Zone 5, Chicago. The building is an E/W oriented, circa 1910, 2 story brick cube with a flat roof on top and a rubble foundation + rat slab underneath. Approx 1000 ft2/floor. Basement is unfinished, but it gets a lot of use as a workshop. About a year ago we had some maintenance pointing done on the walls and the parapet, but we have not added insulation, blocked leaks - all the responsible envelope tightening goodies that are on the to-do list. Overall, the building is in solid shape. No leaks from the roof or from the foundation, despite some pretty heavy rains that caused flooding in other parts of the city. Knock wood. Or masonry, in our case. No obvious areas of drafts or cold spots during the winter. Our neighbors to the north and south are both 3 story buildings and they provide some shelter when it storms. The current HVAC is smorgasbord: gas furnace blowing hot air through some leaky ducts into the first floor, gas fired boiler piping water to big ole radiators on the second floor. The pipes for the radiators are very well insulated with something.... We have not had any testing done, but common sense suggests that it is not something anyone would want to breathe into their lungs. We also have 2 AC units for the Chicago humidity, a unit outside that works through the ducts serving the first floor and a second unit on the roof that cools through a straight run duct that is located within the building envelope. The hot air furnace is from 1987 and the insulation on the pipes should be removed by responsible people, so this project is on the top of the list. Since we love radiator heat, we originally thought that we would just add radiators back into the first floor, especially since their supply pipes were left as-is in the basement. Then we looked into price, which (surprise!!) was $$$$$. While we thought we would be "saving" the existing hydronic system, it turns out the only saving is the actual radiators. HVAC Guru #1 informs us that everything else would need to be replaced - we need a new boiler (current one is undersized, according to him) and all new piping to the existing radiators, plus new radiators with new piping for the first floor, plus thermostats and so on. What HVAC Guru #1 suggested instead was to do a whole new conventionally ducted blower system to blow hot air in the winter and cool air during the summer while keeping the existing AC on the roof for a separate zone upstairs. Much less expensive. Also much less desirable to us as we have never been fans of forced air heat. Since we are also ripping up our nearly worn through hardwood floors, we thought hydronic heat in the floor might make some sense. It did not make sense to HVAC Guru #1 (possibly because he had not installed this type of system?) so we sought a second opinion from HVAC Guru #2. HVAC Guru #2 has extensive experience with in-floor radiant & proposes a multi-zone system with tubing encased in gypcrete underneath the final floor surface. AC on the first floor would be high velocity/space pak & 2nd floor would continued to be cooled by the existing until. His price is higher, but still in the same ballpark as the ducted forced air plan from HVAC Guru #1. I understand that tightening up the building envelope is the A Number One priority for any responsible, environmentally conscientious homeowner. We will be addressing the obvious holes and then rechecking with a blower door to find what we missed *after* the pipe insulation in the basement is removed. In a few years, the roof will need some attention, and the plan is to replace it with a healthy R value sandwich. We love our old building, which has held up pretty well for the last century. So, while we do plan to improve the airtightness, we will not be covering our handsome old brick with a high R value stucco exterior or furring out the original plaster and woodwork interior walls in order hide them under foam and drywall as per Building Science's recommendations to tighten up older masonry buildings. In short - we live in a leaky building and it will continue to be a leaky building, even after we plug the obvious holes. Our preference for in floor radiant is comfort, followed closely by the need to make a major change in our existing HVAC. Any reason we should proceed along a different path? Any obvious detail I am omitting? Any wisdom is greatly appreciated.

It's pretty rare to find boilers that are actually undersized. It's far more common to find boilers 3-5x oversized for the actual loads. Unless they did a careful room-by-room heat load calculation on the "after" picture of your planned building upgrades (a process that would take a few hours of meaurin' & figgrin' to come up with) their statement is meaningless. Don't leave it up to them without running your own numbers.

Start with the heat load calculation using realistic indoor temps and the 99% outside design temp for your area (which is about 0F in Chicago- don't be surprised if someone uses a much lower temp, but make them re-calculate if they do. There may be a case for -2F in some of the 'burbs, but not -10F. http://www.energystar.gov/ia/partners/bldrs_lenders_raters/downloads/Outdoor_Design_Conditions_508.pdf )

If the contractors don't have the print out of their heat load calculations and you find it too cumbersome to run the numbers using an I=B=R method, you can still put a fairly accurate stake in the ground on heat load based on fuel use and weather data.

Since all of the heating systems used the same fuel, find a mid or late winter gas bill and the EXACT dates between meter readings, and the amount of fuel used. With that and a ZIP code we can look up accurate heating degree-day data, and use the fuel-use per heating degree-day to put a reasonable upper bound on the true as-is (pre-retrofit) heat load at your 99% outside design temp. (Don't choose a billing period where you turned the thermostats down to 50F and hung out in Belize for 3 weeks, eh? ;-) ) With the BTU/hr in/out numbers on the boiler & hot air furnace we can approximate the operational efficiency and use that to come up with a BTU per degree-hour number, and apply that number to the difference between a presumptive balance point of about 65F and your 99% outside design temp to come up with a whole-house heat load number. If that number is less than the DOE output of the boiler, it's not oversized. If it's just a bit larger, the fact that you're lowering the heat load with improvements means it's probably still not undersized, but more analysis would be in order.

Encapsulating the insulation on the piping is usually safer than removing it, since removing it puts friable fiber in the air. In most places it's illegal to run blower door tests on houses that have had asbestos insulated piping even AFTER the insulation is stripped, unless the pipe itself is removed, and the chases vacuumed out with the proper equipment by certified remediation techs.

Retrofitting in-floor radiant is expensive- almost an order of magnitude more expensive than adding low-temp panel radiators. And the higher the heat load per square foot, the more expensive it is. In a leaky building it's almost always better to spend the money on fixing the building envelope than putting in radiant floors. Comfort is a matter of the average radiant temperature in the room, and even though that cushy under-floor heat is nice, it becomes a frying pan hot-foot feel if you have R1 brick walls and need to be pumping 40-50BTU/square foot out of the radiant floor, but a panel radiator on that wall would raise the average radiant temp from that direction, and putting an inch of closed cell foam on the brick with a fiber insulated 2x4 studwall would be even better (after which radiant floors are more comfortable, the way you like them.)

Gypcrete hasn't any "give" to it, and is a somewhat less comfortable floor to walk on. After you've fixed the heat leaks to the point where it can be heated at less than 30 BTU/foot of floor an under-floor retrofit using extruded aluminum heat spreaders can work, and is quite cushy under all heating conditions. It's not cheap, but it's nice, and you don't have to replace the finish flooring or rip open any ceilings on the first floor, assuming the basement ceiling isn't closed up, with the joist-bays accessible.

Alternatively over-the-subfloor systems such as WarmBoard (tm) can put out a lot of heat fast, but are usually even more expensive.

You may have to use a different web-browser to do it, but paragraph breaks make your posts more readable. (I'm not sure which browsers have the issue, but both chrome and firefox seem to work-mostly from formatting point of view.)

Dana has you covered. You might want to do a quick existing building energy usage analysis to just ballpark your current whole building heat loss to have a reference point.

Borst Existing Building Energy Usage Analysis

Then you should do a room-by-room heat loss analysis based on your planned building improvements as Dana suggested making sure that you get the exposed floor heat loss for each room that will have hydronic radiant floor heating so that this can be properly designed.

All this wisdom is greatly appreciated. Off to sharpen my pencil & dig out the old bills.

You also need to be very careful when making old buildings more airtight. While doing this is great for reducing heat loss, doing this can also result in unintended and undesirable consequences such as creating a moisture problem inside your building envelope assemblies where you did not have one previously. So one really needs to know what one is doing before building or remodeling to achieve low air infiltration rates. Many times the only reason an old building does not have a building assembly moisture problem is because of the increased drying effect caused by the existing high air infiltration rate through the building assemblies.

Alrighty, then - boiler and building calculations first:

I found scans of an old 19511st edition IBR Guide on the internet. Since our building has not changed much since 1951, I figured it should be pretty accurate. So, I broke out a spreadsheet and ran the numbers for the parts of the building heated by our boiler & per the IBR, our existing needs are: 61,300 BTUs.

Almost immediately after finishing the IBR, I found a much easier plug-n-play calculator over at builditsolar.com & came up with load of 57,654.

As another point of reference, I ran the utility bill/weather data numbers with the help of the handy efficiency use calculator link above, and came up with:
Nov/Dec = 3057 BTUs/day (814 HDD @ base T 65 / avg T 40)
Dec/Jan = 4906 BTUs/day (1245 HDD @ base T 65 / avg T 27)
Jan/Feb = 4957 BTUs/day (1238HDD @ base T 65 / avg T 25)
Feb/Mar = 3996 BTUs/day (1043 HDD @ base T 65 / avg T 30)

The boiler is a 2007 Crown 95,000 BTU with 83% efficiency. In my calculations, I used 80% with the notion that the boiler may not be running a peak efficiency all of the time.

Right now, this boiler is heating what it was intended to heat - which is not the whole building. When I input the numbers for the whole building - if I did it correctly - the IBR sez we have an existing load of 143,850 for the entire building while the Solar calc. sez 137,945.

The nice thing about the Solar calc. is being able to upgrade with the flick of the iPad. I virtually improved the building envelope from "1.0 -- leaky -- typical existing construction??" to "0.5 -- tight -- new, careful construction" and virtually replaced our single pane glass windows (9 of 'em, in the basement) with something that provides a minimum of R3, thus reducing our load to 115,767. Our future (2015?) R60 roof will get us down to 105,551.

So, assuming I figured this correctly, our boiler works for what it was intended (radiators on the 2nd floor), but will not be enough handle the whole building.

As the folks say, garbage in = garbage out. I think I tallied everything up correctly, but I have no idea or reference point against which to check myself. The numbers from the efficiency calc. are especially opaque to me.

HVAC guru #2 quoted a 155,000 BTU Weill McLain condensing boiler. The next model down has a capacity of 105k, so I think 155k is fair based on the numbers above, but again, I am not sure I fully understand enough to evaluate with confidence.

Am I on the right track?

Your numbers appear to be converging to the same area of the ball park. This is always a good sign and I am sure spending the time to better understand this subject is personally rewarding/satisfying too…nicely done!

Your existing energy usage heat loss BTUs seem way low to me?  I would have expected them to align better with your existing boiler.

BuilditSolar is a wonderful DIY resource and website, and they also point to our company and many of our calculators too. Just for grins, you might want to exercise our heat loss software (and perhaps our cooling load software) too:

Borst Heat Loss Analysis Software

Borst Cooling Load Analysis Software

You may also want to check your building assemblies to ensure that you are at low risk of creating a moisture problem. Please feel free to try our free DIY building assembly moisture analysis software which is still under-going beta test:

Borst Building Assembly Moisture Analysis Software

IBR has been obsolete for some time.

Your existing energy usage heat loss BTUs seem way low to me? I would have expected them to align better with your existing boiler.

What do you think they 'should' be?

I am having a hard time establishing a generic baseline to measure against - what is an 'acceptable' amount of heat loss? What is 'normal' usage?

....even though that cushy under-floor heat is nice, it becomes a frying pan hot-foot feel if you have R1 brick walls and need to be pumping 40-50BTU/square foot out of the radiant floor

Now, this scares me. If I figured the Solar calc numbers correctly, in our 3000 ft2 building (1000 ft2 / floor, including the basement) we would be using 38.6 BTUs / ft2 to heat the building after making improvements to building envelope leakiness & replacing the flimsiest windows.

And 38.6 is just the average for the building - the sun porches would draw about 200% more & half of the other rooms need approx are hovering over 100%.

So, what should BTU/ft2 be? Ideally? Generically?

As Dana mentioned, after about 45 btu/sq ft people aren't going to want to stand on it. Look at ceilings or walls if you need more. Or more insulation.

I guess I would have expected your existing energy usage to be closer to the IBR (61,300) and builditsolar (57,654)...which are in fairly close agreement Your future building numbers also seem to be in fairly close alignment and in line with one of your HVAC pro recommendations too.

The required upward heat flux (BTU/hour-sf) depends on the actual room heat loss BTU/hour minus the floor downward heat loss, all divided by the actual room floor area used to provide the actual floor heating. The floor surface temp needed to provide this required heat flux is equal to the desired room temp plus half this required heat flux. You normally don't want floor temps to exceed 85F for rooms with prolonged foot contact (e.g., bedrooms, kitchens and living rooms), but bathrooms (except near toilets which should NOT be heated at all), hallways and entry foyers may be as high as 92F. I would recommend that you just run your numbers using our free DIY hydronic radiant floor heating design software:

Borst Hydronic Radiant Floor Heating Software

Please be sure to carefully read the associated instructions as using this one is not nearly as easy as running the heat loss analysis.

This was very helpful for us. Thanks to everyone who weighed in.

Here is where we are leaving it to hibernate until spring:
-non-profit provided, City of Chicago approved, energy auditor coming next week;
-replace basement windows;
-insulate roof to R60 - I'm sure I will be asking about this in another forum.