I never use an outdoor reset. It's only needed if you set the boiler delivery temperature so low it can't heat the house in cold weather. There is no good reason to do that unless you want a constant circulating system. That theory of operation is both less comfortable and less efficient. It also means the recovery rate is painfully slow which means you have a hard time adjusting temperatures in different rooms at various times. It means there will be a higher average temperature to compensate for the extremely slow recovery rate, so the losses are higher. Less comfortable and/or higher operating costs.
Back in the '50s and 60's a lot of people with radiant were told to just set it and forget it, with simple thermostats. Wrong. Then people would open the doors to blow off excessive heat in the house, they had cold evenings where the temp would dip and the recovery was too slow and they slept in often too warm bedrooms. Bills were extremely high. Comfort was much lower than is should have been. Some of those old places had outdoor resets too. Eventually the resets were all abandoned because they did nothing to improve the comfort. A better idea was needed, but the builders had told the original owners to set it and forget it, as though the heating was so good that it was invisible. Well, it wasn't. It was almost, but not quite. I'm talking thousands of homes with radiant heat that were almost a whole lot better.
Different zones have different needs. Radiant works best and is most efficient when a setback thermostat is used and the recovery rate is reasonably fast. I am ALWAYS able to reduce the operating cost and increase the comfort with a setback thermostat. And I always have a high enough delivery water temp to do the job. The closer you match the boiler temp to the required floor temp, the more time you must circulate and the less you can have various room temps at various times. This means there can be more losses because the floor must always be warm. Warm bedrooms are less comfortable to sleep in and rooms only used part time are warm for no reason except to accommodate the low delivery temp. Very low delivery temps mean near constant circulation and, again, poor comfort where different zones want different temps at different times.
The concept of "Oversizing" the boiler has no clear meaning if it has no numbers attached or if the design of the boiler is not mentioned. If the house requires 75,000 BTU to match the heat load, a 175,000 BTU boiler seems too large. But if it's a condensing boiler with a 5-1 turn down rate, it can match very well. Further, a larger boiler when throttled is more efficient because the heat exchanger is oversized for the burn rate and the boiler condenses and has a cool exhaust. Condensing boilers should be condensing all the time they're running, so matching the load at full boiler output, or not "oversizing" it reduces efficiency because the boiler spends most of it's time at full throttle with a smaller heat exchanger and less condensing. Also, with modulating, the boiler can really send a lot of BTU's to the floor initially and then throttle back, so you get better recovery in the beginning of the heating cycle without overshooting. This allows better programming, greater comfort and better efficiency.
A proper setback program will greatly enhance the comfort while reducing the bill and stabilizing the interior temperature. But it must be a specific radiant program and not a generic forced air program. Avoid using a Nest thermostat on radiant heating unless you know how to go inside and change it's operating strategy.
Higher insulation values lead to lower floor temps. Greater floor mass stabilizes the interior temp, but reduces the ability to program, somewhat. This is where balancing the manifold loops to the needs of the house make up the difference. Balancing defines the character of the heat, such as generally warmer in the bathrooms and kitchen and cooler in the sleeping areas.
A good average floor temp for the general living areas might be 70-74 degrees. Bathrooms about 74-80, bedrooms about 65-70. There are several stages of floor temperature. One is cold! Yikes. Next is invisible where the floor temp goes unnoticed. And three, warm, where the floor greets you when you enter a room, such as the bathroom or kitchen. These are achieved through programming, balancing and adequate delivery temperature from the boiler. It also means greater efficiency because the lower the temp the less the heat loss. So some rooms can be left off or only heated in the morning. Others only in the evening, such as a living room.
My house has an 8" thick slab and is about 3,000 sq ft overall. We are at 5,000 ft elevation in Nevada. My primary heat source is solar with (6) 4X10 flat plate collectors and 600 gallons of storage. This does all the heating from February till December unless a cold snap occurs with no sun. My backup is oil in a conventional cast iron boiler that I have modified to increase it's efficiency. My total oil use last year was 38 gallons and we were completely comfortable.
Solar and in-slab radiant are a magic combination. Solar has several important design considerations that must be followed for best performance. One, decide what kind of freeze protection you intend to have and make sure it is foolproof. No failure of any kind can lead to a freeze up in the panels. This is a must! Don't make it "unlikely" to freeze, make it impossible to freeze. Two, design the system to cool the collectors, not heat the water. Solar efficiency is based on temperature, the lower the better and the greatest amount of energy will come from the panels if they are running cool. Steam is a ridiculous display that will do nothing for the house. This is why it works so well with in-slab radiant, because the floor is running so cool compared to baseboard radiators, for instance. The design must have relatively large storage, relatively high flow rates and probably no collectors in series. Use plain water with only minimal water treatment that won't change its viscosity or make it toxic.
My system heats the floor and the hot water while preheating the 400 pounds of iron in the boiler. Oil is the backup but can only run if certain criteria are met. All of the controls are very simple and reliable. There are no outside sensors for the radiant, as I care about the inside temp and there is always enough temperature to do the work. There are no floor sensors either because the wall thermostat is better suited to comfort. Floor sensors are required if you have a radiant floor inside a forced air zone, but not for radiant only. Again, it's the overall comfort we're interested in and air temp is a better measurement than floor temp because heat can also come from lighting, cooking, people, a fireplace, the sun, etc. Floor temperature sensors know nothing about those other influences on overall comfort, wall thermostats do.
