for not getting enough heat out of the boiler could be that for some reason or other, you are not putting enough (amps) in.
Does anyone else figure that 6000 watts of heat is not enough?
Even if its in the boiler set up and radiant floor it's still adding 6000 watts. And the boiler has it's own losses.
This would be the same as heating the entire cabin with 4 1500 watt electric heaters. Looking at it that way I would say that is way undersized. Right?
Stu
I'm heating 1600 sq ft with 9kw that puts out ~30kbtu. 110* supply in the shoulder seasons, 120* below zero...so it can be done...depends on how optimistic he is with his heat loss calcs. It may be undersized, but he's got bigger fish to fry before he will be able to find out.
"Does anyone else figure that 6000 watts of heat is not enough?"
Yes. I do.
IF it was built really tight AND to an appropriate -20F design temp AND was only single story, I'd roughly calc:
864 sf x 90F x 0.1 BTU/hr/sf/F = 7,800 BTU or 2.3 kw
BUT, it IS two stories and no one has described it as super tight. So I'd estimate:
2 x 864 sf x 90F x 0.3 BTU/hr/sf/F = 46,656 BTU = 13.7 kw
Those 0.1 and 0.3 factors are rough rules of thumbs. 0.1 for really tight and well insulated. 0.3 for well-insulated. 1.0 for hardly insulated (e.g. 1960's house). 3.0 for an old leaky house.
For this particular house, we know what the performance is. At 5F outside, it can't maintain 60F on 6 kw.
So 6 kw = 20,500 BTU = 2 x 864 sf x 50F x factor. Factor = 0.23 So it is well insulated and a little bit tight.
So what should the heater be sized at to perform at -20F?
2 x 864 sf x 90F x 0.23 BTU/hr/sf/F = 35,800 BTU = 10.4 kw
So yes, they undersized the heater. By almost a factor of two.
Yes, the pump will last longer if it weren't cavitating. So get the pressure up. But the heat is being moved. Very clearly by the other loops coming up in temp when one or more are removed from service. The electric "boiler" is running all the time. That heat is going somewhere in the house.
David Thomas Overlooking Cook Inlet in Kenai, Alaska
Dave,
Great info, thanks. My thoughts are along the same as yours. I am certainly going to get the pressure up, be sure the air is completely bled out of the system, correct the leak by the pump. And see what the result is.
But, I also think its clear that if I shut off 25% of the heating area, my temps jump to the 120 degrees that it's designed for. That would seem to me I am 25% to small on capacity. A 9KW boiler would get me about a 50% increase in capacity and would probably fix the problem.
Here's my problem with that though. The 6KW boiler requires 10 guage wire while the 9KW requires 8 gauge. It would darn near impossible to run new wire inside to the boiler at this point. Will 10 gauge be OK?
Let me ask a couple of questions on your calculations. When you say 90F is that the temp difference between -20 and 70? And this house is really 1.5 story with the upstairs not being heated by radiant. So that might make it slightly smaller. But even at a factor of 1, (1x864x90x.3) it shows me being under powered.
Again, thanks for the great info.
"But, I also think its clear that if I shut off 25% of the heating area, my temps jump to the 120 degrees that it's designed for. That would seem to me I am 25% too small on capacity."But that was at 5F. You can get colder temps than that. That's why I usd 90F = 70 - (-20)"And this house is really 1.5 story with the upstairs not being heated by radiant."So is the upstairs heated some other way? If not, it is being, indirectly, being heated by the RFH.If wire size is the only issue, a couple of thoughts:There's always a way to run wires. Maybe along the outside of the house, maybe through a crawl or attic or some where. Think of it as a remodel and not as new construction with the wiring phase passed.Also, you have a variety of 15-20 amp, 12-AWG convenience circuits, right? You could plug in 1500-watt electric resistance heaters into those (one into each). Four such units will double your heating capacity, more than double the responsiveness and set you back about $130. Maybe $75 is you live near a Wall-mart. Usually a lousy way to heat a house due to the high cost of electricity, but you've already gone down that path. Electric is electric in terms of operating expense. And as long as you have some RFH going, the temps will be pretty uniform from top to bottom.Why the heck didn't you get a propane-fired HWH? The smallest available would be 40,000 BTU/hour, cost $139 and do the trick. I heat my entire 1700 sf house (space and hot water) with a normal natural-gas fired HWH. Propane is cheaper than kWh and it works during a power failure (if you have 200 watts through an inverter to run the circ pumps).David Thomas Overlooking Cook Inlet in Kenai, Alaska
Dave,
The upstairs is heated with a base board heater. So while it indirectly may draw some heat from the radiant, it shouldn't pull much from it. It's just a 250-300 sq ft bedroom and small bath.
Yes, I could always run wires outside and that has been considered. If I could get them from the breaker panel to the ceiling without having to tear up too much sheetrock, then it would be a pc of cake.
And, in the living room we will have 2 6' baseboards heaters as back up since the slab heat is on off peak. So those will actually be connected next weekend when we go up to paint. That will probably be enough to keep it all warm, especially since it will already be at 60+ to start with due to the radiant heat.
Why didn't we just use a gas hot water heater? In MN you cannot use a water heater to run radiant heat. At least that is what I was told by the coop up there. And I didn't want to use a gas boiler as I won't be there that much. And the off peak program is quite cheap while the shut offs are very rare.
Thanks!
Tubby...
Well, you've gathered quite an audience here...please be sure to let us know how things go this weekend.
Actually, I won't be up there until next weekend (17-18) but I wil let everybody know what happens. I did talk to my wifes uncle (the electrician) and he said we should be able to get larger gauge wire to the boiler source without too much trouble if we do end up going bigger.
But I'll work on the leak, get the pressure up, try to get any remaining air bubbles out of system, and see what happens.
what ever happened? I'm in Duluth and have been curious for quite a while...
Sorry about the very long delay in getting back on this. Just found this old thread. The boiler seems to be doing a good job. The summer after I started this thread we laid 2" thick pink foam out from the slab 4' all the way around the cabin and thenplaced rock on top of that. Also spent some time getting the boiler to hold pressure. Now it runs exactly at 120 degrees and keeps the cabin quite comfortable. The only issue we have is when we get real sunny days, we get a lot of passive solar and the cabin heats up enough that the boiler shuts down. Of course if it's a real sunny day then that means its going to be a real cold night and the floor heat can't react quick enough to keep the house at the set temp, but the water temp is still right at 120.
Also, as we continued to finish the cabin we got the T&G ceiling put in, got the window trim up and just continued to plug airleaks. So now all is good.