kennyb79
Registered Users
New Member
Posts:8
 |
| 03/27/2009 8:27 PM |
|
| I wish I would have found this forum sooner. heres my problem, I have a takagi tankless heater as a heat source, heating 1000' of 3/4" (4 lengths of 250' each) pex tube in my 1300 sq feet insulated slab. the water goes in at a nice 131 degrees, but it losses so much heat that the warmest I can get out of it is 75 degrees. If I shut off 3 of my lines and run a single 250' line, then I can get 90 degrees coming out of my slab. I contact the radiant company I bought it from, they concluded that I had to much head pressure and my pump isn't circulating...so I bought a bigger pump, 25gpm, and it does the same thing. My question is this for all of you...what temp does your water go in the slab, what temp is it when it comes out of the slab. How does "head pressure" play a part in what may be going on with my system. thanks for your help... |
|
|
|
|
|
|
NRT.Rob
Registered Users
Advanced Member
Posts:733
|
| 03/28/2009 2:13 PM |
|
| how long have you been running the system, what make/model pump do you have, and can you post a sketch of your system layout? not full tubing loops or anything, but a sketch of your piping and where the major components are located. |
|
-=Northeast Radiant Technology=-
NRTradiant.com |
|
|
Brock
Registered Users
Basic Member
Posts:444
|
| 03/28/2009 5:26 PM |
|
Ya that is weird, I am running between 5-10 gpm on four 250’ 1/2 inch pex going in at 90F and returning at 75F and it will just keep warming up if I let it.
Something seems wrong with your flow, maybe pinched pipes or something plugging the lines somehow? Maybe a filter or not fully open valves? Air trapped in the lines somewhere? |
|
Green Bay, WI. - 4 ton horizontal, 16k gallon indoor pool, 1.8kw solar PV setup, 3400 sq ft |
|
|
jmagill
Registered Users
Basic Member
Posts:237
|
| 03/28/2009 5:33 PM |
|
| Do you have a remote on the Takagi? What is your flow rate if you do?
I would bet it is air in the line. My system heats 1200( with a Takagi-Jr) and has no problem heating.
Is the slab insulated? |
|
|
|
|
Dana1
Registered Users
Advanced Member
Posts:702
|
| 03/30/2009 10:43 AM |
|
I'm betting you're getting nothing like 25 gallons per minute of flow, you're probably not even getting 5. Is 75F the return water temperature? (Not just the temperature of the surface of the slab.) Knowing the delta-T between the output & return water and the flow you can calculated how much heat is being delivered to the slab. At a true 5gpm firing even at half-modulation you'd be roasting in most 1300' buildings much of the season. If you're pumping 5gpm with a (131F-75F=)56F delta-T, that's ~140KBTU/hour- higher than the output of a Takagi-Jr at full fire. (And enough to roast you out of most 1300' buildings unless it's completely uninsulated & leaky or minus 55F outside.) At 10gpm @ 56F delta-T it would exceed the output of any Takagi tankless. Assume you're getting something like 1gpm- maybe less.
Takagis all fire up just fine at flows of ~0.75gpm, and most will stay lit as low as 0.5gpm. Takagi recommends between 1-2gpm in recirculation mode to reduce wear on the system. In general you're better off running it at lower volume, higher output temp to get the firing rate you need. If you need more heat, bump up the output temp. (There's a dip-switch for setting the default output temp on most models, if you don' have a remote.) The combustion-efficiency is determined primarily by the return water temp, not the output temp, and tankless heaters are designed to tolerate 50-75F delta-Ts just fine. If you're getting 100F+ return water, which you would be if the slab were running 90F, 150F out isn't going to stress it much. (If you're using the same heater for domestic hot water you'll need a tempering valve to control the scald hazard if you go over 130F though.)
Its seems likely that something is making the head unusually high. (Crimped bend or flattened section of pipe? Filter full o' crud?) |
|
|
|
|
kennyb79
Registered Users
New Member
Posts:8
|
| 03/30/2009 11:12 AM |
|
So yesterday I go out to the house and start the heater….I decided to open up just 2 lines out of the 4 going into the slab. I let it go for 20 minutes, until the temperature coming out the floor had risen from 54 degrees to 77 (water temp coming out of the slab). So I closed those 2 valves and opened the other 2 valves…and it was the same, after about 20 minutes it was 77 degrees. So I opened all 4 valves, and when I went back out to check after 20 minutes, the temperature was reading 65 degrees…it had cooled off! What I know is that the GPM when using 2 lines was 2.7GPM, with all 4 lines it was reading 2.5 GPM (yes, even when the temp dropped to 65), the temp of the water coming out of the Takagi was always reading 131 degrees.
I cleaned out the takagi filter before I started, there wasn't much crude in there, I could still blow through it, and it didn't seem to increase the flow afterwards.
okay, so what should be my next step here? I'm sitting on a bunch of plumbing that just doesn't work, the company in vermont says its my installation thats the problem, the heater is to far from the pump. I'm new to all of this and real eager to learn, and to get this fixed before my wife shoots me. . Do must of these systems push more than 2.7 gpm's thru their heaters? It seems to me that I have to find a pump big enough to recirc the water. I've upgraded to a bigger pump with no change in the results...should I go to an even bigger pump? it makes sense that the more water you move thru there the more you should heat, and it seems that my pump isn't moving what it should. any ideas on what size pump I should be using? |
|
|
|
|
jmagill
Registered Users
Basic Member
Posts:237
|
| 03/30/2009 11:25 AM |
|
I have two zones. Running I am pushing 2.2 gpm through the Takagi.
Are you saying that the return is 65 with all four running? If so that seems right. The same temp is going in but it is running through more tube and coming back cooler. I do not see the problem? |
|
|
|
|
NRT.Rob
Registered Users
Advanced Member
Posts:733
|
| 03/30/2009 11:32 AM |
|
Kenny:
How long have you been running the system consistently? If you're just turning it on and off, it's never going to heat up. You turn it on, and leave it on. Slab startup can take a long time.... many hours, even days on the first round of startup.
There is no way you should be pushing LESS flow with four loops open than with two. You are doing something wrong. That's against the laws of physics, so something else is happening here. opening loops cannot reduce flow rates.
with your flow and that temperature drop you should be more than able to heat the space, and I assume you just haven't let the slab fully heat up yet. What happens if you let it run for a full day? |
|
-=Northeast Radiant Technology=-
NRTradiant.com |
|
|
kennyb79
Registered Users
New Member
Posts:8
|
| 03/30/2009 11:59 AM |
|
I’ve resorted to turning it on everyday because I couldn’t afford the propane to leave it on. When I first fired up the system in November it just ran and ran, but the water temperature coming out of the slab never got any higher than 70 degrees. I expected to see the water temp coming out of the floor rise past 70 anyway. In the meantime thats all I saw was the propane bottle empty out quite quickly. I was running through 5 gallons a day and not seeing any heat. I let it run for a couple of days like this, but it never seemed to warm up, so I’ve resorted to manually throttling it like I do to try to keep the house warm. I fire it up and let it run for an hour or so in the morning and then after I get home from work for 4 or 5 hours. On the weekends it runs 8-10 hours a day while I’m working out there, and the water temp still never gets above 68 or 70 degrees. If you think that I'm doing something wrong, I'll try turning it on and letting it run and we'll see what happens...like I said, this is all new to me, but I expected to see a continual increase in the water temp coming out of the slab until it got so warm I couldn't stand it, at 131 degrees going in how long should it take to fully warm up? and what temperature is the water coming out of you slab that heats you house comfortably?.
It’s a house that I’ve been building and I am in the finishing stages. Its slipform stone, construction. The walls are 12” concrete/stone and I’ve studded and insulated the inside with 6” fiberglass. The slab is 6” thick, 2” of insulation below, and I insulated around the edges of the slab as well. I’ve got a massive stone/concrete wall inside to act as a heat sink. The house is completely sealed an insulated. I’ve even been running 4 small ceramic heater inside there to keep the temperature up, and the interior air temp, never gets below mid 50’s, but never really gets above 60 either.
I know something is wrong but what? Is that the right amount of flow that should be going through my takagi…2.7 gallons? Why does it seem to get to a certain temperature and then stop heating when I’m running all 4 lines? The takagi always indicates it is putting out 131 degrees…
I’m still waiting for that aha moment…
|
|
|
|
|
jmagill
Registered Users
Basic Member
Posts:237
|
| 03/30/2009 12:06 PM |
|
My house is 1200 sq feet. The heat only runs a couple of hours at a time and usually only at night. I have an elevated slab and the first winter there was no insulation between the floor and the crawlspace and it still did not run that long.
With radiant and a slab you need to set the temp and leave it there. The cycling from lower temp to higer temp(steback) does not save money.
My outgoing temp is 110 my incoming will ussually start at 65 and rise as the slab heats. |
|
|
|
|
jmagill
Registered Users
Basic Member
Posts:237
|
| 03/30/2009 12:08 PM |
|
Is this a closed system? Where are you located(climate)
|
|
|
|
|
jmagill
Registered Users
Basic Member
Posts:237
|
| 03/30/2009 12:14 PM |
|
I have reread your last post.
I think the problem is you have yet to bring the whole house up to a set point. All that mass needs to reach a certain temp and then it should maintain easier. You can not cycle the heat source unless you get a great portion of your heat from solar. All that mass sucks up the heat at first.
I also think that massive stone wall is likely connected directly to the earth with no insulation. You are heating the earth! |
|
|
|
|
kennyb79
Registered Users
New Member
Posts:8
|
| 03/30/2009 12:29 PM |
|
I’ve got an open system, but I’m not really using much water in the rest of the house because were not living in it yet. But I have run a lot of water thru the system, I flushed each line individually ensuring there was no air trapped in the system.
I’m in upstate NY on the shores of Lake Ontario. We get a lot of snow and it gets cold in the winter.
I know I shouldn’t keep controlling the temperature like I do, but the heater ran for so long and cost so much, and never warmed up enough, I felt something must be wrong. I also felt that if I controlled it I could get the slab up to temperature myself. I still find it strange that the water only gets so warm and then seems to stop warming.
I’m going to turn the system on right after work and leave it on…then I’ll monitor things, and report back. Either I’ll have positive results or I’ll run out of propane, but either way I’ll report back my findings… thanks for all of your help.
you may be right about the mass in the center being uninsulated and heating the earth...let me know if your heat bill goes down a little!!!
|
|
|
|
|
jmagill
Registered Users
Basic Member
Posts:237
|
| 03/30/2009 12:36 PM |
|
Yes send that heat my way!
The whole problem( other than the flow going down) is the cycling. You will have this problem until you move in full time. High mass homes are not meant to change temp quickly( not good for weekend homes).
|
|
|
|
|
NRT.Rob
Registered Users
Advanced Member
Posts:733
|
| 03/30/2009 12:42 PM |
|
no, your initial warming is just the first charge of heat making its way back to the heater. After that, you're drawing heat out at X rate from a Y setpoint water temperature at Z flow rate.. it will hold steady until you really start heating the mass up.
the first time you start a slab, especially if you have a big "heat sink", it can take several days. the slab may have moisture in it that may make it all worse. |
|
-=Northeast Radiant Technology=-
NRTradiant.com |
|
|
Dana1
Registered Users
Advanced Member
Posts:702
|
| 03/30/2009 12:59 PM |
|
Posted By jmagill on 03/30/2009 12:14 PM
I have reread your last post.
I think the problem is you have yet to bring the whole house up to a set point. All that mass needs to reach a certain temp and then it should maintain easier. You can not cycle the heat source unless you get a great portion of your heat from solar. All that mass sucks up the heat at first.
I also think that massive stone wall is likely connected directly to the earth with no insulation. You are heating the earth!
Ding ding ding ding ding ding ding! WE HAVE A WINNER!
If you don't get the entire thermal mass up to temp, you won't get higher return water temps. When running the single loops the return water temps rise because the flow is several times faster (through those individiual loops.)
Five gallons of propane per day being burned at ~83% combustion efficiency isn't a huge amount of energy- it's 3.8 therms, delivered to the slab, an average of 15.7KBTU/h for 24 hours. That is a realistic heating-season heat load- it's about what my temperature-stablized heat load is during 25-30F outdoor temps. Pumping a large thermal mass up to temp from a cooler setback it'll run higher the first day or so then level out relative to the whole structure's heat-loss. You can't get it up to temp for free (unless you wait until summer. :-) )
2.7gpm is probably more flow than you need to maintain temp once the slab is up to it's steady-state. Coming from cold start it'll get to temp quicker if you bump up the output temp of the heater- just keep the delta-T under 75F to keep from thermally stressing the heater. (I'm not sure what limits Takagi recommends for continuous operation, but they make 120F water out of 40F inlet water just fine- an 80F delta-T. I'm pretty sure you can toast 'em runinng over 100F deltas- better to be nice to 'em if you can.) Flow rates over 2gpm are above the recommended rate- only go there if you HAVE to.
|
|
|
|
|
BadgerBoilerMN
Registered Users
Basic Member
Posts:311
|
| 03/30/2009 1:08 PM |
|
| What an excellent argument for professional design! |
|
MA
www.badgerboilerservice.com |
|
|
jmagill
Registered Users
Basic Member
Posts:237
|
| 03/30/2009 1:13 PM |
|
Do you have an auxilary heater? Fireplace, wood stove?
We have a small Thelin stove( looks like an old time heater) that is propane. When we used the house as a weekend place while building, we set the radiant at 55 degrees and on the weekend used the Thelin to bring the temp up quickly. Now we only use it once or twice a year.
Also. I would recomend running the outgoing temp lower if you can. That first heating cycle is when the slab is drying out( can take a whole season in a tight home). The lower the temp the less cracking. Slow drying is always better.
|
|
|
|
|
jmagill
Registered Users
Basic Member
Posts:237
|
| 03/30/2009 1:15 PM |
|
Posted By BadgerBoilerMN on 03/30/2009 1:08 PM
What an excellent argument for professional design!
More knowledge of his system would have been better. It does not matter how well the system is designed if you don't operate it right. |
|
|
|
|
BadgerBoilerMN
Registered Users
Basic Member
Posts:311
|
| 03/30/2009 1:20 PM |
|
Slabs do not "dry out". Curing concrete is actually a chemical process which is not enhanced or improved by the addition of heat. In fact applying heat before the concrete is reasonably cured is detrimental to the process and will lead to a weaker slab and unintended cracking.
Before taking advice from your peers, seek the council of experienced professionals, e.g. people who design, install or service radiant floor heating systems for a living. And keep in mind; for every professional in the field, there are 10 "experts" on the net. |
|
MA
www.badgerboilerservice.com |
|
|