I'm installing radiant heat in a 1320 sqft remodel.  Had been planning to go under the floor, but now I'm looking at Warmboard or a DIY variation, which needs lower water temps.  My hot water usage will be low, as I live alone, and run just a few loads of dishes and laundry a week.  I've taken out the bathtub (haven't used mine since I bought a hot tub), so it's just a shower. 

As I understand it, I need a boiler with the pressure to move the water through the Pex.  Can I run my household hot water off the same unit?  Since the Pex needs water around 100-110, but my dishwasher needs hotter water, how does that work?  (I guess I'm asking a separate question here:  how does a radiant system control the temp of water flowing through the tubes -- just mix with cold water like any faucet?)

My energy source is electric, I'm not interested in using propane (and natural gas isn't available where I live).  I'd like to add some form of passive solar heat collection later.  But for now, do I need a tank and a boiler, or just a boiler?
Thanks!
Kim

Kim,

the standard solution would be a boiler with an indirect storage tank.   This would take care of both radiant heat and hot water for domestic stuff.  There are severla ways to get different temps in the hot water as oposed to the radiant floor heating.

 

do you know what the design heatload of the house is?  This will help in determining what system would work best?  also what is you kwh rate for electricity?

Cheers
Eric

You might be able to get by with a tank and a heat exchanger (skip the boiler), but you'd need to know your actual heat load to know whether the elements of a HW tank are going to be able to keep up or not. If you need the radiant to run at lower temps a mixing valve on the heating loop works. (Don't run it as an "open" system with potable water in the radiant- it would be just BEGGING for legionalla to take hold. Stagnation temps need to be either above 120F, or below 85F to limit that potential- a 100-110F radiant floor running all winter is smack in the middle.)

Do you have a good history of power/fuel use against degree-day data to be able to calculate what the peak power load will be? (With the kwh usage monthly bills from last January/February- we can look up weather history on for the time period, and calculate the peak load in kilowatts from the kwh/heating-degree day). Shelton WA isn't that cold (compared to the northern midwest), I'd assume a outdoor design-temp of no lower than ~10F (it's ~12F for Seattle) but there's no knowing how well insulated the space is, or how warm you like to keep it. (I've got a brother in Port Orchard- I'm quite familiar with the foggy-dew damp winter mornings you're used to.) If you've added/subtracted windows &/or insulation this could be off by a bit, but it'll probably still be a closer estimate than a Manual-J calculation.

Bottom line- if the tank can keep up with the design day heat load with a bit of margin you can use it for both. If the margin is slim you may want to opt for a larger tank.

But if you went with a boiler it needs to be a fully modulating version handle low DHW flows. Set it to whatever you need for the DHW and use a mixing valve to set the lower temp for the radiant. The peak DHW load is likely to be considerably higher than what you'd need for heating-only, which is why a tank might make more sense if it can handle it. Recovery times will be longer (ergo a bigger tank). A typical big-tank heating element is 4.5kw, which is about 15KBTU/hour, which is about the heat load of my antique non-superinuslated house about 1.5x the size you're talking when it's 20F out- there's some chance it'll work.

I do need to find a way to calculate the heat load. How do I do this? Is there free or inexpensive software to use? Or formulas to plug numbers into? I'm replacing all the insulation, so the ceiling will be R38, the walls R13, and underneath R19. (An increase from mouse-torn R15, wimpy R5, and nothing, respectively.) I'm replacing the aluminum windows with vinyl low-e3 argon. I did add skylights, so I've got some loss there. (Still low-e3 and argon.) This house is going to end up snug and comfy.

There isn't really a way to use the home's history, since the insulation was SO inadequate. I do know I spent $1100 in oil last winter keeping it heated to 55 degrees. (No one was living there, though it was full of drapes and furnishings.) I bought one 100 gallon batch just as gas prices were topping $4/gal -- cost $425 for 4-6 weeks of heat. Painful! AND! As of today, I'm now an electric heat customer, with temp baseboard heat until I get the radiant system in. Now I qualify for great insulation rebates and never need to buy oil again. Ahhhhh....

DHW = domestic hot water, yes? (hot water for taps, laundry, shower, etc.?)
Would love advice on calculating heat load. ~Kim

I haven't used it, but it's an IBR method freebie, gotta be better than a wild-assed-guess:

http://www.heatpro.us/execs/htprolite.html

A more detailed version lives here:

http://www.heatpro.us/downprog/documents/0CD3A75D54D9AAF7D3A12937B72D224044ADDD80.html

Haven't used this one either, but it's a cheapie Manual-J type:

http://www.comfort-calc.net/price.html

For outside design temp, use 12F (could be couple degrees higher or a couple lower but it'll be close.) Design temp is the 97th-99th percentile of binned-hourly weather data (the 1-3% coldest hours of the average year). Since temps colder than that are typically short in duration and occur in the pre-dawn hours of the morning, it's the generarally acceptable to use that as your maximum heating capacity. (If it doesn't quite keep up when you're in bed, who cares if it's a couple degrees low?)

DHW=domestic hot water (potable), as distinguished from boiler/heating water, which should be kept isolated and "dead" (never replaced with fresh unless absolutely necessary, to avoid introducing new & corrosive oxygen.)

BTW: In much of the US oil burned at 86% efficiency is still considerably cheaper than heating with electricity (but in the PNW electricity is cheaper than the national average, and heating customers often get a rate break. A gallon of oil contains 142,000BTUs, which is the energy equivalent of 41kwh, so burned at 86% efficiency, you get 122000BTU/gallon or ~36kwh into the house. $4/gallon is about the same as 11 cents/kwh.) $2.50/gallon is about the same as 7 cents/kwh.

Another considerable cost in determining costs benefits/savings is using programmable thermastats in conjuction with the radiant warm-board heat. If you are consistanly firing up your 2 circulation pumps to "hold" the tempareture as well as firing your DHW, it can add to additional operational costs. A programming thermastat can be used to heat your home during nightime hours, as well as allow enough time for recovery to have hot morning showers. During the day you can gain all the solar energy possible before you have to heat during your night time hours. However, there is a fine line with low temp radiant warm-boards and home heat recovery (could take a long time). This is all based on heat loss in your structure.

I am currently working on a 900 SF home in Montana using the same concepts; however, we have a concrete floor to add thermal mass to the structure (as well as being well inside from all sides at ~R-40, plus insullation underneath and on the side of the concrete slab). I am looking into using solar thermal in conjuection with an 80 gallon tank (tall and skinny to provied a temperature gradiant from top to bottom). The solar loop will connect into a heat exchanger within the bottom of the type. A 4.5kw element will heat approx. the top 40 gallons and the solar loop will preheat the rest. A heat exchanger will be used for heating the radiant floor as Dana1 suggested above. I know this adds a varying level of complexity, but it could also provide serious cost savings espicially when used with heating. I attached a link to an example tank made by Rheem: http://www.rheem.com/product.aspx?id=A1C5BA24-7D0D-4272-99B1-261DEB045D16.

For larger systems used in heating, I would recommended using a larger tank. I only used an 80 gallon tank because of size constraints in the mechanical room.

You should look into more local incentives on renewables. Montana allows a project with renables to be financed at 3.5% for 10 years up to $40,000. When this paired with the fedral tax incentives of 30% as well as few additional state tax incentives, the solar becomes financially feasible. I may be wrong and it is a stretch, but you may be able to deduct the whole system since you are using renewable energy to heat your home. I am not sure where the "line" lays.

Cheers,
Jon

Embassy industries offers a fairly good heat loss program for free. If you go to there web site you can find it and download it and it is fairly easy to use for the home owner. As was stated above if I were to use a W/H for a heat source you would want to use a heat exchanger or keep the water heater above 140 deg. I do use my water heater for this purpose but keep the temps in the 160 deg range and run my small radiant floor and wall 20 sq ft running all year round.
good luck Mars

Oh, the gal at the local electric utility is so good to me... As part of my energy audit, she ran a heat loss calculation and came up with a 24,000 BTU load. I believe my current price per kwh is 6.5 cents. So, boiler? Hot water tank with heat exchanger? I so appreciate your help on this! ~Kim

24KBTU/h is ~7kilowatts.  Some tanks come with dual 4.5kw heating elements for a total of 9kw, so you could get there with a tank, with the caveat that recovery times after showers & tub draws could get painfully long during cold-snap days.

If you can find a 2.5-3.5 ton air-to-water heat pump it would have less than half the operating cost of an electric boiler or tank whenever it's above ~20F, less than 1/4 the operating cost whenever it's above 40F.  The Daikin Altherma line of air-to-water heat pumps get good reviews in Europe- not sure where to get 'em in the US.  At 6.5cents/kwh with a 24K design load in Shelton's 5200HDD climate, with nice low-temp radiation like WarmBoard it's likely to be worth it to go with an air-source heat pump system rather than cheaping-out heating with a hot water heater. (At half the HDD, half the peak load, half the electricity price, or lower average winter temps that might not be the case.)

Assuming that 7kw peak heat load occurs at ~12-15F outside, you'll be using on the order of 3.5kwh/degree-day. Multiply that times 5200HDD/year you get ~18000kwh/year, x $0..065= ~1200/year for heating, with an electric boiler or HW heater. 

With an air-to water heat pump, even if it only averages a coefficient of performance of 2.5 (which would be on the low side, in your location, with your low temp radiation), your annual heating bill would be ~$480.

 If the average COP is as high as 3.0 (it may be even higher in a well-designed system) you'd be looking at $365/year.  You'd need quotes to do a real present-value financial analysis, but looking out 10 years methinks this may approach "no-brainer" land from an after-tax return on investment point of view, comparing differences in installed cost between heat pump vs. boiler systems.  $700-800/year in after-tax savings is enough to rationalize a significant investment.

You might try to chase Daikin Altherma here:

http://www.daikin.com/company/kyoten/index.php#a_usa_and_canada

http://www.daikinac.com/landing.asp

Toshiba Estia is a competing air-to-water heat pump system designed for use with radiant floors:

http://www.toshiba-estia.com/estia/data/estia_english.pdf

With a heat pump you can probably use the WarmBoard for cooling as well, since the dew points summer cooling loads are low enough in your area that you won't have serious condensation issues with chilled-floor cooling the way you would in the FL or MI.