Optimizing the Slab Radiant Heating System

[600SL]

I now have my system up and running and it seams to be working with the default settings of just about everything. But there still seems to be a lot of tweaking that should be done. First off after about 12 hr of running I find the tater temp at the input and output of the manifolds to be ~ 140° and 70° F which I thought would be too high. So I increased the circulator speed from medium to high but there are flow control valve on the manifolds. So how do I balance that out. So there are several setting a need to know how to optimize:

1) Max heating temperature, Currently set to default of 180°F
2) Circulator pump speed High Medium or Low. Currently set to high.
3) Manifold metering valves. Currently running at ~ 0.6 GPM

 

[TurnipTruck]

Patience.
How large is your (assumed) slab? First heat might be 24 hours.
180* into the slab!?! NO. Should be more like 130? max. Or less.
How well insulated? Tell us more.

 

[600SL]

Patience.
How large is your (assumed) slab? First heat might be 24 hours.
180* into the slab!?! NO. Should be more like 130? max. Or less.
How well insulated? Tell us more.

The building is 39'x30'. It has two 7" thick slabs one is 28' 4" x 16' the other is 28' 4" 21'. Both are on 2" insulation. It is a steel building construction with 4" thick insulation on the walls an 6" thick in the ceiling. Three garage doors and one man door.

For some reason it wont let me post pictures but hears a PDF of the slab plumbing using 1/2" PEX AL.

 

[PoorUB]

What type of boiler? Cast iron, set the temp to 140F.

High efficiency, se the temp to 120-130.

Temp going into the floor should never need to be over 120F. Flow depends on size of tubing, but your .6 GPM is fine for 1/2" more for 5/8" or 3/4", but less than 1 GPM.

Like the other post. 12 hours on floor heat for the first heat up in nothing, you are still heating up the slab. It might take a couple days, or longer. Once you get it set up look for 20 degrees across the manifolds and set flow to get that temp.

 

[600SL]

What type of boiler? Cast iron, set the temp to 140F.

High efficiency, se the temp to 120-130.

Temp going into the floor should never need to be over 120F. Flow depends on size of tubing, but your .6 GPM is fine for 1/2" more for 5/8" or 3/4", but less than 1 GPM.

Like the other post. 12 hours on floor heat for the first heat up in nothing, you are still heating up the slab. It might take a couple days, or longer. Once you get it set up look for 20 degrees across the manifolds and set flow to get that temp.

Boiler is a LAARS FT Series Gas Condensing Combination Boiler. The default max temp is set to 180° F but it seams to be running at around 150 and at the manifold inlet I'm seeing ~140. But I did see as high as 175 when I first started. So will that max temp in the boiler setup, should that be reduced, I just assumed that was a safety limit and maybe the running temp will reduce as the slab comes to temperature.

 

[Firebrick43]

Boiler is a LAARS FT Series Gas Condensing Combination Boiler. The default max temp is set to 180° F but it seams to be running at around 150 and at the manifold inlet I'm seeing ~140. But I did see as high as 175 when I first started. So will that max temp in the boiler setup, should that be reduced, I just assumed that was a safety limit and maybe the running temp will reduce as the slab comes to temperature.

You need to lower it so it doesn’t go over 135 and the floor temps ideally under 100. Long gentle cycles are better that fast high temp ones.

That boiler modulates so it will be fine

Don’t turn up the flow, give it time to up up to temp

 

[PoorUB]

Boiler is a LAARS FT Series Gas Condensing Combination Boiler. The default max temp is set to 180° F but it seams to be running at around 150 and at the manifold inlet I'm seeing ~140. But I did see as high as 175 when I first started. So will that max temp in the boiler setup, should that be reduced, I just assumed that was a safety limit and maybe the running temp will reduce as the slab comes to temperature.

That boiler is less efficient the higher the water temp. You can run it at high temps, but you don't need too. Set it for 120F-130F max temp. I would turn down the boiler now, but wait for the floor to stabilize before you mess with anything else.

 

[fitter30]

Size pex, loop lengths, pics of the boiler and piping so can see how its piped. Glycol? What is the percentage and brand? Pump brand and model?
Systems should be design for a 20° temperature drop per loop. The more gpm flowing through any condensing boiler the design head goes up drastically.

 

[600SL]

Size pex, loop lengths, pics of the boiler and piping so can see how its piped. Glycol? What is the percentage and brand? Pump brand and model?
Systems should be design for a 20° temperature drop per loop. The more gpm flowing through any condensing boiler the design head goes up drastically.

See the PDF in post #3 for piping. Glycole is Hercules 35260 Cryo-tek. I believe it is a 40% premix based on the freeze point. I believe the pump is a Granite GP7FRC. For some reason GJ is not letting me post pictures today.

 

[600SL]

You need to lower it so it doesn’t go over 135 and the floor temps ideally under 100. Long gentle cycles are better that fast high temp ones.

That boiler modulates so it will be fine

Don’t turn up the flow, give it time to up up to temp

I was just looking into lowering it and I noticed there is not only a default maximum of 180F but there is also a default minimum of 130F. Seems strange that LAARS would set these defaults so high if they were not optimum for there system. Could this system be different? See options 16 and 17

 

[fitter30]

956' 1/2" pex = 30,592 btu's = 32 btus per ft.
279' loop 8.5' head 4 loops still 8.5' piping 3'
1 gallon for 10k btu's flow
200' loop 6400 btu's 6.4 gpm
279' loop 8928 btu's. 9 gpm
Need the complete model of boiler for flow/ head chart
Before the concrete pour were the loops pressurized to 30 lbs?

 

[dscheidt]

I was just looking into lowering it and I noticed there is not only a default maximum of 180F but there is also a default minimum of 130F. Seems strange that LAARS would set these defaults so high if they were not optimum for there system. Could this system be different? See options 16 and 17

they're sensible defaults for radiation that was designed around a conventional cast iron boiler, which is an awful lot of houses with hot water heat. I suspect they also expect the installer to know how to set water temperature up.

 

[600SL]

956' 1/2" pex = 30,592 btu's = 32 btus per ft.
279' loop 8.5' head 4 loops still 8.5' piping 3'
1 gallon for 10k btu's flow
200' loop 6400 btu's 6.4 gpm
279' loop 8928 btu's. 9 gpm
Need the complete model of boiler for flow/ head chart
Before the concrete pour were the loops pressurized to 30 lbs?

Model # is MFTCW140N. Loops were pressure tested but I don't recall what pressure.

 

[600SL]

they're sensible defaults for radiation that was designed around a conventional cast iron boiler, which is an awful lot of houses with hot water heat. I suspect they also expect the installer to know how to set water temperature up.

If I set the max temp to 135, what should I set the min temp to?

 

[Firebrick43]

I was just looking into lowering it and I noticed there is not only a default maximum of 180F but there is also a default minimum of 130F. Seems strange that LAARS would set these defaults so high if they were not optimum for there system. Could this system be different? See options 16 and 17

Dschiedt has it right.

Cast iron radiators, radiant panels, and many hydronic baseboard systems are sized to work with 140 and up water. Most cast iron boilers are not designed to work in condensing temp ranges as it will destroy them. When replacing these older boilers if you turn the temp down you don’t have the surface area then to effectively heat the building envelope.

In floor heat if designed right has plenty of surface area and mass. No need for high temps and if it’s a condensing boiler it’s good to run it in the condensing range or your just throwing a lot of heat up the stack and wasting fuel. Depending on altitude condensing cycle is below 140 and lower as your altitude rises.

It won’t hurt the boiler to run above 140, only your wallet.

 

[Firebrick43]

If I set the max temp to 135, what should I set the min temp to?

the lower that it will do the job the more efficient it will be. My floor rarely needs above 87 degree water. It’s 6” of foam on the walls and 3” below the floor (ICF). But this will depend on btu load and loop spacing/sizing

Monitor the temp in the building during extreme cold. If it’s running full time and not maintaining temp, raise the Min a few degrees. Long cycles are better than short ones.

 

[fitter30]

they're sensible defaults for radiation that was designed around a conventional cast iron boiler, which is an awful lot of houses with hot water heat. I suspect they also expect the installer to know how to set water temperature up.

Cast boiler the return temp has to be above 140°. Cast can't be lower than 135° are they can condense and cooler water can crack a section.
The 5° difference is a safety factor.

 

[600SL]

Its now been almost 48 hr. I will see what its running at today and set max temp to 135° and min temp to 85°. And see how that works.

 

[600SL]

I set the max upper temp to 135F. The lowest I could set the min temp was 86, so I decided to set it to 100. Its a warmer day today ~ 37F. The boiler is running at ~ 106F the manifold inlet is 102F, Manifold outlet is 72F. Is that good or bad?

 

[Kezorm]

Check your manifold outlet sensor. Are you sure it isn't stuck? Seems to be ~70 regardless of inlet temperature. Assuming reasonable flow rates (~0.8 GPM per loop), a 30F drop is too much given the outdoor conditions and I would question the reading. The original 70F drop (140F to 70F) is certainly not to be believed.

 

[Kezorm]

As someone else mentioned, my garage / shop floor rarely needs over 80F water to hold 70F in the space in Minnesota. I think a 10F drop is the largest I've seen and that was at an extended period of never getting above -10F outside. If your inlet and outlet temperatures are real, then it either means that you have an extremely high load (e.g. no insulation, leaky structure, etc.) or the flow rate much lower than you think.

 

[600SL]

Check your manifold outlet sensor. Are you sure it isn't stuck? Seems to be ~70 regardless of inlet temperature. Assuming reasonable flow rates (~0.8 GPM per loop), a 30F drop is too much given the outdoor conditions and I would question the reading. The original 70F drop (140F to 70F) is certainly not to be believed.

Manifold outlet sensor is working. It will drop down to about 62 in between cycles. Back when I was at 140/70 it was about 25° outside and the system was just stared. Today its ~ 35F outside and been running 48 hr.

 

[Firebrick43]

It might take a few more days to even out. Is the boiler still running 100% of the time? 50%. 10%?

If it’s running for short burst then lower the temp more.

 

[600SL]

As someone else mentioned, my garage / shop floor rarely needs over 80F water to hold 70F in the space in Minnesota. I think a 10F drop is the largest I've seen and that was at an extended period of never getting above -10F outside. If your inlet and outlet temperatures are real, then it either means that you have an extremely high load (e.g. no insulation, leaky structure, etc.) or the flow rate much lower than you think.

The minimum I can set this boiler to is 9F above the minimum. Lowest possible minimum is 86F therefor the lowest mas is 95F. The garage is not leaky at all and. insulation is about R15 walls, R19 ceiling with 3 insulated garage doors.

 

[600SL]

It might take a few more days to even out. Is the boiler still running 100% of the time? 50%. 10%?

I would say it runs about 10% - 20% of the time now, But the weather just warmed up significantly.

 

[Kezorm]

What temperature are you keeping the building at?

Are garage doors insulated? Reasonably sealed around edges?

Based on drawing, total flow is ~3GPM? If so, 30F drop at 3GPM = 45,000 BTU. Based on the description of your building (unless it's EXTREMELY drafty), it's unlikely that it needs 45,000 BTU when it's 37F outside.

 

[Kezorm]

I would say it runs about 10% - 20% of the time now, But the weather just warmed up significantly.

Ok, running at 10-20 percent can make sense. This works out to 4,500 - 9000 BTU/h based on 3GPM flow rate.

 

[Kezorm]

At current outdoor conditions and assuming you need 9000 BTU/hr, boiler temp should come down even lower. It is a mod/con boiler, so the Ideal temperature is such that the boiler runs 100% of the time. I would guess that a boiler temperature closer to 80F would do it at current outdoor conditions. It probably won't let you go all the way down to 80F, but try as low as it goes. You should see it run considerably longer with smaller delta. Again, ideal with a mod/con boiler is it runs 100% of the time at just the right mod setting to match the load.

Look for outdoor reset settings in the boiler. Set up properly, this will automatically change the boiler set point based on outdoor temperature. Lower outdoor temps -> higher boiler set point. Higher outdoor temps -> lower boiler set point. You can get one good data point right now based on current operating conditions.

 

[Kezorm]

Based on the data you have so far, I expect that the 95F boiler setting will work year round in Connecticut. The 140,000 BTU/h boiler is probably oversized for the space by 3x or more. Luckily, modulating allows it to run efficiently even at less than maximum.

 

[600SL]

Temp is 35F today but has been down below 5 for two days this just last week. Currently keeping the building at 58F. Compared to the average garage in CT, This one is extremely well insulated. Compared to the average house in CT roof insulation is much lower a ~R19. Garage doors are as good as they can be in a 1.25" thick panel with well sealed edges and floor. But there are 3 doors. There seem to be a few outdoor temp settings but nothing called reset. The attached file has all the settings. The highlighted ones are the only ones I changed so far.

 

[Kezorm]

Do you have an outdoor temp sensor hooked up to the boiler? Page 59 of manual. That said, I'd be surprised if you ever need more than 95F from the boiler. I would try with it just set at the lowest you can. If outdoor conditions are ever such that it is running 100% of the time and not holding temperature, that's a sign that you need hotter water. From a standpoint of best efficiency and lowest wear on boiler, lowest water temperature that just matches load is best (i.e. boiler runs 100% of the time at low firing rate to match load).

 

[600SL]

Do you have an outdoor temp sensor hooked up to the boiler? Page 59 of manual. That said, I'd be surprised if you ever need more than 95F from the boiler. I would try with it just set at the lowest you can. If outdoor conditions are ever such that it is running 100% of the time and not holding temperature, that's a sign that you need hotter water. From a standpoint of best efficiency and lowest wear on boiler, lowest water temperature that just matches load is best (i.e. boiler runs 100% of the time at low firing rate to match load).

Not sure what all this means, but I do have an outdoor temp sensor. Are you suggesting I reduce my operating temperatures even more, maybe 100F max and 86Fmin for settings 16 and 17. I can give that a try but it will have to be a few days now as I need to turn off the system and let the slab cool over night so I can fire it back up and check it with a thermal imaging camera so I can drill holes for my lift.

 

[u3b3rg33k]

If I set the max temp to 135, what should I set the min temp to?

can you do an outside sensor for outdoor reset?

I've got a hydronic fan coil system and a PVC vented Rinnai combi, 104F min (lowest allowed) to 140F max (at 0F or so). I tried running it without outdoor reset and I was NOT pleased. Most of the season I'm in the 110-120F water temp range (in-floor should be lower). if I wanted to I could lower the max flow temp, but for combi-boiler specific reasons (simultaneous DHW+CH) I've chosen not to.

also re: flow rates, make sure you're giving the boiler adequate flow, don't choke things down unnecessarily, just enough to balance zones. You're better off with a SLIGHTLY higher return temp if it keeps your outgoing flow temp significantly lower.

 

[Kezorm]

The 4 parameters in the graph (6, 7, 16, 17) define the outdoor reset curve. Parameter 15 defines the warm weather shutdown temperature (boiler doesn't run at all above this outdoor temperature). Based on the numbers that you've shared, I would start with the following.

These settings set up the boiler to command 95F if the outdoor temperature is less than 0F. If the outdoor temperature is greater than 10F, then it commands a boiler temperature of 86F. Between 0F and 10F outdoor, the boiler set point will be somewhere between 86F and 95F. Boiler shuts down completely at 50F outdoor temperature (you can decide if you want it higher than this).

 

[u3b3rg33k]

Found it.

 

[Innovate1]

Is there a mixing valve in the system? That allows the boiler loop to run at a higher temp than the floor loops.

 

[Firebrick43]

Is there a mixing valve in the system? That allows the boiler loop to run at a higher temp than the floor loops.

Why would you want to do that with a modulating boiler with a 10:1 turn down ratio and is also condensing? There is no positive reason that I can think of?

 

[Innovate1]

Why would you want to do that with a modulating boiler with a 10:1 turn down ratio and is also condensing? There is no positive reason that I can think of?

Perhaps it's not needed. I'm just asking the question as others have brought up the need to keep the supply temp up to avoid condensing but maybe those were just general comments that don't apply to this case. I did see pics in the OPs other thread that looked like it was condensing so wondered why all the issues and questions about temps.

 

[PoorUB]

Perhaps it's not needed. I'm just asking the question as others have brought up the need to keep the supply temp up to avoid condensing but maybe those were just general comments that don't apply to this case. I did see pics in the OPs other thread that looked like it was condensing so wondered why all the issues and questions about temps.

You are confused about the boiler. A condensing boiler can run at any temp. There is no minimum temperature. I believe you are thinking of a cast iron boiler where you need to run the temps higher to avoid condensation.

 

[dscheidt]

You are confused about the boiler. A condensing boiler can run at any temp. There is no minimum temperature. I believe you are thinking of a cast iron boiler where you need to run the temps higher to avoid condensation.

Even then, it's only the exhaust temperature that has to stay above condensing. If you're willing to throw money up the chimney, like the knuckleheads who installed the boiler in my house were(it's ~200% the size of the radiation that was attached to it when we bought the house. I've added a bit since then. It's getting towards 3 times the actual heat loss.), then you can run lower return temps. When I bought the house, the boiler would turn on, run for a bit, and then cut out out because the output temperature reached the 185F high limit, even though the return water was still just lukewarm. the block would cool down pretty quick, the burner would fire another couple minutes, and repeat. a guru adjusted flow (increasing gpm through the boiler a lot) and added some balancing valves, and now most calls are satisfied with the boiler firing once. It should have a protection loop installed (the very strange near boiler piping suggests there used to be one, but it could have been for a previous boiler.), but the cost of doing that is substantial, and since I'm going to replace and relocate the boiler at some point, we haven't done it.