No, the controller does not need to know what the actual weather will be...although the controller does use 36 hour forecast outdoor temp data parsed from the internet...so those couple of PNW 70F days we often get in February are not a surprise and can be handled automatically.

More importantly, the controller accurately determines in advance what the irradiance heat gain will be into each zone given the sensed irradiance magnitude voltage and the building latitude, longitude, orientation, roof overhang geometry and fenestration geometry parameters. By knowing in advance what both the irradiance heat gain and building heat loss will be, the controller and hydronic heating system have adequate time (i.e., phase lead) to align the thermal mass zone temps so as to be able provide the required zone heat gain/loss at the required time to ensure precise temp control of all the zones.

I'm really lost on the concept of 36 hour forecast data having an input to your radiant controls. That would be more useful in a solar heating system that had some significant storage capability, where you had to make a weighted guess on how fast to use your store of heat.

A simple system just waits until the sun peeks out from the clouds, warms the room a bit and tells the hydronics to stop delivering heat. All that happens really fast. There is no way weather predictions are anywhere near that accurate here.

Exactly right, I don't know that weather forecasts (e.g., local clouds/precipitation) are accurate anywhere. However, 36 hour outdoor temp forecasts are usually accurate for most locations, particularly for just determining whether a significant cold front or hot front is coming. The phase lead that this data provides is very useful when controlling a slow reacting thermal mass system which can take several hours to change only a couple degrees.

For the same reason, it is also useful for the controller to know in advance if the irradiance period will be clear sky or/and how much less than clear sky, and thereby determine/know in advance what the solar heat gain BTU profile will be into the zones during this irradiance period for this day of the year (i.e., by determining/using the solar altitude angle profile for this day of the year given the building latitude/longitude to determine the real time fenestration solar aperture area given the aforementioned building passive solar design parameters while using the sensed real time irradiance magnitude data).

A simple reactive hydronic controller may turn off fast, but a thermal mass system may not be able cool off fast enough to prevent a significant overheat event when faced with a large solar heat gain without having the benefit of this additional phase lead provided by the more intelligent controller.

A simple system controls insolation through the use of fixed overhangs, window apertures and glazing. In passive, there is no need for computer control of that. I still don't see how a computer can guess irradiance in advance considering cloud cover is everything. Here, it varies literally by the minute.

I doubt there are too many radiant systems more massive than what I put in and the fact it is slow to react is what works in it's favor.

Exactly right, you don't need a controller for "passive" solar heating as there isn't anything it could control. The building daily passive solar heat gain is purely a function of the actual daily irradiance magnitude and the aforementioned building design parameters. Nevertheless, there is considerable variability in this building passive solar heat gain given the day of the year (i.e., solar altitude angle effect) and the actual daily irradiance magnitude (i.e., weather effect...clear sky versus cloudy sky). There is also considerable variability in the building heat loss given the daily outdoor temp variance. A "passive" solar heating system does nothing to address any of this variability. You just get what you get and if the "passive" solar heating system was designed right, you get a nice net positive heat gain from the fenestration and you don't get any overheat events. However, you will get indoor temp swings if you don't have any other heating system that has the thermal controll authority to stabilize the heat gain/loss.

You do need a controller to pump heat into a thermal mass hydronic radiant floor heating system. There is considerable variability in the building heat loss given the daily outdoor temp variance. A simple controller using outdoor temp reset can handle most of this variance. If you don't have significant solar heat gain, this simple controller will likely be adequate.

The fundamental issue is that thermal mass can only absorb a percentage of the passive solar heat gain that enters the building. A "passive" solar heating system doesn't do anything to address the additional heat gain that results from the unabsorbed passive solar heat gain that has entered the building. However, when you have BOTH a passive solar heating system and a properly zoned hydronic radiant floor heating system, you have the choice to either have the hydronic radiant floor heating controller properly compensate in advance for this additional unabsorbed passive solar heat gain or you can simply do nothing about it and just accept the resulting indoor temp swings. The absorbed passive solar heat gain will get utilized by the floor zones calling for heat gain in any event...so all goodness here. However, if there is enough building temp overshoot or overheating, there will be zones other than just the irradiated zones that will stop calling for heat and the thermal mass temp of these other zones will be inefficiently reduced (i.e., additional heat will eventually be needed to bring them back to normal temp for the conditions). An intelligent controller solves this issue, maintains precise zone temps at all times, and allows you to obtain the full energy savings possible by properly integrating the passive solar and hydronic thermal mass systems.

I still don't see how a computer can guess irradiance in advance considering cloud cover is everything.

a slow reacting thermal mass system which can take several hours to change only a couple degrees.

Both are correct, at least in some cases. So you end up with some over/under shoot. Enough for anyone to notice - perhaps not. Maybe someday someone will put WEL data online for such a room. Air temp, slab temp, hydronic in/out temps. That would be much better than unsubstantiated vendor claims.

A "passive" solar heating system doesn't do anything to address the additional heat gain that results from the unabsorbed passive solar heat gain that has entered the building.

In that regard, neither does any other residence. What usually handles it is the regular ventilation. That's yet another reason I am a big proponent of active ventilation in homes without classic heating ducts. A passive solar is a leg up on a number of counts. First, because the additional mass begins absorbing any temperature increases. Second, because the envelope is engineered in such a way that overshoots are much less likely simply because the sun CAN'T get in during likely overheating periods.

I am a big proponent of active ventilation in homes without classic heating ducts.

I agree. It's vital if you want to maximize the use of internal thermal mass without having to open and close windows several times/day.

Mechanical ventilation is always required in new buildings these days if they are well insulated and sealed like they should be. If you care about building heating efficiency, this ventilation will be accomplished using HRV/ERVs which don’t have the capacity and were never intended to be used for controlling zone temps in a building.

So it sounds like you are using an additional mechanical ventilation system to move any overheated air from your hotter passive solar heated zones to your colder zones. While we have never found that approach to work well for us relative to avoiding temp swings, you should be very happy if it works for you. However, I would suggest that it would be much preferable to just use the existing hydronic radiant floor heating system to avoid ever creating this situation. Moving heat through a hydronic pex circuit is much more effective and efficient than moving heat through an air duct. Most folks also prefer hydronic radiant floor heating over other heating systems precisely because there are no drafts caused by moving air.  Of course, if your thermal mass receiving the passive solar irradiance was not zone isolated properly, using our approach would be a mute point anyhow even with an intelligent controller.

You seem to be very happy with what you have, so again congrats!

HRV/HEVs which don’t have the capacity and were never intended to be used for controlling zone temps in a building.

Actually, the closer you get to passive house specs, the more likely you can use an HRV to do exactly that. You are kidding yourself if you think there is no moving air in a home just because it doesn't have a forced air system. All homes will have natural ventilation pathways and judicious use of an HRV, or, even simple code-mandated exhaust fans in smaller homes can help define that pathway. For example, if you have a two-story home, it is more often preferable to have an open pathway to the second floor. In this way, overheating on the lower floor will naturally be moved upstairs. Once there, it could be drawn off through the HRV exhaust vents in bathrooms. The simple expedient would be to leave the bathroom doors open when not in use to facilitate this. The same thing would be accomplished with simple bathroom exhaust fans set to run intermittently.

There is no way a hydronic system can respond quickly or effectively enough to remove transient over heats. And, with proper construction and design, transient overheats are about all you should get. I first noticed this while trying to design passive solar in which maximum insolation was captured, and it was very difficult to do. The motion of the sun assures that it does not irradiate any one patch of the floor for any length of time. A glass-roofed sunroom might be an exception to this because of the wide open aperture. I can get the slab exposed to the sun for such a long period of time that it becomes nearly totally heat soaked, and the pex tubing contained within can actually start carrying the heat off for use elsewhere.

Glad to hear that you got it sorted out and are happy

Sailor - if all the energy coming into the room hits the radiant mass, I suspect the water can move that to another area. What happens when a portion or a large portion misses and say hits an inside wall. Would you not need to move this heat with air either mechanically or by windows?

•••For example, if you have a two-story home, it is more often preferable to have an open pathway to the second floor. In this way, overheating on the lower floor will naturally be moved upstairs.•••

Yes, it sure will BUT it is terrible from a heating point of view. You greatly increase the stack effect and your upstairs radiant will not come on except in extreme cases. If you have a reverse setup where you live on the second and sleep on the main, it's not so bad because you can keep the main a few degrees colder than the second.

In my case if I leave the door at the top of the basement stairs open, you can stand on second storey stairs and feel the air moving both up and down i;e; both side of the convectional path.

Open staircases are great from an architectural point of view but not so much for heating.

Oh and btw, unless you have three or four 300 cfm plus fans for bathrooms, you will never move that much hot air out. I have two 22 x 48 skylights over the stair case and they will do well in cooling the main floor is I have windows open on the main floor but then will not cool the second. Closing the main floor windows and opening the second floor windows will cool the second floor but not as aggressively as the main floor. Opening the shady side basement windows and windows on either of the upper floors tends to cool things okay if I start soon enough. And we don't have much heat problem to start with.

What happens when a portion or a large portion misses and say hits an inside wall

How is it going to "miss"? One thing we've been good at for thousands of years is knowing where the sun will be on any given day.

You greatly increase the stack effect

My house is all about the stack effect.

your upstairs radiant will not come on except in extreme cases

Is that a bad thing?

both side of the convectional path.

Right. Hot goes up and stays there, cold is pushed down. In the Summer, you want to exhaust the hot. In the Winter, you want to run it through the HRV to recover the heat.

unless you have three or four 300 cfm plus fans for bathrooms, you will never move that much hot air out

If you have a 110 cfm fan and a smaller 80 cfm fan in two bathrooms, you can exhaust the entire upper floor of an average home in less than an hour of running. Realistically, you don't need to exhaust even that much if you keep the bedroom doors closed to minimize the space receiving the hot air.

We have full open staircases from basement to the top of the atrium, a total of about 45 feet. It's part of the heating and cooling plan for the home.

ICF is correct that we can pre-determine exactly where the irradiance will strike the thermal mass for every minute and every day of the year. We have done floor designs such the irradiance highlights a specific design aspect at the desired dates (e.g., wedding anniversaries and birthdays, etc). So no, we don't need to move any air to maintain precise temp control with our design approach. However, I agree with you FBBP...I also have a strong disdain for multi-story houses, open stairwells and cathedral/vaulted ceilings which just scream heating/cooling inefficiency whenever I think about them or see them. However, some clients like this showy architecture and they don't mind this tradeoff.

However, some clients like this showy architecture and they don't mind this tradeoff.

My home was designed specifically with these things in order to make it MORE efficient.

So no, we don't need to move any air to maintain precise temp control with our design approach

But, how do you stop it from moving?

if all the energy coming into the room hits the radiant mass, I suspect the water can move that to another area.

Not unless you cover the surface with glass (ie, turn it into a solar collector). The room air will heat up significantly unless you have a source of quite cool water to circulate in the floor.

Correct, we don't need to move ANY air from one zone to another because we maintain precise temp control of all the zones so as to never create a situation that would require this. Perhaps some thought examples might help illustrate the physics and control methodology of how this works.

A simple reactive hydronic controller just changes the heat gain into a living space based on what is actually happening at the time using real time indoor temp and thermal mass temp (and perhaps also using real time outdoor temp if outdoor reset is used). So it should not be too surprising that this simple controller can't easily and always keep up with a slow reacting thermal mass system when faced with huge heat gain resulting from aggressive passive solar. So the indoor air temp begins to increase beyond what may be desired in this area of the living space. You apparently designed your house such that you move this higher temp air from this area of the living space to another area so as to avoid significant overheating when this occurs. I would describe this as the classical approach for dealing with passive solar indoor temp swings. The magnitude of these swings can be reduced by designing "passive" thermal mass into the design so as to immediately absorb and slowly release some of the passive solar heat. We have free DIY software on our website that allows one to figure this all out. You significantly benefit by having a hydronic radiant floor system that is able to absorb some of this passive solar heat gain and then use it to offset your overall hydronic system energy usage.

An intelligent controller designed to integrate passive solar and hydronic radiant thermal mass heating systems operates entirely differently. An intelligent controller knows and takes advantage of the fact that the indoor temp in a living space area will not change at all if the TOTAL heat gain into this area is aligned so as to be exactly the same as the heat loss through the building envelope in this area. I suppose I should mention at this point that the building envelope is the most important aspect of getting to an energy efficient building. If one keeps the building heat loss very small, the resulting heat loss variability under various climatic conditions is also proportionally smaller which makes precise precise temp control off all the zones that much easier.

As described previously, an intelligent controller has a pretty good handle on what the outdoor temp will be 36 hours in advance of the passive solar irradiance period for this area. So the controller has a pretty good estimate of what the BTU/hour heat loss will be 36 hours in advance of this passive solar irradiance period for this area. As described previously, the controller also has a pretty good handle on what the range of the passive solar BTU/hour heat gain during the solar irradiance period will be 36 hours in advance of this passive solar irradiance period for this area. By "range", I mean the maximum and minimum passive solar heat gain that can physically occur given the building design parameters and depending on whether the irradiance will be clear sky or non-clear sky conditions. The controller also knows what percentage of this passive solar heat gain will be absorbed by the thermal mass in this area (which will then be used to offset the overall hydronic system energy usage) and what percentage will NOT be absorbed. It is this percentage of passive solar heat gain that will NOT be absorbed which must be addressed and properly compensated for by the intelligent controller and hydronic system.

An intelligent controller uses this information to determine what the thermal mass temp set point will likely need to be at this time so as to provide this much less BTU/hour heat gain so to not change the indoor temp in this area. The controller now has enough information to begin aligning the thermal mass set point temp in the appropriate direction to begin addressing much of the situation that will be coming it's way, which gives the controller more control margin to keep the thermal mass actual temp within the known thermal authority of the system so to be able to maintain precise temp control off all the zones given the condition variability that still remains. As time gets closer to the beginning of the irradiance period, the forecast BTU/hour estimates transition to accurate BTU/hour real time values which allows the thermal mass temp set point to become much more accurate. At dawn, the controller starts receiving sensed real time irradiance magnitude data which allows it to transition from using the aforementioned passive solar BTU/hour heat gain "range" to accurate BTU/hour real time values which further allows the thermal mass temp set point to become much more accurate.

The net result of all this intelligent controller action in getting the thermal mass in all the living space areas to the precisely right temp at precisely the right time is to keep the TOTAL actual heat gain in all these areas very closely aligned with the actual heat loss through the building envelope in all these areas so as to NOT cause the indoor temp to change in these areas. This allows us to maintain precise temp control off all these areas and never create a situation that would require us to move hot air around the building. Yes, minimal ventilation must still be provided as required by code and common sense, but this can be efficiently accomplished just using HRV/ERVs and without creating air drafts that some people find annoying.

The magnitude of these swings can be reduced by designing "passive" thermal mass into the design so as to immediately absorb and slowly release some of the passive solar heat

It really can't. I can appreciate you are using the word 'some', when talking about the excess insolation, but that belies the fact that the air in the space is immediately heated which is what is making it uncomfortable in the first place. There is no waiting for the heat to "absorb" into a mass wall or the floor. That air is gonna start to move. You can either bottle it up while you wait for the heat to be absorbed, or you can let it move.