DIY Radiant heating between floor joists, questions?

cp1

New here , but looks like a good place for help with a few questions
Putting together a radiant floor heating system, in our older farm house. Going to use a tankless heater for heat source. Putting 1/2 inch Pex between the floor joists from underneath, aluminum plates and insulation. Will have 4 zones, so have a manifold and gathering other parts right now. The manifold has manual gate valves to control flow to the different zones. Trying to build things as simple as possible, so wondering where to put the pump? Common sense says to put it before the heater, so I won’t be pumping hot water , only cooler water from the return. Right?
Also , I have automatic air vents on the manifold, will that be enough or do I need another, since the zones /pex will be higher than the pump, heater, manifold etc. This will be a closed system, so I’ll get an expansion tank to put on the return side. Other than a relief valve, and shut offs, am I missing anything else? I’ll be building a control system with a thermostat etc, to control the pump. Just making it as simple as I can.
Thanks

hot_rod

Tankless water heaters are not a great match for radiant systems, they are designed for low flow rates at high delta T, a radiant system needs just the opposite. To make them behave properly may require primary secondary piping and two circulators, depending on the pressure drop of the tankless.
Yes, it seems like the frugal way out, but it can cause headaches. Search around this topic comes across HH very often.

Alan (California Radiant) Forbes

I've seen a lot of systems with on-demand heaters and they are just not worth the time and trouble. Hot Rod spells it out. They also require a larger pump to push water through their heat exchangers and they don't have outdoor reset controls that adjust the water temperature as the outdoor temperature fluctuates.

Ironman

Have you done a Manual J heat loss calculation to determine how much heat is needed and if the radiant floor can handle it? Chances are, with an old farm house, it can't. Unless the envelope has had substantial upgrades, you probably need at least 32 btus per square foot. The radiant floor will give you about 20 btus output per square foot.

I also concurr about th use of a tankless water heater: it's the wrong tool for the job. If a tankless could take the place of a boiler, there would be no need to make boilers.

Alan (California Radiant) Forbes

Take a look at Ultra-Fin for the joist bays. You can run higher water temperatures with greater output.

ultra-fin.com

Another reason not to go with an on-demand heater is that the maximum water temperate is typically limited to 140F.

ewang

As everyone else states - get a boiler. Its designed to do everything you would want.

I recommend looking at some installation manuals from top brands like Weil-Mclain and Triangle-Tube to learn more about system layout and components. That and using the search feature of the forum should get you up to speed fast enough to start asking the real questions about your planned project.

bill794

I also am a DYI radiant floor enthusiast. I started to put in floor heating around 1995. I've now completed about 6 different jobs for houses I've lived in and also for some friends. I started the way you are thinking of starting, doing a few rooms in a house we were renovating, just to see how we'd like it. I ended up needing to remove some of the the early work I did and replace it. I think you'll quickly find that you really like radiant floor heating. How much you'll like it also depends on where you live. I am in upstate NY where it gets cold in the winter. It is great to go into a house with warm floors. I'm telling you all this because I'd recommend that you do some engineering of your floor heat to avoid later problems.
There are several questions I could ask you that might help come up with the best recommendations.
First, are you planning to heat the house solely by radiant floor heat? What kind of heat do you now have in the house? Looking at your present heating system and how much energy you're using will give you an idea of how much output you need from the radiant heat.
I agree with other comments that you really need to calculate how much heat you need in each zone. This is pretty easy in new construction where insulation R values and window R values are well known. That task is a lot more difficult with older construction, and that is why some historical heating bills would be helpful.
In the latest house where I installed RFH, I calculated the heat loss from each zone and then sized the heat output of the floor heat to match the heat load. This can be very important if you are interested in energy efficiency, since the weakest zone in the house will set what water temperature you need to use. One common mistake is to undersize the heating needed in a bathroom (that people like warmer and that is often not set back at night, plus a lot of the floor area is covered by a vanity, shower, jacuzzi...), so that a small zone can end up dictating the water temperature used in the rest of the house.
I also would recommend using a modulating, condensing boiler. Selecting the boiler manufacturer is tricky for people who are not licensed plumbers or electricians. There are several boiler companies (like Bosch) that will not provide technical service or answer your questions if you are not a licensed plumber. Once you've picked the boiler company you'd like to use, first call their technical support number and ask if they'll work with you when you have questions - because you will. I've used the Weil McLain Ultra series for some projects and have had good luck with them. I am planning to build a house in Canada in the near future and have found the situation even more difficult there as far as finding a Canadian boiler manufacturer that will work with you as a DYI.
I have had very good luck using Tekmar house controls (like the 402 and 403). These house controls are a bit pricey, but they do a great job coordinating the calls from up to 4 zones. Also Tekmar provides the piping details that they recommend using. I think they now have a free design service. My son is an architect and he has gotten them to complete RFH designs for houses he is designing. They have a good Web site that I'd recommend you check out.
One of the reasons that RFH has not caught on as much as it could is that solid design/engineering work needs to be done in order to get the heat to work efficiently and correctly. When I tell people I have radiant floor heat, I'll hear some of them say that they have it in some part of their house and it doesn't work. That is primarily due to the fact that some installers without experience wing it with RFH and it ends up not working very well.
Good luck!

bill794

One other thing to look for your project - is to carefully inspect the underside of the subfloors. All the pitches I've seen about PEX show the aluminum plates that you can clip the PEX into, and they are shown being attached to the underside of new construction, and almost always as the first or second trade that gets access to the house. If you are planning an RFH retrofit, you should look closely at how realistic that is for your situation.
If you see a lot of impediments to installing the aluminum plates, you should definitely use Onix tubing instead of PEX. In addition to other utilities you will need to negotiate around (sanitary drains, supply plumbing, gas lines, electrical, forced air ducts...), are there a lot of nails penetrating through the sub-floor, etc... I've used Onix for several RFH retrofits and it is so much easier and faster to apply. This is especially true of you are doing the installation by yourself or with a single other person. Onix does not use aluminum plates, and is instead stapled directly to the subfloor. It is also much more flexible than PEX, and it tolerates being kinked, so it is easy to negotiate around all the pre-existing obstacles sited above.
Finally, if there are places that have big window areas or other large heat losses, you can put 3 or 4 runs of Onix in a 16" joist bay, and that boosts the heat output in areas where you'll have more heat loss. Onix is more expensive per foot, but what you really need to look at is the cost of PEX plus the aluminum plates, plus the additional installation time...

hot_rod

As @Ironman mentioned, you need to figure out the required Btu/sq ft load. Bare tube stapled up is the least powerful or efficient way to install radiant. More often or not it requires high SWT to cover a reasonable load. A little math goes a long way towards a sound system design.

TAG

Agree on the room by room heat load ... only then can you determine if the radiant can work alone. In an old house where you can't upgrade wall insulation .. small corner rooms tend to be a problem as they have two exposed walls and small floor footprint. They often need some other radiator --- I have used tall thin Runtal panels --- Buderus panels under a window .... and radiant wall in another.

My current project: I used heavier plates to get the water temp down to better match the other water temps needed in the house to make the system work w/o mixing vales

bill794

cp1 asked for advice about retrofitting radiant floor heating into an older house. I suggested that he look closely at the condition of the underside of the subfloor before deciding to install aluminum plates and PEX tubing. My experience is that the aluminum plate/PEX approach can be very difficult to implement in older houses. Onix staple up is much easier to apply when you need to work around obstacles and thread the tubing into place. These problems with PEX become even more difficult if you need to work in a crawl space, or you have selected some of the less expensive oxygen barrier PEX that is stiffer. cp1 didn't comment, for example, if the house has oak wooden flooring or other floor covering where nails are sticking through the bottom of the sub floor, resulting in the plates not being in contact with the sub floor. (These nails are often hardened metal, so good luck trying to clip them all off, or pound them down without disturbing the floor!)
hot_rod commented that Onix staple up requires a higher supply water temperature, and should only be considered as a last resort for that reason.
I've attached a picture of an Onix staple up installation in front of a 16' by 18' window wall. The R value of the wall is ~4. A design day in this area is -10 F. This is new construction, but it does prove that staple up provides enough heat output to heat areas with lower R value.
This space is solely heated by radiant floor heat, and is very comfortable all winter long. I've also attached the outdoor reset curve for the house. You can see that the supply water temperature is almost always low enough for the condensing boiler to operate efficiently. Additionally, the mix water temperature is limited to 155 F, so the house controller can boost the water temperature about 20 F above the typical maximum operating point. The boost allows room temperatures to reach a new set point in 15 minutes or so.
So I'll renew my recommendation to utilize Onix staple up for your project if there are a lot of places in your house where you have interferences.
Good luck with your radiant floor retrofit!

TAG

I took the advise on here to use the 3/8 PEX .... It's working well.

Yes -- you need to grind off the nails. A bit of a pain -- did not take long

hot_rod

The bottom line is you need to get the heat energy from the fluid to the floor surface, dozens of way to do that. It the floor surface temperature that is moving the load, regardless of how you warm it.

So the more efficiently you can connect the tubing to the floor the better the performance. With direct tube staple up you are limited by that small contact patch, since we prefer conduction to move the energy into a radiant panel. Also the tube wall thickness has a bit to do with the energy transfer.

Put some copper tube in aluminum transfer plates to behold some powerful transfer!

While this article addresses tube in a slab, the same thermodynamics is at play with dry systems. Get the energy to the floor.

For a period of time "suspended" or dangled tube was also heavily promoted, it too can work. UltraFin enhanced the dangled tube method

IF the goal is to cover the load with the lowest supply providing the highest boiler efficiency, then the use of a "conductor" is your best bet. Don't let convenience dictate the appropriate design If its worth doing, its worth doing right.

Ironman

Not to be unkind, but I wouldn't use Onyx tubing if you gave it to me and did the install for free.

Search this site and you'll find an extremely high failure rate and very low opinion of it by pro's.

bill794

There are a lot of good suggestions to help cp1.

TAG, really nice PEX installation!

However, the "old farmhouses" I've worked to renovate (as old as one built in 1835) have basements that look more like the attached photos. Plywood wasn't even used in houses until around 1960. The subfloor could be either 4/4 sheathing (starting in ~1920) or, more likely uneven rough swan lumber - often thicker than 4/4. An uneven subfloor means that aluminum plates do not make contact with the subfloor in a lot of places. Construction before 1900 often used joists that were wider and shallower - and more variable in dimension. It is possible to retrofit RFH into these older structures, but it takes a lot more effort. The upstairs of these houses (almost) always looks a lot better than the basement!

Would be good if we heard back from cp1 about exactly what he means when he says "old farmhouse..." What does his basement (or crawl space) look like?

Ironman - I would guess I will not change your mind about Onix, but just for fun, I injected two tubes of Sentinel X400 cleaner into my heating system today. The system hasn't been cleaned in 5 years, so it needed it anyway. I'll catch the water I flush out in a white 5 gallon bucket and show you how clean it is, versus the expectation of a lot of "black gunk".

bill794

TAG, et al - I just noticed from your picture that the aluminum plates for the PEX end about 2' from the outside wall. We've all been talking about the benefits of the aluminum plates for transferring a lot of heat from the PEX tubing through the floor. The the floor along the walls in this house is going to be cold zone, don't you think? In a way, you've made the problem worse by making the heat transfer so much better in the rest of the zone that the water temperature is lower, so once there are no plates and no PEX in contact with the subfloor, plus adjacent to outside walls and windows = a cold floor!
There are places in the houses where I install RFH that are right next to an outside wall and that need the floor to be warm. My customers don't specifically ask for, but really want the floor to be warm there. Examples include a kitchen where the counters are on an exterior wall with a view out a window, an office where the desk is next to a window with an outside view, a bathroom where the shower is next to an outside window. With Onix (not Onyx), I just put in more tubing there and that part of the floor ends up being warmer than the rest of the floor. No one has ever complained about that!
I'm only pointing out this problem with the approach you are all using in the hope that you have a way to boost the floor heat in these perimeter areas. How do you deal with this problem?

hot_rod

A little test I did some years back to check how well the Onix actually contacts the floor. Painted it with a thick gooey paint, stapled every 6", tapped the staples to assure they were tight.

This onbservation made it clear why so many of the systems needed to be "cranked up" to meet loads. I'd guess 1/2 of the loops are more like suspended tube, an air space, compared to conduction contact transfer. Assuming it is stapled correctly, which means checking every staple.

Certainly it is possible not every square inch of the plates are in contact either, but the tube is certainly in better contact with the plate.

Then came the failed tube and the chronic sludge issues with some, many? rubber systems.

I've done my share of rubber staple up, had it in 3 of my own homes. Many work fine, plenty slugged, some need much higher swt then the design called out. If the loads are are reasonable, I'd guess low 20 BTU/ ft, and installation is proper it can and does work. Flexibility is certainly it strong point. We drove concrete trucks over it on snowmelts, eliminating pumper trucks, so it does have some unique features.

My current mission is to design all systems to work at 120AWT or lower, so the best transfer money can buy is where I'm headed.


Here are some IR comparisons.

Ironman

I've said for years that those last two pics are worth a thousand words. Even if the Onyx continually contacted the floor, it doesn't begin to have the conduction area that plates do.

TAG

Bill: Project is a converted stone church. This is the upper floor about even with the lower roof line .... there is a knee wall above. It's a big room .. almost 900 sf surface area. Each row is 6 full plates .... trying to get the last foot on each side would not have added much and made for a difficult install

Jells

Don't know if the OP still cares, since he's not posted again, but sometimes you can't overthink this. I did suspended pex under a 2" terrazzo floor in my kitchen and bath despite warnings from far more experienced guys that it wouldn't work, that the BTU througput would never match the heat loss of my uninsulated brick rowhouse. I figured worst case was I'd have to put in a toe kick heater also.

I installed P-AL-P pipe, in 2 loops with r13 stapled under it, direct as a zone to my boiler. Has worked like a charm for 14 years. Floor is never over 77 or so.