Radiant System Settings for Maximum Efficiency

KramerDIY

I'll state for the record that I'm a newbie. I'm a DIY homeowner. I purchased a radiant system from a company that targets the DIY audience and I installed the system exactly as they instructed me to. The good news is that it works!.....Mostly. But now I'm trying to configure it so it's running as efficiently as possible. So I'm hoping this forum can give me some advice.

About my setup....
We live in a cold climate (northeastern US). The house is newly built and only 950 square feet total. It has one bedroom, a large main room, and a mudroom (ceilings in main room and bedroom are vaulted). These act as 3 separate zones. The mudroom and bedroom are small enough that I was able to install one continuous pex loop for each of those zones. The main room has four separate loops in it that branch off from a manifold. The layout is so small that I was able to home-run these loops (ie. the source and return manifolds are located right next to the boiler itself).

I have a closed system. The heating unit is only for radiant heating (not used for domestic water at all).

I'm using an American GT-240 (http://www.americanwaterheater.com/residential/tankless/gt-240-nih).

What do I need help with?.....
It's only been in operation for a couple of months. Last night was a cold night. We noticed that the system was not able to maintain the thermostat's set temperature in the rooms (they were 5 degrees colder than set).

Our boiler heat output temp setting was 125 F. We raised this to 130 F and I know we can raise it up to a max of 140 F if needed. I've honestly wondered which is more efficient......do I keep the output temp low and run the pumps for a longer period of time? Or do I raise the output temp and run the pumps for less time? Since this is an on-demand system, there is no tank holding water (unless you count the water in the tube-runs as a "tank").

On a semi-related note.....while each zone has its own pump, our setup has a "helper" pump that comes on whenever any zone is called. This pump is near the inlet side of the boiler. There is also a by-pass loop built into this system with a ball valve on it. Originally, I had closed this valve to force all of the hot water to go through the zones. Then I started researching my issue this morning and started reading up on thermal shock. This is when I looked at the system and saw that my inlet temp was down to 82 degrees while my output temp was 125. I've read that the delta T should never be less than about 20 degrees. So I opened up the bypass valve and the input temp jumped up to over 100.

I was told originally that the purpose of this bypass loop was to prevent the helper pump from pulling water through zones that were not calling for heat. (So if zone 1 was turned on but not zone 2 and 3, the by-pass loop would prevent the helper pump from pulling water through zones 2 and 3.) But now, I'm thinking that this is there to prevent thermal shock to the system. Any thoughts on this? Should this bypass valve be fully opened, fully closed, or partially opened?

GroundUp

Yet another RadianTec wreck.... Sorry sir, but you got screwed. That water heater is 10x the size it needs to be if all zones run at once, the single loop zones are killing it. Thermal shock is not a thing with water heaters, remember ground water is 55ish degrees so you won't be able to shock it with radiant temps. Speaking of radiant temps, 125 is probably far too high of a supply temp- what is the under-slab insulation like, how thick is the slab, and what is the perimeter insulation detail? Some pictures of your setup would be great.

Voyager

It would be helpful to know the details of your system. Loop lengths, tubing sizes, tubing spacing, and pictures of your setup. Was a heat loss calculation done for your home? I didn’t see you mention if this is a slab on grade house or do you have a basement and the pex is in aluminum stapled under a wood floor.

And you don’t have a boiler, you have an on-demand water heater. And, unless you leave your windows open during the winter, you probably have a heat source that is 3-4 times larger than needed and maybe even more than that. I heat my 32’ x 48’ x 14’ insulated slab workshop with a 50,000 BTUH boiler and this is more than 50% larger than your house. And my boiler is even slightly oversized as the calculated heat loss is about 40,000 BTUH at design conditions.

It is impossible to say without a lot more structure and system details, but it sounds like you have a system that is very poorly designed.

Gordy

First of all some details about the radiant panel. Tube in concrete? If so was concrete insulated? If so how much?

Or is there a different radiant floor detail you are using?

Loop lengths, and pex spacing?

Flow rates through each loop?

The heat source is not ideal as mentioned above, and needs to be plumbed primary secondary.

To others commenting I would be cautious about bashing a certain company. Law suits have been threatened in the past to others on this site. Tongue in cheek approach is best.

Gordy

After rereading your post it sounds like your discription is primary secondary piping. Some pics of the setup would help.

KramerDIY

Here is a pic of my setup. Sorry for the confusion. Yes. It's an on-demand water heater. I used 1/2" O2 barrier pex for all runs from the unit.

It was my understanding that this heating unit is designed for structures up to 1200 square feet. (Granted, it's designed for combined domestic water and radiant usage).

It is not in a slab (we don't heat the basement). This is stapled up to the subfloor of the main living floor from underneath with continuous heat plates.

Bedroom is 168 square feet. Total tube length is ~252 ft. Flow rate is 0.75 gal/min.

Mudroom is 160 square feet. Total tube length is ~240 ft. Flow rate is 0.75 gal/min.

Main Room is 616 square feet. Total tube length is ~924 ft (but uses 4 ~230 ft runs). Flow rate is 3 gal/min.

SuperJ

It does appear that your tankless is rated for space heating.

Zman

I am assuming that insulation was installed in the floor bays?

Most homes today which are reasonably well designed and insulated will have a BTU loss of < 20 Btu/sq ft. You can do the math on your water heater. If you could get the water to flow through the constricted heat exchanger, you could heat a typical 8,000 square foot home with the unit you have.

Radiant heat is much nicer with outdoor reset on the supply water temp. This feature is not available on water heaters. They are specifically designed to heat domestic water.

For now, I would suggest setting the water heater temp to the lowest setting that will heat your home. Leave the bipass valve open. Save your money to buy a very small and efficient condensing boiler when you get tired of repairing the oversized POS you have been sold.

Voyager

It would take a fair bit of analysis to really understand what is going on, but I can say a couple of things from a quick look at your pictures.

1. You have a primary/secondary system, but I don’t see an expansion tank. Is the expansion tank built into the water heater?

2. Your primary/secondary system is improperly designed as you do not have hydraulic separation between the zones. That requires closely spaced tees or a buffer tank or a purpose built separator. I am not sure if that is causing you any issues, but it can as the zone pumps can interact with each other in undesirable ways. I believe both Taco and Caleffi have great videos on their web sites and YouTube that address hydraulic separation and why it is a good thing.

3. You effectively are heating your basement as you have no insulation under your aluminum panels. Remember that radiant heat is not subject to gravity the way that convection is (well, at least not in a meaningful way and we don’t want to get into Einstein’s theories here). So, your aluminum panels are radiating heat downward as well as upward into the floor. Insulating your stud cavities will help force more heat upwards, though you lose the benefit of having some heat going into your basement. I am not sure what proportion of heat is going down vs up and the upward is getting both radiant and conduction, but I would not be surprised if you are losing 20% or more of your BTUs downward into your basement.

How thick is your subfloor and what floor coverings do you have?

Voyager

Here is a picture of my system which shows the closely spaced tees that connect the primary and secondary loops. Sorry this is not a good picture as I was not just getting the boiler and near boiler piping, but rather my entire system including water and gas feeds. And the window in view messes up the lighting, but if you zoom in I think you can see the essence of the hydraulic separation. Please don’t take this as necessarily a good example overall as this was my very first installation and I am sure the pros here will be able to point out at least a few flaws, not the least of which is crappy looking soldering ... but nothing leaks at least!

Gordy

Those plates are less than desirable as they don’t fit the tube snuggly. Omega style channel is the best.

Voyager

Zman said:

I am assuming that insulation was installed in the floor bays?

Most homes today which are reasonably well designed and insulated will have a BTU loss of < 20 Btu/sq ft. You can do the math on your water heater. If you could get the water to flow through the constricted heat exchanger, you could heat a typical 8,000 square foot home with the unit you have.

Radiant heat is much nicer with outdoor reset on the supply water temp. This feature is not available on water heaters. They are specifically designed to heat domestic water.

For now, I would suggest setting the water heater temp to the lowest setting that will heat your home. Leave the bipass valve open. Save your money to buy a very small and efficient condensing boiler when you get tired of repairing the oversized POS you have been sold.

Question: With no hydraulic separation, even with the bypass valve open, isn’t the primary loop pump likely to draw at least some flow through the “non calling” zones given that all three returns are pretty close to the suction side of the primary circulator? That likely isn’t causing the problem of failing to maintain temperature and the water heater should have BTUs to spare, but it might put some heat in rooms that aren’t calling for heat, right?

SuperJ

I think what you have should be capable of heating your house. Even though it's far from text book. On demand heaters can be tricky since they heat to a fixed setpoint and rely on a flow switch to start heating. And have a high pressure drop.
Open the ball valve in your primary fully.
Can you measure the supply water temp in each of your circuits? Make sure it is the same as the boiler supply, if it isn't you need more primary flow. Your pump probably has three speeds if you need to more flow make sure it's set appropriately.

You primary secondary piping isn't ideal. It looks like you went out of your way to add pipe and fittings 😀. You should confirm all your zone circulators have check valves except for the primary pump. Otherwise flow can go backwards.

In your case I think you could install a hydraulic separator fairly easily to give you proper separation. Or perhaps a buffer tank to tame the oversized tankless heater.

The biggest problem as others have said is the lack of insulation. To prove the point, fully insulated one small zone, you should see a huge difference and with an empty basement like that it should only take a hour or two and a couple hundred dollars. I prefer mineral wool to fiberglass personally.

Voyager

Yes, it would not be too hard to make a few modifications to get a lot of benefit with not too much cost. I hate all forms of fiber type insulation, but it is cost effective. If it were my house, I would probably cut 2” XPS foam to fit the cavities and just wedge it into position below the top flange of the I-joists. That would be a quick installation with little breathing hazard, but might be a little on the pricey side.

DZoro

You absolutely need a pressure tank. What lb pressure relief valve is that?
The relief valve should be placed immediately coming out of the boiler on the hot side.
I'm surprised your not asking why it is releasing water.... Actually more concerned why,
Looks like you don't have any make up water either.
If this is the case you need to address those issues immediately, as in turn the system off type of immediately...
D

Gordy

With out reading the other comments. If you piped it primary secondary you would only need two circulators. If you want zoning then add zone valves to the the loops you want to zone.

One grundfos 26-99 for the tankless loop, and one of the extra pumps for the secondary loop. If zoning with zone valves use a delta p circulator. Should give you 6 gpm for all loops together. Shouldn’t need that much though.

Get an expansion tank installed in a location where pumps pump away from it. Add a good air separator.

Get the floor joists insulated if it has not yet been done. Pay extra attention to sealing the rim joist detail with spray foam, and Rigid xps insulation.

The plates are what it is you have already bought them.

Brewbeer

Are those really radiant heat transfer plates? It looks like aluminum roll flashing that was cut to lengths, then stapled around the tubing to the subfloor.

Can you post a closer photo of the plate detail?

Voyager

Brewbeer said:

Are those really radiant heat transfer plates? It looks like aluminum roll flashing that was cut to lengths, then stapled around the tubing to the subfloor.

Can you post a closer photo of the plate detail?

Good eye. I didn’t look that closely before. These definitely were not preformed as the “channels” on some appear to be angled to follow the tubing. And it appears they only have incidental contact with the tubing. Probably will take some really warm water to heat these plates significantly.

Paul Pollets

I'd be surprised if O2 barrier tubing was supplied, which means a boiler would need additional protection and costs to install. Without adequate insulation (R19 minimum) under plates, system will always under perform.