Radiant floor not holding temp

singh

Is the thermostat not being satisfied and not going over 65* ?
Is the edge insulation on the exterior of the building or interior? Is the underslab insulation a ridgid blue board type or a tarp type?
I see you paint . Great work by the way! Do you have some type of ventilation system going while air brushing or garage doors open for lomg periods of time?
How large is the shop? Ceiling heights,etc, was a heat loss calculation done. Glycol added? Perhaps that's all you can get out the floor with out supplemental heat.

The slab seems ok other than the stripping (68*-80*) but that's because of the 16 " o.c. , which is ok for a shop like yours .

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Christy

New system - radiant floor not holding heat

We have finally got our new radiant floor system up and running and it won't hold heat. Outside it is getting down into the single digits and our floor never drops below 60 but it won't get above 65 without an additional heat source.

This system is in a concrete slab, approx. 3000 ft 1/2 inch pex tubing. It is in an heavily insulated shop with insulation also below and around the sides of the concrete slab. We are using an on demand 84M Noritz gas heater, a taco Zbf1 circulator pump. The water temp coming out of the water heater is 140 and the return is 85.

I can provide more info if needed.

Thanks!

Brad White_184

When

was the slab first fired up? It takes some time to warm up all of that concrete.

How many circuits do you have? For 3,000 LF of pex, I would hope you would have at least 10 if not 12 to 15 circuits. If you have less than 10 circuits they are too long and you are getting a reduced flow which may in part explain the wide supply to return temperature difference.

Contact your contractor or designer to ask. If you have installation photos please post them.

If the slab were already warmed I would be concerned about that delta-T (the 140 F. in and 85 F. out).

When first started, I would expect a fairly wide delta-T as the heat is absorbed. Once warm though, I would expect a narrower delta-T, maybe 10 to 15 degrees and at lower temperatures entering the slab too, depending on floor covering.

Your using 140F supply water seems high for what is a concrete slab. What floor finish do you have that might be adding some thermal resistance?

hot_rod

Is there a mixing device

or does the water go right out of the Noritz into the floor loops? Was the circulator sized to the pressure drop in that Noritz?

if you started with a 60F slab, you should be feeling some temperature increase within 2- 4 hours, if everything was designed and piped correctly, specifically the number of loops as Brad mentioned.

Were all the loops purged? Does the heater ever catch up and shut off?

hot rod

Andrew Hagen_2

Flow

Do you have primary/secondary piping? What is the spacing between the tubes in the floor? As Brad noted, it sounds to me like you have a low flow issue. There should be another number on the circulator in addition to the zbf1. Can you post a digital photo of the piping near the water heater?

Weezbo

Christy ,

i live in Alaska,heavy delta changes in supply and returns is not altogether unheard of here ,however, 30 ->40 below zero would be considered a non event in the background of a seasons extremes.....so i answered your post to get some indication of where this might be occurring ...i would be curious as to the piping ,control strategy, depth of the concrete pour ,length of time you have had heat loading the structure ,depth of water table, thickness of the insulation beneath the slab and along the perimeter and how far laterally the insulation extends underground away from the building and its depth....

loop lengths might be something to consider also as variations in lengths + overly long loops in the mix can produce non harmonious *~/:) radiant.

If you own a I R temp gun you might consider taking a reading or few along the perimeter of your building from within 3 4 inches from the slab to about 12 ->16" ,should you be trying to heat something other than the building you might be able to understand where the additional heat is going .

easily you have plenty of heat available however it sounds as though you are heating the great outdoors.or have some mixing strategy that either is confusing the numbers that you gave us or is not indicative of what is circulating within the slab loops. any air possibly still in or is there some chance that there are loops turned (valved )off within the slab?

i type slow two more replies:) since i began plinking at the keys *~/:)

Christy

more info

The slab was cold before it was fired up with the waterheater. There is supplemental heat with a large wood stove but the water heater has never been able to catch up. We have 1 zone with 12 different circuits. spacing of the tubing in the floor is approx 16" on center. All of the loops are appro. 250 ft long - we were very careful to keep these the same length. I should be able to get some photos in the morning. Took readings with a digital infrared heat gun. The floor had some hot and cool spots from 68-80 degrees but the output and return pipes are about the same temps respectively (130• - 87•). Also took some readings outside the building. Around the base of the building it is about 5 degrees warmer than the rest of the ground but this is only right next to the insulation of the foundation. I am beginning to think it is probably not properly purged of air.

Brad White_184

A few thoughts/questions

At that 16-inch spacing density and 3000 LF of tubing, I am guessing that your floor slab is about 4,000 SF, correct? If at say 30 BTUH per SF that is about 120 MBH. The Noritz has a BTU input of about 236 MBH, is that also correct?

If so, you have plenty of capacity but I have to ask if it cycles and what you are doing to buffer the flow. Random thought. Do purge each circuit and see what you get.

The 12-circuit layout is great to know and at between 0.50 and 0.75 GPM per circuit, your flow rate would be right in there for the Noritz 84, although I have never used one. Just comparing raw numbers.

All for now.

Brad

heatboy_2

What is a Zbf1 Circulator?

Since the water heater has a high pressure drop, your issues may center around the pump.

Mike T., Swampeast MO

I seriously doubt that your problem is "a radiant floor not holding heat". From everything you say, the problem is that your "water heater" cannot move enough heat into the floor! You've likely discovered why such heaters, designed for HIGH temperature rise at LOW flow are not well suited for space heating--especially an inherently low temperature system like tube-in-slab!

If the Noritz instructions for a radiant floor system were followed, you have a single Taco 0014 circulator driving the system without any form of primary/secondary. e.g. the single circulator moves water through both the heater and the floor. In this configuration it is a physical impossibility for the heater to deliver anywhere its rated output to your system!!!

At best it would appear the circulator can move about 5 gpm through the system. I'm not sure what to make of the dual head loss ratings for the heater, but it would appear that if your "set temp is 135F or higher" that the most flow you can expect is only about 2.5 gpm! [With 12 * 250' circuits of 1/2" tube @ 5 gpm your head loss in the floor tube itself is nearly inconsequential at 1.75' or less.]

Primary/secondary piping and some form of mixing valve and/or buffer tank appear to be absolute requirements for this system. Even then, if your goal was an efficient radiant system, you won't achieve such with your choice of heat source. Please take this in the spirit intended, but it looks to me like someone cheaped out on the choice of heat source. Tankless water heaters are NOT boilers--and certainly not mod-cons that thrive when driving low temperature systems! If you used a professional installer who provided a guarantee of suitability, then it's time for a serious discussion. If not, then it's time for some serious thinking: "Do I want to put in a bunch more money to get the system working marginally or do I want to use a proper heat source and have a comfortable and highly efficient system?

Mike T., Swampeast MO

From a Google search I'm almost postive that it's a Taco 0014

My thoughts exactly Heatboy. I don't see any way for that circulator to work for this system given the extremely high head loss in that water heater. The so-called "instructions" that Noritz provides for connecting a space heating system to this appliance look to me to be a guarantee of poor performance--especially for tube-in-slab.

ALH_4

Flow Rates

My thoughts are that the circulator is very likely undersized. The head loss through that heat exchanger is very high. Based on the bronze circulator, it also sounds as if this may be an open system where potable water is circulated through the floor?

There should be model numbers on the circulators in addition to the zbf1. The model numbers probably start with "00". Unfortunately, the head loss curve goes off the charts at 8 gpm @ 50ft of head, which results in a 60°F temperature rise at full output. Your circulator is almost certainly undersized to provide a decent temperature rise. I think the water heater is boucing off the temperature setpoint and the circulator cannot provide enough flow through the water heater to get the heat to the floor.

Edit: The Taco 1400-50 would provide the 8-10gpm for a 60°F rise, but that consumes 1150 watts per hour. Really this system should have had a boiler instead an instantaneous water heater. I dont think you will get acceptable operation from the Noritz.

Christy

I must have miscalculated something because the shop is only 2500sf. They must have been 200 ft sections of pex because I know we still have some left. The designer of this sytem said that this tankless heater would be more than enough, but that is why I am here asking because something isn't right.

The thermostat has been set at 85 the last 7 days to try to bring it the floor up to temp. the insulation (pink foam) is between the slab and the foundation walls. The insulation under the slab is the tekfoil Reflective Foil Insulation for under radiant floor slabs. Not sure if this is the brand name but I know this was the type of product we used under the shop.

We use a recirculating filtered downdraft paint booth for large jobs (once or twice a month) but for the small airbrush jobs a simple mash is all that is necessary. The paint is already semi dry when i comes out of the brush so there is very little overspray fumes. We do have 2 big overhead doors but the are very seldom used, have a walk through door for people. About 1000 sqft of the shop has 16ft ceilings with fans but the rest is standard height. We also put 12 inches of R49 Blown-In Cellulose insulation.

This is a closed system, not sure why they chose this circulator. The full numbers are 0014-ZBF1.

Thanks for the compliments on the painting. I love my job:)

ALH_4

Circ

The Noritz has plenty of output, but the 0014 pump cannot move the water through it at a high enough flow rate to get the heat from the Noritz to the floor. I estimate you have approximately a 5gpm flow rate through the Noritz (assuming primary/secondary piping), which would indicate a 90°F temperature rise, not far from what you are seeing. If you do not have primary/secondary piping, you have an even lower flow rate.

I hate to say it, but I dont see how the Noritz can be made to work properly with your system without a very large circulator or two circulators in series. Instantaneous water heaters are designed for low flow, high temperature rise applications, typically 50°F to 70°F rise. Radiant floors are relatively high flow and low temperature rise, typically 20°F to 35°F rise. It's just not a good match, even though it saves a few thousand bucks on the installation.

EDIT: Since you have a return temperature of 85°F, you would need a supply temperature of 175°F to get full output. I dont know if the Noritz will allow a supply temperature that high. That is also too hot for a comfortable radiant floor unless you install a mixing device and separate circulator for the floor.

Dennis Lee

NORITZ

Was wondering what the true r-value of the underslab insulation is? I saw in last post you mentioned a reflective product.If it is just bubble foil type product, that will not help matters.

Christy

foil insulation

http://www.rima.net/pdf_files/TB101-UnderConcreteSlabs.pdf

Don't know if this link will work but this is where we got some of our info. We were afraid to put just the foam core underneath the slab because of the "big rigs" that we will have in the shop. It would have offered better insulating properties but we decided on the foil instead.

singh

Christy

This Is why I asked earlier if you and ridgid insulation or tarp type.
That "tekfoil" bubble stuff has very little R-value. And should not have been used.
I supposed your losing a great deal of heat below slab, eventually it will catch up.
I don't see why the tankless would not work, although I would not use it for heating. But others can comment on that.

Good Luck

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Unknown

Christy,

Do the floor manifolds your tubing connects to have flow meters on each loop?

They are typically clear plastic cylinders with a graduated scale up the side. Typically ranging from 0.2 to 1.5 or so.

Mike T., Swampeast MO

Search a bit here and you'll find some good information regarding "bubble foil" insulation under slabs. General concensus is that while better than nothing, it's not much better...

At least you have slab edge insulation which is required. I must ask though, is there a thermal break between the slab and any driveway(s) for those overhead doors? If not, the driveway(s) will be sucking out serious energy from the slab.

Unless ground water is contact with the slab, even with very little insulation value underneath there shouldn't be too much of a problem as the ground eventually achieves a sort of thermal equilibrium with the earth itself acting as the insulation. (Ground water in the "insulating" zone of the earth will cause serious problems however.) Thousands upon thousands of older slab radiant systems were installed without insulation underneath, but they ALWAYS included edge insulation.

While I can't see your system, I still believe that the real problem lies in the inability of the water heater to move sufficient energy into the slab.

If as someone else suggested, that bronze circulator was chosen because this is a combined DHW/space heating system with the two not separated by some sort of heat exchanger, then you'll probably have a hard time finding any true professional willing to work on such a system as they pose genuine and potentially deadly health concerns. In this case I would most sincerely suggest getting a true boiler--preferrably a condensing/modulating boiler--for the space heating and using the "water heater" for its intended purpose--domestic hot water.

Does the PEX tubing used have an oxygen barrier?

Unknown

what?

this is a 2500 sq ft space. It is highly unlikely the load is any more than 75kBTUs/hr.

That much heat could be drawn out at 5 GPM with only a 30 degree temperature rise... totally realistic.

That said, a mod/con would pay for itself relatively quickly on a system of this size with the likely water temperature requirements.

Christy

"If as someone else suggested, that bronze circulator was chosen because this is a combined DHW/space heating system with the two not separated by some sort of heat exchanger, then you'll probably have a hard time finding any true professional willing to work on such a system as they pose genuine and potentially deadly health concerns."

What? You lost me - Deadly health concerns? I work out there daily, is there something I am missing?

Driveway is not in, still gravel and up against the shop it is only about 5 degrees warmer than the rest of the outside ground. Ground water is not a problem our well is approx 350ft deep (we live in high desert).

I am not sure about the pex tubing oxygen barrier. It is bright orange/red in color. Would have to ask the husband and he isn't here. I know it was the more expensive of the 2 pex tubing selection we were looking at.

Mike T., Swampeast MO

Does the water heater provide hot water from faucets as well? If so, you should see a sort of "box" somewhere with four pipes--two leading in, two leading out. This is a heat exchanger that separates the domestic (from the tap) hot water from the water contained in the slab tubing.

The main health concern is legionella. This is an EXTREMELY nasty bug that lives and grows in stagnant (or nooks/crannies in slow moving) water below about 140F--just the sort of conditions that will occur during the non-heating season in your slab. While not incredibly common, legionella is not rare--I personally know three people (one a seasoned pro here at "The Wall") who have had the disease. None died, but they all came awfully close... By the time you figure out that "this isn't the flu" and seek medical attention you already have one foot in the grave!

Christy

legionella

This is a closed system with glycol sp? antifreeze in it. Can this bug grow in here or is it only a concern for open loop systems with potable water? Thanks!

ALH_4

Fixing the problem

It sounds as if you have a few things working against you. The insulation is essentially non-existent. Reflective insulation has no effect when in contact with the materials being separated. You need to run the Noritz as hot as possible and use primary/secondary piping and a mixing device to prevent the floor from overheating. The pump for the Noritz should be *at least* a 0014, and I bet a 007 would work for the radiant floor tubing.

As far as "open" systems (shared potable and heating water) go, Legionella bacteria can live in the floor tubing and cause sickness if mist is inhaled or tainted water is ingested, particularly during the non-heating season. Generally open systems are discouraged due to the possible health concerns. You can add a heat exchanger to reduce the possibility of sickness. Ideally, potable hot water chould be delivered through insulated pipes at over 150°F to kill the bacteria in a reasonable time (approx 2 min) with point-of-use tempering valves at the fixtures to protect from scalding. This is almost never done in residential or commercial plumbing and there are rarely problems, probably because the pipes are regularly flushed. However, with stagnant water in the radiant tubing, the possibility of bacterial growth is amplified.

Edit: I see you have a closed system. There is no issue, however I do question the use of a bronze circulator if you have tubing with oxygen barrier. Usually the tubing will have "O2 Barrier" or just "Barrier" printed on it. If it doesnt, it probably isnt, and you would need the bronze circulator. From my limited knowledge of your system, Primary/secondary piping with a mixing valve will help your problem. You might get the opinion of a second contractor. Unfortunately, I disagree with the manufacturer's diagram for heating applications. 2 gpm will not deliver much heat unless you have a very large temperature rise.

Mike T., Swampeast MO

Not a concern in a closed system supplying only space heating.

I must say that the glycol is only adding to your problems as it cannot carry the same amount of heat as plain water.

In all honesty, I see your heat source as the true problem.

Is there a single pump for the entire system?

Unknown

you're incorrect. while I am not a fan of instantaneous heaters, there is little doubt that this heater can do the job, as long as you call the job "heating the space", not "heating it as economically as possible".

the non- p/s piping might be an issue.. air bubbles might be an issue.. heat loss might be an issue.

But if you're ever going to use an instantaneous heater, the time to do it is on a fairly large slab. like here.

A mod/con would still be a much better choice. But the problem here is very much most likely flow related. I would wager p/s piping, or a good air purge will likely solve the underheating issue.

Then the question is entirely one of economy.

ALH_4

Instantaneous for heating

I see the application for an instantaneous heater as a heat source is where there is a small heat load. I see the head loss through the Noritz heat exchanger as a very difficult hurdle to clear unless the heat load, and therefore flow rate, is relatively low. I dont know what the heat loss is in this case, but it would be extremely difficult to extract the full output of the Noritz unless a large circulator was used.

Noritz recommends sizing the pump for 2gpm @ 10ft plus system loss. A 15-58 would do that. Even at a 40°F delta-T you can only get approx 40,000 btu from the unit. I'm not sure what the head loss of other brands of instantaneous water heaters is, but it can be a major problem if it takes 50ft of head to force 8 gpm through the heat exchanger.

Am I way off base? Is there another brand that has less head loss?

Unknown

There is no application for an instantaneous water heater small loads are better served with tanks, and bigger loads like this are better served with mod/cons.

That said, sure, you can't extract the full output of the heater with a small pump.. but, you don't have to, not even remotely. Only about half. Coincidentally, the basic output of the water heater assumed about a 70 degree dT... getting half of that with a bit less than half of the delta T should be completely achievable, no?

The 0014 will do 5 GPM at 20 feet of head. Probably just about right if this were p/s. As is, it's probably undersized. I would convert to p/s, keeping this pump for the "boiler pump", or maybe upsizing one step and reusing this pump for the "system pump".

I'd probably install a mixing valve body on the secondary side as well, just in case mixing is in fact necessary.

That's assuming there isn't money to put in a mod/con. That would be my first choice, and for a system this size in a real heating climate, I would assume payback would be real for such a switch out.

Unknown

There is no application for an instantaneous water heater small loads are better served with tanks, and bigger loads like this are better served with mod/cons.

That said, sure, you can't extract the full output of the heater with a small pump.. but, you don't have to, not even remotely, the full output is over 200kbtus!!! Only about half. Coincidentally, the basic output of the water heater assumed about a 70 degree dT... getting a bit less than half of that with a bit less than half of the delta T should be completely achievable, no?

The 0014 will do 5 GPM at 20 feet of head. Probably just about right if this were p/s. As is, it's probably undersized. I would convert to p/s, keeping this pump for the "boiler pump", or maybe upsizing one step and reusing this pump for the "system pump".

I'd probably install a mixing valve body on the secondary side as well, just in case mixing is in fact necessary.

That's assuming there isn't money to put in a mod/con. That would be my first choice, and for a system this size in a real heating climate, I would assume payback would be real for such a switch out.

This is absolutely not 15-58 territory, and given that we like to push homes three times this size with 15-58s, it is somewhat wasteful. But that doesn't mean infeasible.

hot_rod

Noritz offers the NH hydronic series

their website lists it for @ 2200 bucks and shows an 82% efficiency.

If in fact you need a high head circ to get anywhere near the full output (150,000 X 82%= 123,000 output at sea level)

Now if you take it to the high desert altitude, and add the glycol mix, that output get's hammered again.

Seems a small mod con, most include the boiler circ, and offer modulating and OD reset, listed HX in most cases... I'm trying to see where this type of appliance actually fits the hydronic market?

hot rod

Unknown

absolutely with you HR.

good catch with the glycol/altitude as well. that will make it worse.

Mike T., Swampeast MO

Rob: Have you looked at the head loss specs for the heater in concern? Much less the incredibly low water content in the HX (0.3 gallons)?

Do you have any idea why there are two different head loss curves based on ultimate temperature setting? I sure don't but I have a sneaking suspicion that the head loss "floats"--dare I say "modulates"--between the two values. Why the warning (e.g. "contact us") against using with fin-tube baseboard unless the heater intentionally changes restriction? You can pack an awesome number of BTUs into pure water at 80+F delta-t, but when it comes down to tube-in-slab where even 20F delta-t is considered high during maintenance there no way on earth nor in heaven for that water heater to deliver ANYWHERE its output if the installer follows the provided instructions unless they use an absurd amount of energy just to circulate the water for reasonable design conditions--to include the ONLY way with primary/secondary. This is basic, simple, utterly confirmed math and I can only assume that the installer was a basic plumber with essentially zero understanding or experience with hydronic heating systems. I have a terrible feeling that this is a thick slab with the tube at the very bottom--even the best have a hard time keeping it higher than 2/3 below the top.

Mike T., Swampeast MO

Let's add another downer. Is this LP?

Unknown

Again.. it does not have to provide anywhere near its listed output here. Nor do they ever, in these applications. Their rating is ludicruous for heating, yes. Luckily, no 2500 sq ft shop requires 200kBTUs/hr.

This is a shop. We design around 20 degree drops standard in a home. 30 degrees in a shop is routine. But that's neither here nor there; this should definitely be primary/secondary. The piping is a problem; the HEAT SOURCE is not the problem. With a piping fix, she could meet her load with this heat source.

That said, she shouldn't, she should get a mod/con. But that, one more time, is not saying that the noritz cannot do this job. I am not running design quality numbers here, but I would be utterly shocked if, once the piping is rectified, that this heat source could not heat the shop.

That means the heat source is not the problem. Follow?

Christy

Solutions...

Wow, thank you everyone for the input. I am trying to work through it all now. I am coming up with a few possible solutions.

1.make sure all the air is purged first.
2.get a bigger circulator pump
3.ask my installer if we can take out the Noritz and price some mod/cons.

Of course the third solution will be the most expensive/biggest change but we might have to look at this option. The system has yet to be finalized so I am hopeful we will be able to come to a solution before this is all said and done.

If we were to change out the pump/circulator what size would be best matched for a system of this size?

If we were to look into a mod/con where do we look and what size would be appropriate?

Of course these are just options, but we have A LOT of money tied up in this shop and we need it up and running. Once again I appreciate all the input, wish I would have found you guys first.

One other thing I need to mention, the temp does lower at nighttime by 5-10 degrees. Don't know if this indicates anything or not.

Oh and one final thought, we do have an additional 700ft of shop that will be eventually added to this system. Might need to consider this as well in the options.

Thanks,

Unknown

not "get a bigger pump".

"Repipe for primary/secondary piping, add a pump, AND maybe get a bigger pump at the heat source".

That is, there should be one pump going through the heater and a separate pump handling flow through the radiant.

That should be a fairly simple piping change in the mech room. You would also have to do this if you put in a mod/con, so that could be a nice interim step to see if the noritz will handle the load, while you save up for a more efficient heat source.

MechTech_2

Radiant floor not holding temp

Like everyone else I'm not a big fan of tankless units for space heating, but I have about 50 installs under my belt, and not by choice! Most of the units have a max flow that they can handle before velocity becomes an issue, so let's do some math. If the unit has a max capacity of 8 gpm and our radiant is running at a 20 degree delta T, the max output of the tankless unit will be 80,000 BTU's, lower the delta T to 10, and our output is 40,000 BTU's. At 8 GPM I typically have to have a primary pump that can overcome 40 feet of head! Install a high temp heat emmitter on a tankless unit, and watch you domestic hot water problems start. If you set a tankless unit at 160 for your air handler, you will install a thermostatic mixing valve for the domestic. Remember that minimum flow rate that's needed to trigger the flow switch (around .7 gpm) before the unit will fire up, it has now jumped. You may open a tap at 1.2 GPM and have .6 gpm cold through the mixing valve, and .6 through the tankless, and these flow rates will fluctuate as the mixing valve reacts, the net result will be cold blasts of water every time the flow rate drops below .6 - .7 gpm.