Advice for Designing Radiant System for Small Cabin

Calling @Alan (California Radiant) Forbes !

And may I suggest looking into air to water heat pumps? One of those might be a good fit…

also consider an electric boiler. It’ll be more expensive to run but so much cheaper to installer. For a small space in a warm climate, it’ll probably make sense to pay higher bills than drop the money on an air to water heat pump.

"Consider using Gypcrete (lighweight) so you can have more of a high mass system. It makes such a difference in "feeling" the warmth of the radiant. Alot of people are a bit disappointed otherwise."

The latest thinking is that you get better comfort from a low heat capacity system that is able to shift temperature quickly and respond to changes in the load. That's what systems like Warmboard pride themselves on. A lot of what was attributed to "thermal mass" in the past turns out to be due to other properties of the material. What makes concrete work is not its nor its heat capacity, but rather the fact that it's a good conductor of heat so it spreads the heat evenly. Warmboard has a continuous layer of aluminum, which is an excellent conductor and gives even heating.

A benefit that is touted of high heat capacity emitters is that they warm slowly and cool slowly, so they emit heat continuously. A low heat capacity emitter with a properly calibrated outdoor reset will give the same kind of continuous heat while also being much more responsive to changes in the heating load.

"I have been considering the SANCO2 air to water heat pump. I had one of the most confusing (on many levels) calls with a rep at Small Planet Supply…"

I honestly don't know how SANCO stays in business. I've heard so many stories like that, of people looking into it and going a ways down the path before eventually discovering it just wasn't going to work for them. I don't know what situations it does work in, but it seems like a vanishingly small confluence of circumstances.

I've had a little bit of sales training, and one of the things I learned was that the first step in the sales process is to qualify the customer. Are they buying what you're selling? Because if they're buying apples and you're selling oranges there's no point in wasting both of your time. Just say, "sorry, at this time we're selling oranges and not apples, I hope in the future you'll consider us for all your orange and orange-related needs" and move on. For some reason SANCO seems resistant to do that.

OK, got the data. Based on the measurements I estimate a design heating load of around 12,250 BTU/hr. 99th percentile design temp is 31F, average annual low is 25F. My understanding is California requires you to size between those two numbers. The lower one would put you at 14,100 BTU/hr, I think in either case you're looking at a 2 ton heat pump. I don't think any 2-ton heat pump on the market would have trouble putting out 14K BTU/hr at 25F, I'm not aware of anyone who makes a unit less than 2 ton (other than SANCO, who I wouldn't recommend).

I'd think about cooling. The weather station data shows a 99th percentile cooling temp of 91F and an average annual high of 103F, in a small building you'll roast in weather like that.

It seems like the meat of your heating season is around 47F. While you want to insure comfort at the coldest temperatures, if you're going to the trouble of doing heated floors you want them to be delivering their effect most of the time, not just on the few coldest days of the year. At 47F your load is about 10,000 BTU/hr, if you were to put heating under the entire 850 square feet that would be 11.8 BTU/sf, which would mean a floor temperature about 6F above room temperature, or 78F in a 72F room. That isn't noticeable. I would instead target a small part of the house to be at 85F when it's 47F out. That would give 26 BTU/SF, you'd need about 400 SF of it. Then I'd have a second zone — which could be another 160 SF of floor, or a radiator or a fan unit — that provides up to another 4000 BTU/sf when the primary zone can't keep up.

Finally, don't take my word for all this. This is just to get you started. My understanding is California will want to see a real Manual J calculation.

The thing about Warmboard is it's really expensive, if I recall correctly about $10 per square foot just for the panels. It is the gold standard, you get even heat with low heat capacity and quick responsiveness. There are lots of ways to install a heated floor, the question becomes whether you can get equivalent results with cheaper materials.

When working with heat pumps you really want to keep the water temperature down, it's the key to efficiency. Which means you really want an even floor temperature, which means you want something highly conductive under the floor. With Warmboard it's a continuous layer of aluminum.

Ultimately it's going to be your local building official that decides.

My understanding is that California has a strict energy code, Title 24. It's not the same as the code used in the rest of the country, it divides the state up into zones. Before proceeding any further you should check in with your local officials and find out what they're going to want from you, both in terms of insulation levels and system sizing. This is something your architect should have already done but they don't always do, they often want to leave it to the HVAC sub and insulation sub. What you don't want to do is go down the road and find out that something that would have been an easy fix in the planning stages is now a big problem.

That actually looks pretty cool.

The picture says "certified," do you know if it is EPA-certified and UL-Listed? We can't install if it's not. Insurance company is even pickier.

Hi: I have a hydronic radiant floor in a 980 sf house and I absolutely love it. I was pressured to go with mini splits but I didn't want the fan sound and also — there's nothing like a warm floor. Of course, I'm in New England (-8 last night) so in CA you could easily get away with (what should be) less expensive mini splits along with a WELL INSULATED floor. Depends on what you want to feel and hear.

One more detail: I have two neighbors who had hydronic piping installed in certain areas, but not connected; they both later went through sticker shock re: the cost of a boiler. In the end, both installed simple electric DHW tank type heaters, as they each had enough excess solar PV production to supply the kWh. And they're both very happy. They got a plumber to install the water heater (after some persuasion). Much simpler and easier than trying to modify a heat pump DHW (with all its pre-programmed control algorithms) to this purpose.

I see they have added some accessories for the Cubic minis. A little water tank for making boiling water and a grill attachment.

Mine has a pipe connection at the air intake. I guess it could get air from outside. I like to crack a window regardless. And keep the CO detector running.

I have one of the heat motor fans also to help dry clothes in front of it.

I visited the factory years ago when I was in Montreal doing trainings. Nice people.

From the website it looks like the stove is UL-listed but not EPA certified.

Ha ha. You think 900 square feet is small. That's twice as big as a few condos I have had in Cambridge.