Posted By sailawayrb on 02 May 2014 09:25 PM
“Thank you! Since the formula is using the air temp delta, would it be overestimating the heat loss at the design temp since the ground temp in the centre of the slab is typically a little higher?”

Well Eljay, not according to Siegenthaler (a professional engineer recognized to be the authority on hydronic radiant concrete slab-on-grade floor heating) and others who have extensively studied slab-on-grade floor heat loss. The vast majority of slab-on-grade floor heat loss occurs at the exposed floor perimeter (which is exposed to the outdoor temp) and not in the center section of the slab (which is exposed to the ground temp). So outdoor temp and exposed floor perimeter is used for estimating slab-on-grade floor heat loss. However, if you have a basement floor that is entirely below the frost line, it would indeed be appropriate to use the ground temp and the floor area for estimating this basement floor heat loss.

Makes sense. I have seen some calculators add both perimeter loss and centre of the slab loss, which made me wonder. Thanks.

Soil is insulation. So while it takes a long time to heat up, it eventually has much less heat loss than if you assume that the other side of the slab+insulation is at ground temp.

Well, I am still pretty set on radiant rather than air-distributed heat. After some more reading and all the articles that indicate that radiant slab maybe an overkill for new well-insulated house, I think I am now leaning away from it. I think that if I can get the envelope to the point of needing only 10-15 BTU/hr/sqft, I'll aim to put together a system designed around 110-120F supply water temperature and use properly-sized low-temperature radiators with thermostatic valves on all of them for zone control. I'm just trying to crunch all the numbers to see whether that approach will save me some cost compared to a radiant slab.
Do I need to be thinking about more than R20 insulation below slab if I don't heat it?

While I'm almost settled on the distribution system, I still remain quite confused about the best upfront/running cost ratio when it comes to selecting the best energy source to supply the water to the system.
I know that I will want to make the house at least solar-ready by changing the roof design, getting the rough-in from roof to the mechanical room etc. However, I'm not sure, which direction to go:
- solar DHW + supplementary solar thermal for heating season with electric backup
- solar DHW + electric backup + future solar PV
- air-to-water heat pump/DHW combo + electric backup + future solar PV

It's hard to get a true comparison of installation and equipment costs as most places would add on varying distribution systems, different assumptions about usage, and, generally, systems/methods with which they are familiar.

Currently, I'm trying to determine that if I went with a solar-based system for both DHW and supplying space heating, how much electricity would I be consuming during the periods without sunshine and how much of that can be offset by using a large water tank as thermal storage. Will that cause overheating in the summer?
My utility company does offer net metering and time-of-day rates. However, to participate in TOD tariff, I'd need some electric thermal storage (not sure if a solar water tank qualifies) and controls etc, plus the base charge is $8.00/month more and the peak rates are 30% higher during the heating season, while off-peak is 50% less throughout the year.
The net-metering requires that the renewable energy generator must be sized to meet domestic consumption. I don't know how much power will we need in the house. We use about 500 kWh in our apartment and I'm sure the house will be more. So, it sounds like I'd need to put at least a 6 kW array on the roof, which is not going to happen. :(

Off to crunch some more numbers, but I'd welcome input on an appropriate heat source for a low-temp hydronic system. Thank you.

Please keep in mind that energy efficient buildings that use HR floor heating typically only need supply temps in the low 80sF range and the floor surface temps are only in the low 70sF range. Not having to heat water to higher temps saves energy. Not having to heat water to higher temps also allows you to more effectively use other heat source solution like solar or geothermal. The downside is that some folks like their floors to feel hot in order to feel comfortable. HR heated floors in energy efficient buildings do not feel hot. However, they do not feel cold either and the living space quietly receives the required heat gain it needs to offset the heat loss without having to blow air around. So I believe HR floor heating makes good sense in energy efficient new construction where a floor concrete slab will be used anyhow.

Posted By sailawayrb on 10 May 2014 10:55 AM
Please keep in mind that energy efficient buildings that use HR floor heating typically only need supply temps in the low 80sF range and the floor surface temps are only in the low 70sF range. Not having to heat water to higher temps saves energy. Not having to heat water to higher temps also allows you to more effectively use other heat source solution like solar or geothermal. The downside is that some folks like their floors to feel hot in order to feel comfortable. HR heated floors in energy efficient buildings do not feel hot. However, they do not feel cold either and the living space quietly receives the required heat gain it needs to offset the heat loss without having to blow air around. So I believe HR floor heating makes good sense in energy efficient new construction where a floor concrete slab will be used anyhow.

Hmm... I thought that since the floor won`t be warm anyway, why bother with PEX tubing and just install a couple of low-temp radiators with TRVs to get better zoning. I know that rads are not cheap, but given zoning costs for in-slab, I thought that would be a cheaper approach to not lay PEX tube in the slab and do three  zones downstairs with rads: main living area, entry, bathroom.

If you have to pour a concrete slab anyhow, about the only heating system that would have a cheaper heat emitter than HR floor heating would be electric baseboard heating. You can purchase 1000 feet of PEX for less than $500. You can spend anywhere from very little to very much for the heat source, but that is the case no matter which heating system approach you choose. I suggest that you get multiple estimates for the various heating system approaches that you are considering. If you are even remotely interested in HR floor heating, it likely makes sense to just place the PEX even if you don’t immediately use it. It will cost a couple magnitudes more to add HR floor heating after the slab is done. There is huge comfort difference between a cold, unheated slab and a slab that is couple of degrees warmer than room temp.

True, true... ah, these decisions are not easy. I just read through a great presentation I found from Siegenthaler on radiant heating in low energy houses.
Putting together a system like that based on solar DHW+solar thermal with electric backup and possibility of running it on solar PV later on sounds very appealing, but I wonder what the cost would be.
I visited one person`s house who has solar thermal on his roof and runs DHW and heating with it and saw a big reduction in his electric bill. His mechanical room was very hot with solar tank water at 180F, so I guess it takes some maintenance and regulation of the temps since it`s not even summer yet.

Yes, Siegenthaler is both the pioneer and the master with all HR things. The fact that you are thinking deeply about all this is goodness. Most folks just do whatever their GC recommends. Keep researching and learning, knowledge will set you free!

Note that Siegenthaler writes:

What kind of heat emitters should be used in these houses?
...
They should have low thermal mass for rapid response to interior temperature changes

Regarding mixing radiant slab with passive solar storage in the same slab:

If maintained at an elevated temperature with auxiliary heat ensuing solar gains cannot be absorbed.
The space quickly overheats.

The future looks good for radiant ceilings (or other low temp, low mass radiators), water thermal storage and heat pumps.

Siegenthaler is correct, like always. However, you do need to read what he wrote carefully (so as to NOT misquote it) and you also need to be capable of comprehending the technical content and significance too.

From chart 29, “Low thermal mass is appropriate when there is a NEED for rapid response to interior temp changes.” This statement is indeed true…and there often is a NEED for buildings with highly insulated envelopes that also have relatively LOW interior thermal mass to be able to provide a rapid response to interior temp changes. While passive solar buildings typically have highly insulated envelopes, they typically also have relatively HIGH interior thermal mass specifically for the purpose of maintaining a relatively constant interior temp, and they consequently don’t have a NEED to respond to rapid interior temp changes like a building having relatively LOW interior thermal mass. There is nothing wrong with using low thermal mass HR emitters from a performance perspective when you have a building design that creates a NEED to use them, but low thermal mass HR emitters can often cost an order of magnitude more than concrete slab emitters that can be economically created during new construction and low thermal mass HR emitters can often be quite noisy too. However, some folks are not concerned about spending more dollars or bothered by contraction/expansion noises common in tube in plate HR emitters, so this is not a big issue for them. Nevertheless, it is always best to consider all these design details before actually constructing your building and limiting your options.

From chart 35,” Direct gain passive solar buildings…if maintained at an elevated temperature with auxiliary heat ensuing solar gains cannot be absorbed…the space quickly overheats.” This statement is indeed true…and this can be the case even if you don’t have HR floor heating if you don't properly design the passive solar heating system. This statement means that if you have both HR floor heating and passive solar heating systems, and if the HR floor heating system maintains the floor temp too high in temp, the space will quickly overheat during the irradiance period. This occurs because any time the floor surface is warmer than the space temp, the floor will generate space heat gain via convection and radiation. Companies that are knowledgeable about integrating HR floor heating and passive solar systems fully understand this and use a control methodology to properly maintain the appropriate HR floor temp in the space where the passive solar BTUs are collected. This keeps the space where the passive solar BTUs are collected at the desired temp and comfort level, and allows using the HR system to move the collected passive solar BTUs to other locations in the building where they are needed.

As we have discussed many times before, not many companies understand or have the know how to properly address the technical issues associated with integrating HR floor heating and passive solar heating systems.

sailawayrb, does this mean that the heating system would need some clever water temperature control mechanism to account for solar heat gain to the slab?

If I'm understanding this correctly, in my floorplan, there will be some solar heat gain from the SW side windows around the "dining area" next to the kitchen. So, when this happens, the objective would be to lower the water temperature being sent to the slab, so that the slab does not overheat AND that the water in the PEX can absorb some of the gain and distribute it to other parts of the slab/space? Does that happen effectively in slabs?
I'm trying to get this straight in my head since I'd think that when random periods of solar gain occur, I'd want a system that can respond quickly during the irradiance (to lower the output and allow for absorption of the gain) and during no irradiance (to increase the output) period. So, from that, I'd conclude that I'd want low thermal mass emitters rather than a slow-responding slab.
In fact, I visited a house of a contractor with an Altherma system on a sunny afternoon and he experiences exactly that: solar gain overheating. He uses a mini-split to maintain a comfortable temperature and keeps the slab at an even temperature. I know it sounds silly: keep even floor heat and control the air temperature through opening windows or A/C. So, my question is, how does one avoid that with a slow-responding large thermal mass emitter like a slab?

Thank you.

Yes, if you want to use a HR heating system having a high mass emitter to move captured passive solar heat without overheating your space, you need an intelligent controller that overcomes the slower response of the high mass concrete slab emitter by providing sufficient control phase lead (i.e., a fancy control system design term for creating anticipation) and by providing sufficient control authority (i.e., a fancy control system design term for being capable of moving the maximum excess passive solar heat that can occur in the space). Again, you also have to keep in mind that a passive solar building with a high interior thermal mass also tends change temp much slower than a passive solar building with a low interior thermal mass…and in this case, the slower response is goodness because it gives you more time to create control phase lead (i.e., anticipation) to better control the interior space temp.

So as long as you have a good controller for your design approach, either a low or high interior thermal mass building can be successfully designed. The design choice is more about which approach is more appropriate given your overall building design objective. If you are designing a passive solar building having a concrete slab, you are tending toward a high interior thermal mass design solution and you already have a good start on the required passive solar thermal mass and a good start on a very affordable HR floor heating emitter too. If your building design also includes masonry interior walls (perhaps for aesthetics and increased passive solar thermal mass) and a masonry heater (perhaps for aesthetics, increased passive solar thermal mass, and to have the most comfortable/clean/efficient wood burning solution for an energy efficient building), you are tending even more toward a high thermal mass design solution.  Folks often get in trouble when they try to use a low interior thermal mass control solution to solve a high interior thermal mass design problem or vice versa.

We are not big fans of having to use mechanical ventilation or opening windows to keep a passive solar building comfortable. Why? Because most people who want passive solar heating and HR floor heating do NOT want to have to forcibly move air around in order to maintain their comfort level. If they liked that approach, they likely would have just built a standard, cheaper house with a forced air furnace having leaky, noisy duct work. And just to be clear, we are big fans of having well-insulated/well-sealed building envelopes and using ERV/HRVs to efficiently provide the required ventilation and air circulation.

So, my question is, how does one avoid that with a slow-responding large thermal mass emitter like a slab?

I have a passive solar with radiant slabs and it works quite well. We provided for slab temperature sensors, but right now, only the standard wall-mount air-sensing thermostats are providing the control. I haven't had the time to do a full analysis of why ours works, but there are a couple possibilities.
1) We controlled the insolation that was admitted fairly well. It is important to match it to the building's heat loss. What we see happening is that the radiant system provides heat at the thermostat setting. So, a certain amount of BTU/hr are running into the slab from the heating source to keep it at steady state. As insolation begins, the temperature rises past the setpoint and the system stops providing input to the slab. The energy that was being provided to the slab is now coming in the windows. As the sun moves across the sky it stays relatively steady.
2) We also have a giant atrium in an open plan which seems to help moderate interior temperatures by allowing the warm air to collect.
3) We have a lot of interior mass.

Yes, designing a passive solar roof overhang or using some other means to properly control and align the passive solar heat gain with the building heat loss is the first step toward a successful design. Yes, having a lot of interior mass greatly stabilizes the interior temp and having an open floor plan and/or creating good air circulation further mitigates the risk of overheating. Again, as long as the design provides sufficient control authority to fully address the variability of passive solar heat gain that can occur, all will be well.

Posted By sailawayrb on 11 May 2014 01:21 PM
you also need to be capable of comprehending the technical content and significance too.

I have that capability.... Sailor, a "smart" controller sounds interesting in theory. Can you please point me to a third party hot box or field test report?

sailawayrb, I have a question about calculating the heat loss using your calculators.
I couldn't find any direction on how to approach heat loss calculations for the entire building envelope vs individual rooms.
I'm trying to determine the exact room by room heat loss to determine the size/type of heat elements that will be needed for the upstairs "doored-off" rooms. I understand that heat moves to the coldest place (e.g. the outside wall/window at outdoor design temp), but surely the heat loss to the indoor space is not as dramatic even if there's no heat source in the house, correct? Does the same heat loss calculation/formula apply or would the standard building envelope calculator be overestimating the heat loss? Or is the difference too negligible to be worried about it?

So, to calculate a bedroom's heat loss using your calculator, do I use the wall area of the entire room or just the wall facing outside temperatures? And for the floor losses, do I now assume the joist between-floor insulation levels or simply indicate "no exposed area"? And for the door heat loss should I use the interior doors to the hallway sqft or just enter zero?

Thank you for your guidance.

Eljay, it would appear that you have answered your own questions!

You accomplish a room heat loss analysis in essentially the same manner as you accomplish a whole building heat loss analysis. For the room analysis you just enter the room data for the ceilings, doors, floors, walls, and windows that are actually exposed to the outdoor temp. In other words, you don’t enter room data if the ceilings, doors, floors, or walls are an internal assembly that are NOT exposed to the outdoor temp. Please also keep in mind that this approach assumes that ALL the rooms will be maintained at the SAME design indoor temp. In other words, if you will have rooms that will have vastly different design indoor temps, additional heat loss analysis would be required to achieve an accurate heat loss analysis.

Thank you!
Another quick question: is a skylight in the ceiling entered as "just another window" or is the heat loss larger because it is in the ceiling?

For a heat loss analysis, you can treat a skylight just like a window, just use the appropriate R-value. While skylights are great for adding daylighting to a living space, they are often the worst fenestration element from a thermal performance perspective in a building. They can lose a significant amount of heat in the winter and they can generate a significant amount of solar heat in the summer.

Not to mention in snowy climates a skylight may be covered all winter as mine was last. As most windows in most climates the only good thing about a window is the natural light it will produce in any otherwise artificial environment.

The orientation will make a great difference in the solar gain, so as with any window, climate matters.

Most quality heat load programs account for the potentially significant heating and cooling loads presented by the shaft. This paper covers the subject without pretense or dissimulation.

http://www.stanford.edu/group/narratives/classes/08-09/CEE215/ReferenceLibrary/EDR%20Design%20Briefs/sg-2-design.pdf