a way to get the return water hotter?

synolimit

so i have a pretty large delta on my radiant floor, its going out at 120* and returning a little above 70*. i may have made the mistake of only putting 2" foam under the basement slab as the normal ground temps only 53-55*. i think im losing to much to the ground or i need to change something in the setup.

the system works fine. the basement is nice and warm. it may run longer than i want though so i may be spending to much on propane. the system is 6 runs, under 300' (280' average) of 1/2 pex on a 1/12th hp 3 speed pump. the heating unit is a 199,000btu Rinnia tankless.

ive tried to mess with the "boiler" temps and the pump speeds but the delta is still there. i think the delta was even greater at 140* going out at max speed and it was still coming back in the 70's.

what else can i try?

PS my tubes might be to far apart also in some spots. roughly 1800-1900sqft.

synolimit

here you can see a 4' wide foam board and theres only 4 runs on it. so average may be 9.6"-12" apart as its bunched up in some spots and wider in others.

synolimit

the set up

PC7060

Is the system maintaining the desired room temperature?

mattmia2

Ideally it runs continuously at the temp that matches the heat loss. Slabs take a long time to heat, the delta could be huge for days until it reaches equilibrium. Your swt may be too high.

synolimit

PC7060 said:

Is the system maintaining the desired room temperature?

yes it is. i think it runs to long though, last year when it was 28* i remember it running for 4.5hrs straight. however that is unfair as we had no heat upstairs so all the cold would come down into the basement. we are still building the house.

synolimit

mattmia2 said:

Ideally it runs continuously at the temp that matches the heat loss. Slabs take a long time to heat, the delta could be huge for days until it reaches equilibrium. Your swt may be too high.

if its 68 now and i set it to 68 and its off, the slab isnt up to temp? its always been around 50 delta even last winter. my what may be to high? whats swt mean?

leonz

If you invest in a hot water storage tank you will have created a buffer to aid in heating your basement.
You need additional thermal mass to aid in heating your basement.

Hot_water_fan

What is the head loss through that tankless? Is that a boiler or a domestic water heater?

Jamie Hall

Physics is such a nuisance. Your delta T and flow determine -- very exactly -- how many BTUh you are putting into the slab. The slab characteristics determine, then, whether its temperature will rise, stay the same, or fall.

You have two choices. First, increase the flow rate. That will give you a smaller delta T and, for the same source temperature, a higher overall slab temperature (eventually) -- provided, of course, that your boiler can produce enough power. Second, reduce the heat loss from the slab.

synolimit

Hot_water_fan said:

What is the head loss through that tankless? Is that a boiler or a domestic water heater?

not sure. domestic water heater
https://www.supplyhouse.com/Rinnai-RU199IP-RU199IP-199000-BTU-Super-High-Efficiency-Condensing-Indoor-Tankless-Water-Heater-Propane

synolimit

leonz said:

If you invest in a hot water storage tank you will have created a buffer to aid in heating your basement.
You need additional thermal mass to aid in heating your basement.

not sure i follow. ive never seen anyone use this in any setup.

synolimit

Jamie Hall said:

Physics is such a nuisance. Your delta T and flow determine -- very exactly -- how many BTUh you are putting into the slab. The slab characteristics determine, then, whether its temperature will rise, stay the same, or fall.

You have two choices. First, increase the flow rate. That will give you a smaller delta T and, for the same source temperature, a higher overall slab temperature (eventually) -- provided, of course, that your boiler can produce enough power. Second, reduce the heat loss from the slab.

so right now im on medium flow on the 1/12th hp pump. i need to increase to high flow? i believe that was still only 70-72* when i tried that out but i can try again. would i need a bigger pump? like a 1/6th or something? the garage only uses a 1/25th pump so i could sway the 1/12th over to the garage and install a new pump on the basement. the garage is only 3 runs long.

how would i reduce the heat loss from the slab? i cant since its already installed. your saying i should of done 3-4" of foam under the slab vs the 2" i used?

Hot_water_fan

not sure. domestic water heater
https://www.supplyhouse.com/Rinnai-RU199IP-RU199IP-199000-BTU-Super-High-Efficiency-Condensing-Indoor-Tankless-Water-Heater-Propane

Okay looks like that might be your problem. This is not a central heating appliance, so shouldn't have been installed to heat a floor. What's happening is your pump is not able to overcome the head loss of the boiler, let alone the entire system. On page 66, you'll see the head loss curve. At 3gpm, that's 12ft of head JUST through the boiler. The second link is a 1/12th HP circulator. Not sure if that's the one you have or not, but this one can only get up to about 7ft of head, period. So you'll never get 3 gpm, especially when the tankless is so restrictive. So what you're seeing is a trickle of water going through the boiler and a large delta T as a result. If you google "Caleffi Idronics" journals, you can find resources on how to do this the right way. Ideally, you could probably heat this space with ~ 80 degree water and a 10 degree Delta T.

https://s3.amazonaws.com/s3.supplyhouse.com/product_files/Rinnai-RU199IP-Install-Instructions.pdf

https://s3.amazonaws.com/s3.supplyhouse.com/manuals/1348755618119/81388_PROD_FILE.pdf

synolimit

Hot_water_fan said:

not sure. domestic water heater
https://www.supplyhouse.com/Rinnai-RU199IP-RU199IP-199000-BTU-Super-High-Efficiency-Condensing-Indoor-Tankless-Water-Heater-Propane

Okay looks like that might be your problem. This is not a central heating appliance, so shouldn't have been installed to heat a floor. What's happening is your pump is not able to overcome the head loss of the boiler, let alone the entire system. On page 66, you'll see the head loss curve. At 3gpm, that's 12ft of head JUST through the boiler. The second link is a 1/12th HP circulator. Not sure if that's the one you have or not, but this one can only get up to about 7ft of head, period. So you'll never get 3 gpm, especially when the tankless is so restrictive. So what you're seeing is a trickle of water going through the boiler and a large delta T as a result. If you google "Caleffi Idronics" journals, you can find resources on how to do this the right way. Ideally, you could probably heat this space with ~ 80 degree water and a 10 degree Delta T.

https://s3.amazonaws.com/s3.supplyhouse.com/product_files/Rinnai-RU199IP-Install-Instructions.pdf

https://s3.amazonaws.com/s3.supplyhouse.com/manuals/1348755618119/81388_PROD_FILE.pdf

while i dont understand it all, if the water is leaving the heater several feet down and showing 120* on the gauge, im not sure how its a heater issue. to me the loss is in the floor. ive yet to understand head loss though. the heater being 199,999BTU that can supply 6-8 showers, you'd think that can have any amount thrown at it and be fine. my house water pressure though is 70psi so thats a big difference from 13psi on the pump. wouldnt more psi and a bigger pump help out?

synolimit

Hot_water_fan said:

not sure. domestic water heater
https://www.supplyhouse.com/Rinnai-RU199IP-RU199IP-199000-BTU-Super-High-Efficiency-Condensing-Indoor-Tankless-Water-Heater-Propane

Okay looks like that might be your problem. This is not a central heating appliance, so shouldn't have been installed to heat a floor. What's happening is your pump is not able to overcome the head loss of the boiler, let alone the entire system. On page 66, you'll see the head loss curve. At 3gpm, that's 12ft of head JUST through the boiler. The second link is a 1/12th HP circulator. Not sure if that's the one you have or not, but this one can only get up to about 7ft of head, period. So you'll never get 3 gpm, especially when the tankless is so restrictive. So what you're seeing is a trickle of water going through the boiler and a large delta T as a result. If you google "Caleffi Idronics" journals, you can find resources on how to do this the right way. Ideally, you could probably heat this space with ~ 80 degree water and a 10 degree Delta T.

https://s3.amazonaws.com/s3.supplyhouse.com/product_files/Rinnai-RU199IP-Install-Instructions.pdf

https://s3.amazonaws.com/s3.supplyhouse.com/manuals/1348755618119/81388_PROD_FILE.pdf

PS i believe this is mine. also the garage is a 1/25th pump, 3 runs, still the same delta drop. water heater works fine with the low flow of water going though it.

https://www.homedepot.com/p/Grundfos-UP15-29SF-1-12-HP-115-Volt-Stainless-Steel-1-Speed-Circulator-Pump-59896771/302859589

Hot_water_fan

while i dont understand it all, if the water is leaving the heater several feet down and showing 120* on the gauge, im not sure how its a heater issue. to me the loss is in the floor. ive yet to understand head loss though. the heater being 199,999BTU that can supply 6-8 showers, you'd think that can have any amount thrown at it and be fine. my house water pressure though is 70psi so thats a big difference from 13psi on the pump. wouldnt more psi and a bigger pump help out?

Correct - water is leaving the heater at 120F. The issue is that you want ~3-4 GPM and you're not getting that because the tankless is too restrictive for that pump to flow that rate. You're likely getting <1GPM through the floor. The tankless is not undersized, quite the opposite. The issue is you're not getting enough flow because this appliance wasn't designed to be used like this and the set up isn't sufficient to address this mistake. Right now, you have a minivan with bald tires and 800HP.

Jamie Hall

synolimit said:

Hot_water_fan said:

not sure. domestic water heater
https://www.supplyhouse.com/Rinnai-RU199IP-RU199IP-199000-BTU-Super-High-Efficiency-Condensing-Indoor-Tankless-Water-Heater-Propane

Okay looks like that might be your problem. This is not a central heating appliance, so shouldn't have been installed to heat a floor. What's happening is your pump is not able to overcome the head loss of the boiler, let alone the entire system. On page 66, you'll see the head loss curve. At 3gpm, that's 12ft of head JUST through the boiler. The second link is a 1/12th HP circulator. Not sure if that's the one you have or not, but this one can only get up to about 7ft of head, period. So you'll never get 3 gpm, especially when the tankless is so restrictive. So what you're seeing is a trickle of water going through the boiler and a large delta T as a result. If you google "Caleffi Idronics" journals, you can find resources on how to do this the right way. Ideally, you could probably heat this space with ~ 80 degree water and a 10 degree Delta T.

https://s3.amazonaws.com/s3.supplyhouse.com/product_files/Rinnai-RU199IP-Install-Instructions.pdf

https://s3.amazonaws.com/s3.supplyhouse.com/manuals/1348755618119/81388_PROD_FILE.pdf

while i dont understand it all, if the water is leaving the heater several feet down and showing 120* on the gauge, im not sure how its a heater issue. to me the loss is in the floor. ive yet to understand head loss though. the heater being 199,999BTU that can supply 6-8 showers, you'd think that can have any amount thrown at it and be fine. my house water pressure though is 70psi so thats a big difference from 13psi on the pump. wouldnt more psi and a bigger pump help out?

I'd agree that the power usage (heat loss) is in the floor. Naturally. The problem is that the combiination of the high head loss in the water heater and the tiny pump can't produce enough flow to carry the power required. Now if you could get enough flow through the heater (well over 3 gpm) to carry the power of the heater to the floor you'd be fine.

You have the answer right there, though: your house water pressure is 70 psi before the heater. At the shower head it's at most 10psi; the rest of the pressure disappears in the water heater. If you insist on using the water heater -- wrong tool from the job, but when all you have is a hammer everything looks like a nail -- you will need a much bigger, higher pressure pump to move the amount of water you need. You will also need to change the expansion tank and the pressure relief valves to accommodate the higher pressures. You may need to change the pipe and fittings, too, depending on what you used.

hot_rod

Tankless water heaters need flow to ramp up to full output, but the flow requires a large enough circ to provide it.

here is the pressure drop chart for that heater. I think you will need something along the line of a Grundfos 26-99, etc.

synolimit

Hot_water_fan said:

while i dont understand it all, if the water is leaving the heater several feet down and showing 120* on the gauge, im not sure how its a heater issue. to me the loss is in the floor. ive yet to understand head loss though. the heater being 199,999BTU that can supply 6-8 showers, you'd think that can have any amount thrown at it and be fine. my house water pressure though is 70psi so thats a big difference from 13psi on the pump. wouldnt more psi and a bigger pump help out?

Correct - water is leaving the heater at 120F. The issue is that you want ~3-4 GPM and you're not getting that because the tankless is too restrictive for that pump to flow that rate. You're likely getting <1GPM through the floor. The tankless is not undersized, quite the opposite. The issue is you're not getting enough flow because this appliance wasn't designed to be used like this and the set up isn't sufficient to address this mistake. Right now, you have a minivan with bald tires and 800HP. </p>

i gotcha. it still works though but im coming back to cold and probably spending to much in gas since at 70* return the slab just takes to long to heat and stay hot? the heater would be a pain to replace! but it was only $900 with a bent front door so it didn't cost the $2600 retail. pumps might be cheaper and easier to swap in with the way i have it setup. i mean a 1/2hp pump would be 5 times greater so wouldnt that take me from less than 1gpm to over 3-4gpm? its not cheap either at $600.

synolimit

Jamie Hall said:

synolimit said:

Hot_water_fan said:

not sure. domestic water heater
https://www.supplyhouse.com/Rinnai-RU199IP-RU199IP-199000-BTU-Super-High-Efficiency-Condensing-Indoor-Tankless-Water-Heater-Propane

Okay looks like that might be your problem. This is not a central heating appliance, so shouldn't have been installed to heat a floor. What's happening is your pump is not able to overcome the head loss of the boiler, let alone the entire system. On page 66, you'll see the head loss curve. At 3gpm, that's 12ft of head JUST through the boiler. The second link is a 1/12th HP circulator. Not sure if that's the one you have or not, but this one can only get up to about 7ft of head, period. So you'll never get 3 gpm, especially when the tankless is so restrictive. So what you're seeing is a trickle of water going through the boiler and a large delta T as a result. If you google "Caleffi Idronics" journals, you can find resources on how to do this the right way. Ideally, you could probably heat this space with ~ 80 degree water and a 10 degree Delta T.

https://s3.amazonaws.com/s3.supplyhouse.com/product_files/Rinnai-RU199IP-Install-Instructions.pdf

https://s3.amazonaws.com/s3.supplyhouse.com/manuals/1348755618119/81388_PROD_FILE.pdf

while i dont understand it all, if the water is leaving the heater several feet down and showing 120* on the gauge, im not sure how its a heater issue. to me the loss is in the floor. ive yet to understand head loss though. the heater being 199,999BTU that can supply 6-8 showers, you'd think that can have any amount thrown at it and be fine. my house water pressure though is 70psi so thats a big difference from 13psi on the pump. wouldnt more psi and a bigger pump help out?

I'd agree that the power usage (heat loss) is in the floor. Naturally. The problem is that the combiination of the high head loss in the water heater and the tiny pump can't produce enough flow to carry the power required. Now if you could get enough flow through the heater (well over 3 gpm) to carry the power of the heater to the floor you'd be fine.

You have the answer right there, though: your house water pressure is 70 psi before the heater. At the shower head it's at most 10psi; the rest of the pressure disappears in the water heater. If you insist on using the water heater -- wrong tool from the job, but when all you have is a hammer everything looks like a nail -- you will need a much bigger, higher pressure pump to move the amount of water you need. You will also need to change the expansion tank and the pressure relief valves to accommodate the higher pressures. You may need to change the pipe and fittings, too, depending on what you used.

i wouldnt think id need to change anything but the pump. the system will still only be pressurized to 10-20psi. everything i have is basically made for domestic hot water which is 70psi and maybe around 105-110*. if that doesnt damage anything then a bigger small size pump will be fine. i mean my well pumps 240v and flows 20 gallons a minute with the same parts.

synolimit

hot_rod said:

Tankless water heaters need flow to ramp up to full output, but the flow requires a large enough circ to provide it.

here is the pressure drop chart for that heater. I think you will need something along the line of a Grundfos 26-99, etc.

thats not bad, only 1 size up. i guess ill need 2 though. the house has 6 runs, the garage has 3 runs. currently the house is a 1/12th hp and the garage is a 1/25th hp and has no issues heating the space besides how long it runs for. last winter i remember it ran for 1.5hours when it was 28* outside. the garage is set to only 55*. i dropped that run time however when i installed an in floor sensor so when the doors open its not turning on as much or running as long since opening the doors isnt going to cool the concrete 1" down.

Teemok

"Not designed for this use" but a Giannoni heat exchanger just needs a higher head pump. Per manufacturer.

synolimit

wait so i found my pumps!! my 1/25th can do 0-19.5ft head and 0-17gpm. my 1/12th pump can do 0-18ft head and 0-25gpm. so my pumps arent the issue??? both the heads are over the tank at 12ft head??? so my ground is stealing the heat? more speed may not hurt though. a 1/6th hp pump is showing 0-29ft head and 33gpm max.

hot_rod

Is the entire heater piped with 3/4 Pex? If so  that will be a limiting factor on how much flow you will get also.

It is bit of a crap shoot trying to force a tankless water heater into believing it is boiler🥴

 Larger pumps may get you over the hump, or primary secondary piping where the heater gets a dedicated pump

Teemok

The delta T and flow rate through the boiler determines how may BTUs it can push to the slabs till the boiler is the weak link (Max ouput). I'm guessing 199 BTU input is not your weak link. You are not asking enough of the Rinnai (low flow) balancing the BTU delivery rate of the boiler (flow rate) to the needs of the zone at the lowest possible supply temperature for the outside temperature is the golden key. Some times, a pump sized for the tank-less's ideal flow and a pump perfectly sized for ideal slab flow is best. Look up primary secondary piping. If your setup happens to have flow rates and heat losses for both parts that match and a reasonable pump can achieve both, you can run primary only piping (one pump), reaping highest boiler efficiency. Watch the pump amperage some times two pumps do the job more electrically efficient than a single big one and remember application engineering is fun.

Teemok

Oh, the ideal on cycle is just one while the water supply temp tracks the outdoor temperature so that the heated space conditions are stable while weather conditions vary. Your tank-less can't do that. It's still a decant heat source though not always ideal.

Larry Weingarten

Hi, How much work would it be to install a flow meter in the line? It seems like the information it gives you would be useful in eliminating guesswork.

Yours, Larry

Teemok

@ 8 gpm flow rate and 63F in -110F out you get close max 188kbtu/hr out of the tank-less Design for 5 gpm @ 90F in and 110F out and it's down to 50kbtu/hr. 5gpm @ 100F in and 110F out you only get 25kbtu/hr. Slabs have lots of mass that need lots of btu/hr to begin to deliver heat to the space. If your slab needs 250k to just warm up to 75F from cold and you can only deliver 50kbtu /hr it will be close to 5 hours before the slab starts heating the space well.

Hot_water_fan

wait so i found my pumps!! my 1/25th can do 0-19.5ft head and 0-17gpm. my 1/12th pump can do 0-18ft head and 0-25gpm. so my pumps arent the issue??? both the heads are over the tank at 12ft head??? so my ground is stealing the heat? more speed may not hurt though. a 1/6th hp pump is showing 0-29ft head and 33gpm max.

It’s a curve so as GPM increases, ft head decreases. You’re still undersized. The 12ft is just through the boiler itself, you have the entire rest of the system to consider. By the way, the height of the components doesn’t matter when it’s a pressurized system in regards to pressure loss. Check out the Idronics journals! They’re great resources. 

I gotcha. it still works though but im coming back to cold and probably spending to much in gas since at 70* return the slab just takes to long to heat and stay hot?

No, gas usage won’t change if delta T decreases. 

synolimit

hot_rod said:

Is the entire heater piped with 3/4 Pex? If so  that will be a limiting factor on how much flow you will get also.

It is bit of a crap shoot trying to force a tankless water heater into believing it is boiler🥴

 Larger pumps may get you over the hump, or primary secondary piping where the heater gets a dedicated pump

no just in and out and to the manifolds.

synolimit

Larry Weingarten said:

Hi, How much work would it be to install a flow meter in the line? It seems like the information it gives you would be useful in eliminating guesswork.

Yours, Larry

not hard but what am i looking for? if i know my pumps max speed, wouldnt that tell me my flow rate? or do i want the meter after the tank to see whats coming out?

synolimit

Teemok said:

@ 8 gpm flow rate and 63F in -110F out you get close max 188kbtu/hr out of the tank-less Design for 5 gpm @ 90F in and 110F out and it's down to 50kbtu/hr. 5gpm @ 100F in and 110F out you only get 25kbtu/hr. Slabs have lots of mass that need lots of btu/hr to begin to deliver heat to the space. If your slab needs 250k to just warm up to 75F from cold and you can only deliver 50kbtu /hr it will be close to 5 hours before the slab starts heating the space well.

sounds about right. watch this or skip to 8:10. 4hrs and 35min of run time.

https://youtu.be/Td2keRa6178?si=AxMsu5hANzM3ciyn

synolimit

Hot_water_fan said:

No, gas usage won’t change if delta T decreases. 

i yeah i forgot about all the running. 1800 feet of runs is head space too. the pump manufacture did say it could do those runs though.


it wont? if i run for 4hours lets say by returning at 70*, the concrete is taking forever in places to heat up and hold the heat with a 50* delta. if i could get the water to return at 100*, the concrete, specially in the room where the temp sensor is, shouldnt the system shut off sooner than 4hrs? any amount of shorter run time should decrease my gas usage if the tank is still set to 120*. or are you saying if i increase the flow, the tank will put more BTU's into the water therefor im saving on run time but increasing gas flow usage?

GroundUp

The issue here is most certainly not the insulation.

-You used the wrong appliance
-You piped it wrong
-It's grossly oversized for the load
-You're running the water temp too high

If you're going to use this water heater for space heating, it needs to be piped primary/secondary with 1" ID piping to cover the load you've given it. The outlet temp should be turned down to 90-100 degrees. The unit will run longer at a lower firing rate, thus increasing your efficiency while also decreasing the delta T through the slab. High mass systems running high temps for a short time are terribly inefficient, they should be running 24/7 on the coldest day of the year. Your actual heat load is likely less than 50,000 BTU, so that huge tankless will never be as efficient as the proper appliance would have been, but you can certainly run it until it craps out in a few years then replace it with a proper boiler.

Jamie Hall

synolimit said:

Larry Weingarten said:

Hi, How much work would it be to install a flow meter in the line? It seems like the information it gives you would be useful in eliminating guesswork.

Yours, Larry

not hard but what am i looking for? if i know my pumps max speed, wouldnt that tell me my flow rate? or do i want the meter after the tank to see whats coming out?

No. The flow rate and pressure increase relationship of a pump is determined by the geometry of the pump impellor and diffuser assemblies. Every pump model has a characteristic pressure rise/flow relationship, and the manufacturer's literature will have that information.

Maximum speed for a variable speed pump only shifts that relationship (pump curves are drawn for the maximum speed, for variable speed pumps).

Hot_water_fan

it wont? if i run for 4hours lets say by returning at 70*, the concrete is taking forever in places to heat up and hold the heat with a 50* delta. if i could get the water to return at 100*, the concrete, specially in the room where the temp sensor is, shouldnt the system shut off sooner than 4hrs? any amount of shorter run time should decrease my gas usage if the tank is still set to 120*. or are you saying if i increase the flow, the tank will put more BTU's into the water therefor im saving on run time but increasing gas flow usage?

Neither, it’s the same gas usage if the delta T closes. The hourly btus math is 8.34 x GPM x delta T x 60. Right now, you’re probably a GPM of .75 and a delta T of 50. It should be a GPM of ~3.75 and a delta T of 10. Those both are the same Btu/h. 

 yeah i forgot about all the running. 1800 feet of runs is head space too. the pump manufacture did say it could do those runs though. 

It can do those runs, it can’t do those runs and pump through a DHW tankless. Try the Caleffi Idronics journals