What is best way to control temperature with a high-mass slab and house that has both large solar gain and poor insulation. I know this isn't the ideal set-up for radiant.

I live in Menlo Park, CA (San Francisco Bay Area). Our climate is temperate (no need for heating so far this fall), and in the winter the average high is 60 and the average low is 40.

Last year, I spent ~$150/month on gas heating the house, but I kept things cold, supplemented with space heaters, suffered overshoot and likely wasted a bunch of energy heating the slab each early morning only to have it cool off by night. This year, I'd like to do things smarter, so I am turning to the experts. Some additional house facts:

• construction date: 1971
• heated sq ft: 2600
• slab is 6-8" thick and not insulated (over gravel and a membrane)
• tubing is 3/8" copper spaced 15" (spec'd to be 2" below slab top, likely lower though)
• 2-zone system with a newer munchkin boiler
• original heating plans called for a 160,000 BTU system

The wall and roof insulation are decent for 1971 but the biggest energy issue is the 1400 sq ft or so of single pain glass. The house has 11 sliding glass doors and many fixed pain windows. As a result, with no heating, the house could easily swing between 50 and 70 on a sunny winter day. This poses a challenge for my thermostats measuring air temperature.

From what I've read so far, it seems that the best thing to do would be to keep the slab at a constant temp (maybe in the low 60s since I'm ok with colder). My questions are:

• is this right?
• is a slab sensor enough or should I combine with an air measurements as well?
• tekmar?
• any suggestions on water temp? I think the boiler is set to produce 135 degree water but would lower be better?
• ODR seems unnecessary? (there are only a handful of nights that reach freezing)

Any other advice? Thanks so much for the forum!

tekmarNet 4 w/indoor feedback would be the best bet, set and forget and let the brains figure out what you need. it can modulate the munchkin to provide a water temp appropriate for whether it's "cruising" or "accelerating", and it can "think ahead" a fair amount. You'll overshoot if you have any rapid solar gain areas but there is no way around that with 6"+ of mass and big windows if they face the sun.

but you are never going to run this efficiently with the insulation situation of the slab.

I concur with NRT. As far as slab insulation it still may be possible to insulate around the perimeter. This is where the greatest losses are occurring. How are planning to install slab sensosr after the fact. It can be done but room location, placement and depth need to be considered. Would recommend Tekmar 079 sensors.

As you have seen, thermal mass can work against you and increase heating bills. For example, in the case where mornings are 50F and the afternoon is 70F and you want 70F all the time. The mass forces you to add more heat that is later wasted (hopefully by opening windows, but often with AC). IMO, passive thermal mass is net neutral for the average building (but perhaps not yours) if a) you want a steady temperature or b) one would otherwise use thermostat setback.

Rob, acwizard and jonr, thank you so much for the feedback. I will investigate using energyedge or equivalent to add insulation around slab perimeter.

For control, do I want: 2 tekmarNet®4 538 thermostats, and 2 tekmar 079 slab sensors? Do the two tekmar thermostats understand how to work together, or is there another controller needed? I also wanted to confirm that I didn't need (or want) something like a Boiler Control 256 to perform outdoor reset.

Thanks again!

"passive thermal mass is net neutral for the average building " Now that is a savvy thought.

I start with outdoor reset.

you need more than the thermostats if you want the indoor feedback. Assuming you have a mod/con boiler with no mixing you'd typically want a 420/335 control combo with the thermostats. so it is a pricier upgrade in that respect.

You can do thermostat/sensor only first with just outdoor reset on the boiler and see if you like it as well, and add the 420/335 later if you decide the system needs to "smarten up".

A couple of other questions should be asked. Is the Munchkin boiler serving domestic hot water and are the areas where the radiant zones serve do they have a/c for summer cooling.

What model boiler? What controls the boiler at this time? Do you have an indirect water heater? What direction does the glass face? Are you having a contractor do this? If diy what is your level of competence? It may be as simple as lowering your target water temp to around 90 and a couple of good adaptive thermostats.

Thanks again to everyone. To answer additional questions:

- Boiler is a pre-Vision Munchkin likely 8-10 years old but I'm unsure of the model number or the board version. I'll check when I'm home
- The water heater is separate
- Single circulating pump, 2 zones controlled by non-mixing erie valves (attached to 2 setback thermostats)
- There is glass in all directions (but the least on the NW portion of the house)
- No a/c. The 11 sliding glass doors and backyard pool are usually sufficient for cooling, and overhangs help when the sun is high

I'm pretty handy but I'd prefer to have a professional install the system. I just want to make sure that I have enough information to select the right professional.

I think a Tekmar 400 and a couple of 528 room stats I dont think you will need the slab sensors and would try it without, but the 528 will allow you to add it later. You could use a 552 if youd like a little more features
http://www.tekmarcontrols.com/media/literature/400_d_05.pdf
http://www.tekmarcontrols.com/media/literature/528_dl_06.pdf
If this is more than the budget allows than go back to my original post. should be in 1500 to 2200 range

Adding the last bits of info, the boiler is a 2002 manufacture Munchkin 199M.  From the feedback, it seems my options are:

1) do nothing and continue with my blast of heat when I get home from work; blast of heat early in the morning approach using existing White-Rogers programmable thermostats

2) add ODR by either upgrading to a 925 board on the Munchkin or adding a Tekmar ODR

3) move to a tekmar net 2 system with ODR, house control, new thermostats and possibly slab sensors

4) implement with same with tekmar net 4

Any recommendations for San Francisco Bay Area experts I can work with on this?

For the life of me, I don't know why HTP doesn't send the boiler with ODR. Get it, set it and forget setback designed for scorched air.

199 is probably 300% over sized that is contributing to your over shoot. 1st thing I would do is lower target temp to 90 and see what that does then I stand by my last post. Used to know a contractor in Sonoma but that was 15 years ago. You could try RPA or calling Tekmar..... http://www.tekmarcontrols.com/index.php

You need to find out if the outdoor sensor is connected to the Munchkin. Your boiler has modulating capacity between 66,000 and 199,000 btuh. Unfortunately we are located in Southern Calif. but I could refer you to the Munchkin rep in Ca which is very close to your location.I would like to see how your system is piped . A photo would be beneficial. Your system response to temperature flucuations is never going to very rapid because of the high mass slab.Your gas bill really is not bad for your size home.You may be trying to save peanuts by spending a lot of money on upgrades. It just may not be cost effective to do so. Your boiler is oversized for your home. I would add a n indirect tank and utilize it to also heat your domestic hot water.

I would have to agree with Ilgeo as well, lower the boiler temp. So far in this post dialogue your boiler delivered water temp is unknown.
This is a start point, you may be set at 135 or more, who knows, colder delivered h20 (95) on a condensing boiler will lower fuel cost.
Odds are there is no insulation under your slab and work is copper pipe in floor, perhaps I missed this already. The swing of the slab will be slow, I would lean toward a simpler solution first. You can experiment with boiler temp and find the sweet spot for the winter and spring/fall seasons.
Dan

boiler oversizing will not contribute to overshoot. If anythign the short cycling would contribute to an undershoot period in the shoulder season as the post/pre purge time plus run time may average to an insufficient average water temp.

Slab sensors would be a big help if you have an undershoot period after solar gain. If not, they won't help that much but I recommend them for comfort anyway... you have an uninsulated floor, it probably gets kind of chilly.

You dont want JUST outdoor reset with mass this big if load is variable, oftentimes. the indoor feedback addition would allow the water temp to rise and responsiveness to increase. with outdoor reset limiting overshoot and slab sensors limiting undershoot you'd be pretty close to great on most slab systems. But with a 6-8" slab, I think indoor feedback will give a noticeable improvement.

Wow. I really appreciate all of the thoughtful replies. To recap:

- 2600 sq ft of heated space (1971 construnction)
- 6-8" slab on gravel, no insulation and no edge insulation
- radiant is 2 zone, 3/8" copper 15" spacing
- single circulating pump, 2 erie values for the 2 zones
- Munchkin 199M, no ODR (pre 925 control board)
- water temps currently too high at 135
- temperate climate, average high of 60, low of 40 for winter months

My goal isn't to spend less on heating. Given how cold I kept things last year, that would be impossible. But, for any extra dollars I was hoping to buy more comfort and more efficiency. I'm now looking into edge insulation for the slab (thanks, GBT!) but this will be difficult for half of the perimeter. And, my morning project is to pull the boiler cover and lower the set point. Any thoughts on the best value for the differential?

My last year heat solution was basically full blast radiant for a few hours at night and then a few hours before waking up early morning to make sure I didn't freeze in my sleep. This meant that the slab was alway heating up and then cooling right back off. I used 3 to 4 therms a day and it was cold. The hard to answer question is, with the right tweaks and control equipment, could I use say 6 therms a day and be actually comfortable? The original plans, with room by room heat loss calculations (1971-style) called for a 160,000 BTU system.

Thanks again for the great forum and indulging my newbie questions.

If you really need that much heat, your radiant system would be completely unable to keep you warm... that's like 60+ BTUs/sq ft which would mean your floors would have to be about 100 degrees and you would be hopping around to avoid hotfoot.

If you want to know what it will cost to run this appropriately, you need a real heat load calc. Or you'll be playing "wait and see".