Small Radiant Heat Floor Tankless Electric Hot Water - Help

Everything in the system has to balance. The heat released by the floor is determined by the difference between its surface temperature and the air in the room. That has to equal the heat delivered by the fluid, which is equal to the temperature delta in degrees F times the flow in GPM time 500.

If the temperature drop is not as big as you'd like, with the same heat output you need to reduce the flow. However, this will tend to reduce the temperature of the floor, which will reduce its heat output.

The question is going to be whether the water heater has a minimum output that is low enough to match what the floor actually puts out. If not, it may be easier to replace the tankless water heater with a small tanked water heater.

https://electromn.com/electric-mini-boiler/

Electro makes very small boilers. They can handle high incoming temps.

Which circulator is that? Looks like you have a tiny delta T between the two thermometers. Is that not what you expected? If the flow is quick enough, it’ll be small. Live and learn!

Do you know what the heat loss of that room is? A quick back of the envelope calculation suggests to me that last night it may have been around 8,000 BTUh, unless those are very good windows. Hmmmm…

Now let's look at another number… you have about 5 gpm with that pump. You have a maximum heat loss through the floor of 10,000 BTUh. That should produce a delta T of around 3 degrees. You may not actually be far off that.

The pump is hotter, by the way, because it is water cooled — and the cooling water isn't cool…

So. Going back to the back of the envelope. You are probably putting out as much heat as that floor can put out, assuming that there is no carpet or furniture or what have you blocking part of it. That much heat isn't capable of getting the air temperature much higher, as the heat loss of the room is soaking it all up. The delta T of the water indicates that you are putting way too much water through the pipes…

Try reducing the flow through the floor, as has been suggested — no more than 1 gpm. That will give you a more manageable delta T. But… I suspect that you are going to struggle to get that space much more than 40 degrees or so warmer than it is outside, no matter what you do.

What is the heat load for the room?

Getting an 80° floor surface with wood, carpet and pad? That is going to be a tough go.

Are you wanting to maintain a room temperature or a floor surface temperature?

Slowing the flow the flow rate will not increase the heat output.

Here is a graphic of how that works. The tighter the delta, the higher the average temperature of the loop, the higher the output. Basic law of thermodynamics.

I suspect the carpet is the constipation. Pull apiece back, see what the wood surface temperature is.

Are you sure the loop(s) are purged and actually flowing?

This was a total finger blunder on the calculator by me. 16*144= 2304 BTU/hr. That's 675 Watts.

To get 3500 Watts — 12,000 BTU/hr — you would need 83 BTU per square foot. That would require a floor temperature about 42F above room temperature, or 114F. You'd probably have difficulty standing on that. You're going to need to add some sort of supplemental heat to the room, there's no way the floor is going to be able to deliver that. Most people find a floor above about 90F uncomfortable to stand on. At 90F the floor will deliver 36 BTU/sf or just over 5000 BTU/hr or 1500 Watts.

To get 675 Watts with a 10F temperature differential you'd need a flow of about 0.46 gallons per minute. With the observed 2F differential you'd need a flow of 2.3 GPM. That doesn't sound all that far off from what you're seeing.

one 1/2” pex loop? How long is it? That will be the determining factor on how much flow you can move, and how much heat you transfer. If you can eliminate the carpet you will feel the warm surface better. Good to know you have another source of heat

If that is a 15-58, on high speed it is around 78w. You may need speed 3 to over come the pressure drop in the loop and that heater. That wattage is the least of your cost if you have a 6 kw heater😉

With the loop or or loops length we can work backwards to an expected outcome.

While the heat load is 3.5kW, the floor itself is only going to be able to put out maybe 1500W max and more like 700W at your desired conditions. The heater you have is way oversized, and because of the tankless design it's not able to modulate that low. Definitely send it back. It doesn't do anything for you that a 1500W small tank heater wouldn't do.

@ag27 wrote: "The water going into the floor is about 110 and coming out is like 108."

That really tells us everything we need to know about water flow. I have no idea how to predict the heat flow in this assembly, but I know that unless you modify the assembly the heat flow through it is going to be entirely determined by the difference between the room temperature and the average water temperature. Right now the average water temperature is (110+108)/2= 109F. With source water at 110F you're never going to get an average water temperature above 110F, which isn't going to give you meaningfully different heat flow from the current average of 109F. Increasing the flow might give you return temperatures of 109F, 109.5F, or even 109.9F, but none of them are going to meaningfully change the heat flow — and hence surface temperature — of the floor.

If I'm understanding @ag27 properly, the design goal isn't to provide a certain quantify of heat, it's to keep the floor at a certain temperature. At design temperature the floor is only going to provide about 15% of the heat for the room, the minisplit provides the rest. But heat flow and surface temperature are linked, surface temperature determines heat flow into the air. So if currently the surface temperature isn't what's wanted, the heat flow through the floor assembly needs to increase.

The heat flow through the floor assembly is entirely determined by three things: the average water temperature, the floor assembly construction, and the temperature of the room air. It's a little late now to change the floor assembly construction (although I consider rugs and furniture to be part of the assembly and they might be modified). And presumably you don't want to make the room colder just to make the floor warmer. That leaves water temperature. You need to increase the average temperature. That can be done by increasing the flow, or by increasing the source temperature. We've already looked at — and discarded — the flow. What needs to be done is increasing the source temperature.

One final note: the heat output of a floor like this is pretty sensitive to room temperature. In order to test the system you have to have room temperature at actual room temperature. In much of the east it was unseasonably warm for much of the past month, it was over 80F in DC last week. You can't test the floor under those conditions.

The challenge with a system like this is temperature control

If the mini split maintains the room at say 70, and you want to feel a warm floor, say 80 f, then the floor adds heat at a rate of 2 btu/sq ft/ degree difference

So an 80 floor temperature in a 70 space will add 20 btus to the room and it will over-heat the space

I have tried this numerous ways for master bath floor warming, with dual stats and floor sensors, but never found a way to maintain a floor temperature and not over heat The floor needs to be at or above skin temperature to feel neutral or warm, around 80. Carpet tends to need to be even warmer to notice the warm toe feeling.

Ideally the floor would be the primary heating method, better yet the only heating method. But with all that glass, it may not be possible? The load calculation holds the key to this answer

A two stage thermostat with floor sensing limit might work out best. Let the floor do as much as possible, with an indoor/outdoor reset control, mini split as stage 2. So it does get complicated with two systems.

I may be thinking too far outside of the box here, but I like simple. 😁 https://www.thereviewnerd.net/best-heated-socks?utm_source=bing&utm_medium=cpc&utm_id=519618295&utm_content=1327112850032733&utm_term=what%20are%20the%20best%20heated%20socks&creativeId=&gclid=&msclkid=bcbdbd4d57b514ad21e7da7a2da117db

Yours, Larry

A heated floor is going to put out 2BTU/hr for every degree F difference in temperature between the floor surface and the room temperature. With a surface temperature of 80F and a room temperature of 72F that's 16 BTU/hr per square foot; with 144 square feet that's 2304 BTU/hr.

@ag27 said his design load is 3500W and that he's in the Boston area. Design temperature for Boston is 13F (suburbs are probably a little lower). In BTU, 3500W is 12,000 BTU/hr. The design temperature is 59 degrees below room temperature. The floor will exactly balance when the indoor-outdoor temperature difference is 2304/12000= 0.192 of the design temperature difference, or 11F below room temperature, or 61F.

So when it's 61F or lower outside the floor can be kept at 80F, the heat pump runs to make up the difference, and everything is good. The question is, what happens when it's warmer than 61F? And the answer is another question, what do you want to happen? There's two alternatives: either you have the floor be a little bit cooler, or you let the indoor temperature creep up. For example, if you let the floor temperature drop to 78F, that's 6F above room temperature, 12 BTU/sf/hr, 1728 BTU/hr, 0.144 of design temp difference or 8F below room temp, which balances at 64F.

If you let the room temperature rise to 74F it's the same 6F difference and you end up with the same 8F below room temperature. But since the room is 74F now that balances at 66F.

If you do both, let the room rise to 74F and let the floor drop to 78F, that's 4F of difference, 8 BTU/hr/sf, 1152 BTU/hr, .096 of design temp difference or 6F below room temperature, balance at 68F.

My ideal would be to say at 75F outside the inside is at 75F, the floor is at 75F and the heat is off. From there as the temperature drops the indoor temperature drops, the floor temperature rises, until at 12F outdoor temperature the indoor temperature is at 72F and the floor temperature is at 80F. Below 61F outdoor the floor and indoor temperature stay constant.

Now, how practical is this? For a small system like this, not very. You need some way of sensing the outdoor temperature, and some way of modulating the water temperature and/or sensing the floor temperature. In high-end hydronics this kind of outdoor reset is very common but that's a whole other game. If you like to tinker, you could experiment with outdoor sensors and various ways of controlling the floor temperature.

If you don't want to tinker, the simplest control mechanism is modulating the floor temperature to keep the room temperature constant. This means that at 61F outdoor the floor will be at 80F and at 72F the floor will be at 72F, and at temperatures in between it will be a straight line. So at 67F outdoor the floor will be at 76F and the room will stay at 72F. In this scenario you don't measure or directly control the floor temperature, rather the thermostat turns the circulator pump on and off in response to the room temperature.

A great looking space!

I'd figure close to 400 sq feet of wall at maybe R-5 with the glass an lumber.Doesn't look like much wall framing to insulate. What's the r value in the ceiling and floor? It just seems the loads would be more than 27 btu/ft in a space like that?

It's going to come down to how much $$ and technology you want to throw at it. The small boilers like the Electro MN and Thermo 2000 have outdoor reset built into some of their models, and they modulate.

I used one of these Thermo 2000 mini's in a 160 sq ft tiny home I build. I put a lower wattage element in it to avoid short cycling. I think the smallest they offer is 3 kw.

This gives you a real, low pressure drop electric boiler with the outdoor reset onboard. Plus more KW/btu if you can drive the floor higher. If the carpet stays you might run 85F?

If you can get in the crawl space this JAGA "canal" heaters could run off the boiler and give you the boost you need without the split running. They have some cool corner radiators also.

https://www.thermo2000.com/wp-content/uploads/produits/miniULTRA/miniULTRA-flyer-EN.pdf

https://jaga.com/ex/radiators/

I'd like to see 100% of the heating be hydronic.

When we have rooms that don't have enough floor square footage to cover the heat load, we add supplemental heat. This is common in kitchens where the cabinets eat up so much of the floor space. The floor is a radiant heat emitter. Being low temperature you need a lot of surface area.

Kick space heaters or some sort of panel rads, are often used as supplemental heat. . Jaga makes heat emitters that run at radiant temperatures for that extra output. That simplifies piping as you need just one temperature.

Anything you can do to keep that heat load below the floor lower would help. Is there perimeter insulation around the foundation? That is a big cold spot. The lower the heat load the easier and less $$ to heat or warm the floors.