hey guys
i'm asking because i respect most of your replies
the better half and i have a newly dry'd in icf (tf systems )2900 0r so sqft house in the seattle washington area
we were planning on a lochinvar knight boiler wb50 and a warmboard substrate below the tile to heat the house untill i read?
the floor won't be warm enough to be warm to the feel? hmmmm
it might warm to 78-80f but won't be noticably warm unless it is actually cold outside
in a super insulated or icf house, the hydronic floor temp will be in the low 70's?
anyhow. i have a heating cooling load estimate from energywise in texas
32000-34000 btuh is the number he comes up with for heating and 24k for cooling
so?
we are thinking radiators instead of infloor
is this reasonable?i love to put my feet up on a heater now
rob.dana,dan, badger?
i would truley appreciate your input
signed: lost in unconditioned space

Not warm and not cold are two different things.

With the Knight properly sized and on-board outdoor reset, comfort is a given. I use a condensing water heater and ODR controlled sub-system for a sealed combustion space and DHW loads where the loads are modest due to house size or climate.

If you are really, really desperate for warm floors, you can always reduce the size of the heating element by installing "blanks" under beds and sofas, etc.  This will bump up the temperature of the heated part of the floor because less of the floor will be heated, reduce your efficiency (since you'll need warmer water) and introduce an element of risk (that the heated section will then be too hot) but it will accomplish what you want.

Frankly, I'd never do this.  Besides all of the above, your floors will only be warm (even if you reduce their size) for a fraction of the heating season in any event.

Jeff

well
the waaaaaambulance finally got here and the emt said something about warm feet being for diva's
then the "effeciency" word popped outand something about lower temps and now we're back on board with the warmboard
thx guys for the input
kinda curious though about the condensing water heater vs the boiler fora btu source in this climate
if you care to elaborate? i have spock sized ears

thanks again
keith

Well,
We provide the RHT floor panel system. It is considerably expensive than the warm board platform. Sits on top of the sub floor.
Low temp delivery water. works well with condensing system, installs before or after sheet rock.
We are on Vashon, come on over for a look,
Dan

Posted By kotero on 03 Nov 2010 11:52 PM
well
the waaaaaambulance finally got here and the emt said something about warm feet being for diva's
then the "effeciency" word popped outand something about lower temps and now we're back on board with the warmboard
thx guys for the input
kinda curious though about the condensing water heater vs the boiler fora btu source in this climate
if you care to elaborate? i have spock sized ears

thanks again
keith

The only reason you would EVER go with a condensing water heater rather than a "right sized" mod-con for the load is for the greater domestic hot water output, and even then, unless you're filling monster tubs on a regular basis, an indirect fired tank running as a separate zone would be enough.  Very tight space-restraints might push you there too, in rare instances.  Cost-wise a condensing tankless will usually equal or exceed a mod-con + indirect solution, but will run with comparable efficiency (if the system design is up to snuff.)  Both Rinnai and Navien now have condensing combi-boilers based on their tankless water heater technology, but I'm agnostic on whether that will be a better solution than a small Lochinvar and an indirect in your situation.  

In general condensing TANK type water heaters have somewhat lower raw combustion efficiency in space-heating mode than a condensing tankless or a mod-con since the return water from radiation will be invariably mixed with the stored water at the heat exchanger, but whether that makes enough of a difference to matter in a low-heat load situation kinda depends.  A tank-type water heater won't have the short-cycling issues that a tankless combi-boiler might, but may cycle less often than a mod-con under very light loads.

[edited to add] The Lochinvar WB50's minimum-modulated input is 10KBTU/hour- lower than most water heaters (or most other mod-cons) can go, which should be about as good as you can get efficiency-wise, if set-up properly. With warm-board and a peak heat load under 35K it should be possible to break into mid-high 90s territory- higher than you'd likely get with any condensing water heater. 

<!--[if gte mso 9]> Normal 0 In ICF, SIP and warm climate construction, DHW loads nearly ALWAYS exceed space-heating loads. They are often too small for the smallest condensing boilers as well. We don't even have to consider then, a 199,000 Btu tankless "water heater".

A condensing water heater then is the logical choice. 2" of closed-cell foam - virtually no stand-by losses -sealed combustion, direct vent, quiet, nearly always condensing. Since its design temperature (output) puts it in condensing territory the return temp and ground water assure nearly continuous condensing (98% thermal efficiency) whether it space or DHW heating.

The sub-system is key and should separate potable water from heat transfer fluid AND reset temperature for guaranteed comfort and fuel savings.

This is my go-to system where my precious condensing boilers just won't fit.

There is never a good excuse to use a tankless water heater for space heating, condensing or otherwise.

Have a look at our Mascot II
This is the new LAARS american made/assembled boiler with the Gianonni condensing burner.
Modulates 25-125,000 BTU, domestic H20 on demand or side arm tank.
This is a nice package for residential apps with out breaking the bank.
Dan

To hit 98% efficiency you would need the return water to be at temps at or below room temp- can't cheat the physics. You can hit ~0.98EF with a condensing tankless in water-heating mode, but not so likely in a heating system. But you'll NEVER hit 98% with a condensing tank unless you have a continuous flow of sub-75F water at the heat exchanger, which you CAN do in a thermal-efficiency test, but that would be a big exaggeration of it's as-used efficiency (~10% higher than real-world best-case.

See:

http://www.modsync.com/images/graph2.png

http://www.emeraldenergy.ie/images/cboiler.gif

http://2.bp.blogspot.com/_fR_WhZMEYGE/RYsulbtz5KI/AAAAAAAAABU/KzvbpW-OEL8/s1600-h/Aerco_Boiler_Curve.jpg

http://www.healthyheating.com/Low_Exergy_Systems/Energy5.jpg

You can hit 90% in a well-designed system around a condensing tank, butONLY on a system with very low temp requirements at the radiation. The EF factors of condensing tanks are probably a more realistic representation, and are usually a handful of points below 0.90 , (90% average efficiency.) You won't exceed the EF rating of a condensing tank in a space heating app, but it might come close with radiant slab or WarmBoard at low heat loads. Steady state thermal efficiency in water-heating mode is a lousy number to work from for estimating the combi-average (or even the EF.)

With a mod-con and an indirect you'll probably beat 80% in water heating mode, but mid-90s for space heating with WarmBoard. Instantaneous heat loads for hot water ALWAYS exceed the peak space heating loads, but only rarely does hot water heating BTUs exceed space heating BTUs on an annual basis. If your annual thermal requirements for DHW exceed that for space heating, a condensing tankless might give you better overall efficiency than a mod-con + indirect. With a mini-mod-con you'll have to up-size the volume of the indirect in high hot-water use to buffer enough for high-use DHW applications though.

But if your combined loads are small enough, the efficiency of the systems supporting those loads becomes irrelevant. If you would only burn 400 therms/year at $1/therm with an 80% efficient system, you'd be only looking at saving ~$50/year going for that 90-95%-efficient condensing system, and the net-present-value of the delta in cost will be forever negative within the anticipated life-cycle of the equipment. If the installed cost of a ~0.85-0.87EF condensing tank would be substantially cheaper than a mod-con + indirect or a 0.98EF condensing-tankless based combi that might boost your average efficiency to 95%, the lower efficiency tank might still be a better choice.

No argument.

We are talking thermal efficiency here and it is not all about the money - though ROI should not be ignored.

Most of the radiant heating systems I design incorporate low temperature radiant panels; walls, ceiling, floors and wall-hung steel panels at or near 120°F. The condensing boiler operates without modulation or shorting-cycling which means steady state is easily achieved.

The main benefits of condensing water heaters are great DHW heating performance, space savings, sealed combustion, direct vent, and quiet GREEN operation. This will beat the Combi units of all kinds for efficiency, reliability and economy if designed for the combined load.

Tankless can't hack it, and most of the Combi water heaters are too big for the heating load...even at low-fire.

Give me the condensing water heater for my small projects or forced air over radiant basement applications.

Disclosure: I sell the stuff I am about to comment on. Disclosure II: I used to sell one of the other products already mentioned in this forum.

Alternative to Warmboard = Ecowarm OSB panels. Less expensive than Rehau and certainly less expensive than Warmboard. It will also reduce the surface area (so would Rehau) by not going under cabinets or into closets, which will raise the floor surface temperature a little bit. Ecowarm will still give you a wood substrate, and aluminum top emission plate, even heat spread, nailable surface, etc. It also uses 1/2" PEX, so the plumbing side of your system doesn't have to change much (except for slightly less PEX usage in the floor surface area).

I concur with jbaron regarding the "blank" areas under beds (don't do it),especially if there is any hardwood involved. Mostly because the heated portion will cause the hardwood to react differently than the unheated portion and it's difficult to predict how much.