Boiler for Radiant in ICF

[diesel_farmer]

I am building a 2600 square foot ICF home in zone 5 (central Illinois) on a concrete slab with Anderson 400 series windows and with 3" of XPS in the slab -- so it will be fairly well insulated. I had a local HVAC contractor provide a heat loss calculation of 45k btu, which I am not certain of since sometimes the ICF homes are not properly understood.

Nevertheless, I'm planning to install 7 lines of 3/4 or 7/8 pex tubing in roughly 300' runs in the slab on top of the XPS in one zone.

In the home I currently reside in I have used an electric (Thermolec) boiler with a primary/secondary loop which has worked trouble free for 8 years now with no complaints. It does require heavy electric use (higher power bills), but has provided trouble free usage. I also have used electric on-demand water heaters with trouble free usage.

Reading and discussing my options with a couple of plumbers, the local recommendation has been to install a combi boiler like a Navian for my set up with the added benefit of the hot water from the Navian. I'm concerned that this unit might be more "high maintenance" than what I have experienced with my electric boiler. I also don't want any more 2" pipe through the ICF walls than I have to -- one minor benefit of the electric boiler is the lack of required venting to the outside.

I am open to all thoughts regarding what unit I should be more seriously looking to install for my application -- i.e., an electric boiler with a stand alone on demand water heater OR an LP combi boiler.

Thanks!

 

[Dana]

A load of 45K at 70F indoors/-1F (Peoria's 99% outside design temp ) would be unusually high even for a 2600' code-min house, let alone an ICF with better than code windows, unless you have huge air leakage or a an acre of window area or something. A typical 2600' house built to IRC 2015 code minimum would come in around 35-40,000 BTU/hr at those design temperatures, using AGGRESSIVE input assumptions (as prescribed by the ACCA Manual-J instructions). Your real heat load is probably under 35K, possibly under 30k. Rather than relying on an HVAC contractor for the numbers, have an independent energy nerd like a competent RESNET rater or professional engineer run the room-by-room Manual-J, and stress that you're looking for aggressive rather than conservative numbers. It'll be a few hundred USD, but it'll usually save more than the fees in equipment costs.

Maintenance is less of an issue with a Navien combi than the fact that it's going to be oversized for your heat load, and possibly marginal for your hot water loads. Low mass combi-boilers are better suited to houses with low hot water needs and high heat loads. The smallest Navien you'd be installing for serving sufficient hot water would be the NCB-210 or -240, either of which has a minimum fire output of about 17,000 BTU/hr for space heating, which is likely to half your actual heating load or more. Ideally you'd want a boiler that would modulate down to well UNDER half the design load, so that it will run in a modulating most most of the heating season. More on sizing condensing boilers here.

How are you air conditioning the place?

LP is an expensive fuel, but so is resistance electricity. You may want to opt for an electric boiler to serve the floors operated with floor thermostats, but install ductless mini-split heat pumps to maintain the room temperature. Heating JUST with better class ductless heat pumps with 13 cent electricity would be about 60% the cost of heating with condensing $1.50/gallon propane. Keeping the floors a degree or two warmer than the average room temperature preserves the cush factor, but unless you really crank the floor temps up going all-electric with heat pumps would cost substantially less than heating with propane, and you'd have high efficiency air conditioning to boot.

Electricity, with HSPF 12 ductless:

BTU/kwh = 12,000 , kwh /MMBU (million BTU)= 83, x 13 cents = $10.79/MMBTU

95% propane:

BTU/gallon = 91,600, x 95% efficiency = 87,020 BTU/gallon , gallons/MMBTU= 11.49... x $1.5o/gallon = $17.24/MMBTU

Propane is as cheap now as it's likely to ever be, whereas electricity has downward price pressure on the wholesale markets now that new wind and solar are cheaper than existing nuclear & coal, and becoming more competitive with natural gas power generation every year. (Midwestern wind is cheaper than combined cycle gas, utility scale solar is cheaper than simple cycle gas turbines.)

 

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[diesel_farmer]

Dana --

Thanks for the thoughtful comments. I will search out an "energy nerd" for Manual-J calculations.

AC -- I am planning to install a high efficiency forced air unit with duct work - with probably a 14 or 16 seer rating. I have looked at the ducted and ductless mini-splits, but have shied away from them because 1) they are relatively new/unknown in my neck of the woods; 2) the idea of having 5 or 6 units (that can fail) in various rooms in the residence is not that appealing to me; 3) higher initial installation cost and 4) the difficulty in running line sets in ICF walls or overhead and the possibility of hitting them with nails/screws is not that appealing to me. I surmise you think I should be looking at the heat pump instead of the conventional AC unit, which will also give supplemental heating?

LP vs. Electric -- LP is running around $1.10 per gallon right now with a forecast that it will increase, since oil prices are rising. Electric rates here are $.12 per kWh.

I agree with your forecasted energy assessment in your last paragraph.

 

[Dana]

Apparently the financial analysts at Lazard (an investment bank) think so too. They just released their eleventh levelized cost of electricity report, which shows onshore wind and utility scale solar are both competitive with combined cycle natural gas, and cheaper than most other grid sources.

Those numbers are $ per megawatt hour over the lifecycle of the systems. The $30-$60 range for onshore wind translates to 3-6 cents per kwh, wholesale price. Both wind & PV are getting cheaper year on year, which puts a lot of downward pressure on wholesale prices even WITHOUT the tax credit or production credit subsidies.

 

[diesel_farmer]

Update -- the Manual J calculations were performed using Wrightsoft software (by a very nice/professional fellow in Colorado) based on characteristics specific to Peoria, Illinois, while factoring in my specific floor plan dimensions of 2594 square feet on a concrete slab with no basement, ICF walls, under slab insulation, Anderson 400 series windows/doors and cellulose blow in the ceiling. The results are:

-- heating equipment load is 27400 Btuh
-- cooling equipment sensible load is 15342 Btuh
-- latent cooling equipment total cooling load of 17967 Btuh.

Now I can do a better job of sizing my boiler. Obviously the local HVAC fellow didn't quite hit the nail on the head on his calculations.

The Manual J report has a graph on Hourly Glazing Load that I will study up on. It says the maximum hourly glazing load exceeds average by 16.3%.

Thanks for the previous comments, Dana.

 

[Dana]

Hey, at least the local HVAC guy didn't overshoot by fully 2x, eh?

A heat load/square foot ratio of ~1o.5 BTU/ft is credible, for a house like that, as is the cooling load of ~1 ton per 1700'.

An electric boiler running the floor controlled by a floor thermostat, and a 1.5-2 ton (max) heat pump for higher heating efficiency + AC controlling the air temperature could be the sweet spot. If you size the boiler to be able to manage 100% of the load you won't have any comfort issues with the heat pump's declining capacity when outdoor temps drop to single digits and lower.