New home is in progress and I'm working on load calcs for sizing the heating system this week.  I bought the homeowner version of "HVAC-Calc" and have entered everything in, but I think the heat loss is too low.  I know these homes are more efficient that standard framing construction, but it seems a little hard to believe..  The windows were all added to each wall, specifically I'm using Serious Windows rated at R8.  The house is 4000 sq feet, garage is full icf up to the gables, w/ walkout basement.  I attached a screenshot of the results, if anyone is familiar with this please let me know what you think.  The A/C is actually not too far off what I was guessing, I figured 4 tons and it comes in at 3.5 tons.  The heat loss of only 33k btu/h seems too low to me.

Thanks!

If you're doing ICF with serious windows, I would not consider this CRAZY. low, perhaps, but you're making a "serious" investment in your envelope. It's impossible to know if it's a problem or not without a fairly intense review of your calcs. some gotchas:
1. did you account for the walkout wall in the basement, i.e. not call it a foundation wall?
2. does your second floor have "ceiling" elements?
3. did you allow for a 0.15 ACH minimum in winter?
4. you can't put R value in on HVAC calc for windows, how did you approximate the serious window performance? if you look at the detail for one of those windows, are you getting roughly Area x (room-outdoor temp) all divided by R value per window?
5. are you missing any major windows/doors?

I would expect 10 BTU/sq ft as a ballpark for this kind of construction but a lot depends on the amount and quality of glass of course. so you're in range. congrats, if you've got good passive solar you might be near passivehouse

Posted By NRT.Rob on 12 Jan 2012 09:23 AM
If you're doing ICF with serious windows, I would not consider this CRAZY. low, perhaps, but you're making a "serious" investment in your envelope. It's impossible to know if it's a problem or not without a fairly intense review of your calcs. some gotchas:
1. did you account for the walkout wall in the basement, i.e. not call it a foundation wall?
2. does your second floor have "ceiling" elements?
3. did you allow for a 0.15 ACH minimum in winter?
4. you can't put R value in on HVAC calc for windows, how did you approximate the serious window performance? if you look at the detail for one of those windows, are you getting roughly Area x (room-outdoor temp) all divided by R value per window?
5. are you missing any major windows/doors?

I would expect 10 BTU/sq ft as a ballpark for this kind of construction but a lot depends on the amount and quality of glass of course. so you're in range. congrats, if you've got good passive solar you might be near passivehouse

1.  I did account for the walkout as an ICF wall with the same values as the main floor and second floor walls, not calling it a foundation wall.

2.  The second floor does have a ceiling element as well as insulation added.

3.  I used .2 ach for summer and .3 ach for winter

4.  I didn't approximate the window performance, just used double pane and vinyl frame with no shade and clear glass.  If anything, this probably under-values my windows.

5.  I went through and checked all windows and doors, they are all there.

I have to say I'm still pretty stunned at the heat loss values.  I've done commercial heating and air for about 15 years and have never seen any numbers like this.  Prior to deciding to build with ICFs I had figured  picked out 2 furnaces to split the zones into basement/main floor and then the second floor.  I still plan to split it up, just with smaller equipment.  The water heater for the radiant is going to be much smaller as well, looking at the AO Smith Vertex to cover both Radiant and Domestic needs.

Thanks!

with that total load in michigan I would still prefer a mod/con boiler unless you do have a significant passive solar element to offset usage further.

wait, I'm a liar. Your infiltration is probably double reality (0.35ACH is kind of like "good quality" fiberglass 2x6) and your windows are too (half real R value), easy. Your real load is probably significantly less than what you are currently calculating, actually. maybe closer to 20k. in that case the water heater route is more worthwhile.

You are not using "mass enhanced r-value" on those walls, are you? Just specified ICF wall with X amount of foam or however HVAC calc does that?

Posted By NRT.Rob on 12 Jan 2012 10:59 AM
wait, I'm a liar. Your infiltration is probably double reality (0.35ACH is kind of like "good quality" fiberglass 2x6) and your windows are too (half real R value), easy. Your real load is probably significantly less than what you are currently calculating, actually. maybe closer to 20k. in that case the water heater route is more worthwhile.

You are not using "mass enhanced r-value" on those walls, are you? Just specified ICF wall with X amount of foam or however HVAC calc does that?

Right, just the ICF wall w/ 22-26 R value insulation.  The homeowner version didn't have the "mass enhanced" option.

As for the water heater, I should have been more specific.  The models I'm looking at the the HTP Phoenix and the AO Smith Vertex, both are modulating (5:1 turndown) and go up to 100K btus.  From what I've found the Vertex is less than half the price of the Phoenix. 

The radiant will also be backed up by forced air heat.  I'm installing central air, so instead of just using air-handlers I'm using furnaces.  The added benefits being humidifiers (rather than needing steam models that guzzle power), plus backup heat.


I'm also putting up a separate 3 car garage as well so just for giggles I ran hvac calc on that 2x4 building including insulation and 1" foam board on the exterior- at 832 sq ft the heat loss is greater than the house.  Lol

you were specific, but usually I would prefer a modcon to a water heater because the modcon can run to lower temps than the water heater can. You don't need forced air backup here. save the cost of the furnaces. I'm not even sure you'll need humidifiers, I would "wait and see" on that and put in a good ERV in the meantime.

Funny you say that, I really like Weil Mclain boilers- been working on them for years and was talked out of going that route by the company that designed the radiant heating system.  I was told the water heaters work substantially better in this application due to the lower operating temps.  I had spec'd a Weil Mclain Ultra originally, I'll have to go back and look through everything again- especially now that I know how much less heat I need.

Which ERVs do you prefer?

you can probably run radiant, if you still use radiant here, less than 100 degrees easily. you cannot do that in a water heater.

The loads do not surprise me. When you control your infiltration it is easy.

Our business is mostly modcons, indirect water heaters and radiant floors but in block loads below 50mbtuh a condensing water heater is the usual choice. Forced air in a basement - walk-out or not is hardly ever justified as cooling is not needed and you have a hydronic heat source already. If you must have forced air for whatever reason you can make up, then properly sized it would handle the upper floor only, for cooling and scorched air with a radiant floor zone for the basement. An ERV is mandatory and will keep enough moisture if two people are living there.

I am a great believer in modulation but you don't have to buy a modcon to get it.

Better to have a professional help with the design (usually little more than a good heat load program) before spending hard cash on two furnaces, one water heater, dehumidifier, humidifier, ERV etc. etc.

Without a plan in front of you, it is just guesswork, techno-speak about "mass-enhanced R-value" not withstanding.

I would say your numbers are OK with a fair bit of fudging on the high side even. I just finished an all ICF house with a design temp of 3°F , 3000 square feet and heat load was 36,000 BTU with large windows on the south wall. NRT Rob could confirm those numbers as he designed my in floor heating. Went with an electric boiler, cause we don't have access to gas.

In 2017 I built a new 1375 sq ft full ICF house with full walk out basement. The basement slab has 4" of insulation under the slab with radiant in slab heating. The slab was completed with four 250' parallel loops. The slab is heated with one 3k electric boiler that is on time day and only operates from 11pm-7am each day. The slab is heated to 20 deg C by morning with an outside temp of -15C. The space temperature is normally 20.5 early morning. By 10pm the slab temp has dropped to 19-19.5 and the room temp is 19.5. The basement has 3 double pane low e argon windows @ 7.5 sq ft each. One insulated 42" x 6'6" door with a 8 sq ft double pane low e argon lite. I plan on installing kw monitor on the boiler but have not completed that part of the project yet. If the boiler operated for 8 hours each day which from the utility bills is not the case the heat loss for the basement would be 3412 BTUs per hour for a 24 hr period. The main level of the home is open design concept and is heated by on 12kBTU air to air heat pump. The home has small electric baseboard heaters in each room that are only used if the doors to the main area are closed. So if these two heat sources are taken into consideration the total heating capacity for the building would only be 18Kbtu per hour. There are 100 sq ft of glazing on the main level with two 4'6" entrance systems with side lights(entrance system glazing included in the main level glazing total). The home was tested for energy efficiency by an outside rating agency and received an Energy Star Rating with ACH rating of .59. According to the report the total energy consumption for the building per year including heating and occupant use would be 16458KWH per year. Total for 2020 14637.

I have worked on several projects where heat load calculations for residential ICF construction appear to be high after the building is occupied and commissioned. During my working career I was responsible for the commissioning of new commercial properties and reviewed heat loss calculations during building design stages. I will admit that I never had to complete heat loss calculations myself.

I take it your in Canada. Would be good to know your heating degree days as well as if you used a standard thickness icf and what did you did about ceiling insulation . It's good to get real world info.

Posted By smartwall on 13 Dec 2020 11:27 PM
I take it your in Canada. Would be good to know your heating degree days as well as if you used a standard thickness icf and what did you did about ceiling insulation . It's good to get real world info.

IMHO Canada makes better windows then the US....We imported InLine Fiberglass windows to Michigan from Toronto about 4 years ago and love them. Great company.