I am building a workshop in central NC, 30x36, and I'm considering using in slab radiant heating.  I recently saw articles that showed systems where a single HW heater was used for both DHW and radiant heat.  In calculating the heat load and comparing with the heat output of various heaters, however, it seems that all HW heaters have much greater heat output than what I need (for space heating).  How does one size a HW heater in this case?

Here are my specifics:
Only first floor is heated with R11 in the walls, R30 in the ceiling, 3 inches of exp. foam at perimeter of slab, ground compacted, moisture barrier, and 2 inches of exp. foam under 4 inch slab.  Doors and windows factored into Heat Load calc.
Heat load: 13K - as calculated using John Siegenthaler's Modern Hydronic Heating formulas for Slab Radiant heating using an outside temperature of 20F and inside temperature of 70F (75F for bathroom and Finishing room).
DHW requirements are very low as this will only supply HW to the workshop for cleanup purposes and the occasional shower.
I am considering an Open Direct system due to its simplicity and the fact that the DHW is only used for cleaning purposes - not concerned about potential contamination issues.
Gas source will be LP.

I am impressed with the Polaris hot water heaters and can understand why they are more expensive than alternative heaters but their lowest BTU rating is 100K BTU which seems way overkill for my needs. 

So three questions:
1. What minimum size HW Heater should I use?
2. How much will using an 'oversize' HW heater affect my cost of operating?
3. Are Polaris HW heaters worth the extra money for my application?

Tom

Things that affect my decision on which appliance to use for an application are;
Btu's; you are correct in starting there and not wanting over kill. If you are grossly oversized, you may need a buffer tank to avoid shortcycling.
Code approval; In MI for instance water heaters used for space heating must be listed and labled for theat purpose.
Simplicity; eeking out the last % of efficiency may cost geometrically more to purchase and maintain on a relatively low operating cost system....not a good trade.
Additional requirements; if you do want to make DHW as well you'll need extra capacity over the heating btu's.
Fuels available/cost to operate; Electric providers often offer a discount for space or water heating with electricity, and this can often (again in MI) be cheaper than propane, throw in the bonus of a really inexpensive water heater and it's win win.
Good luck,
Joe

Open Loop systems are bad, bad bad bad.
Why?
Legionaries disease
perpetual stand by heat loss
The polaris is an expensive appliance. For about the same cost why not buy a nice wall mounted condensing boiler that has isolated domestic on demand and a primary system pump built in that will handle the pipe in the slab. you are talking about maybe 1,250 feet of pex pipe.
Your cost is a wash, efficiency greater, install easier.
When you are not heating or using H20 the system is off.
Dan

Dan,

any suggestions on a appropriate wall mounted condensing boiler for my application?  All the pricing I've seen shows these more expensive than even the Polaris HW heater, especially those that would supply DHW.  I think if I were to go the boiler route I'd probably just go electric DHW given my low HW usage.

Tom

Around here a .075 KWH rate for space heating trumps a propane 90+boiler at $2.50gallon.
J

Tom,
I see Polaris units go for about $3,600 + freight. You will then need additional parts, pump, controls and more all to be built on externally, difficult and time consuming to do a proper clean job.
We design for and distribute the Laars Mascot with a lot of our systems. Retail price is about the same but it is a self contained wall mounted unit, no add on parts. They are reliable, diagnostic applications are good, modulation is 25,000 BTU - 125,000 BTU, in your application you could easily expand to a low temp radiators for the upper floor. The built in pump and expansion tank are more than adequate to drive your whole system. These are not a difficult install if you have some mechanical skills and/or access to a gas/boiler tech to help with commissioning.
The boiler core is standard, same as what is used in the Munchkin and several others.
I can send you a mechanical PDF when I am back in the shop Monday.
Dan

Dan,

thanks for the lead.  I got the info from your website.  It looks like a good fit for my house in Maine (hydronic radiator) being totally self contained as it is. 

Unfortunately, I see no data on the efficiency curves versus output water temperature.  The data shows the maximum temp at 176F but this is way too high for a floor radiant system.  Do you have any data that shows efficiency vs output water temp?

Tom 

Hi Tom,
The heating side of the boiler can be adjusted from the front of the boiler as can the domestic H20, Heating range is 85 degrees - 180 degree. As with any condensing boiler they really are only at there maximum efficiency when condensing (less than 135 degrees). Set point on your floor loop would likely be in the 110 range or less, in this mode it is running at about max efficency that would be in the 95% according to the manufacture. If a cold snap requires a warmer delivered floor temperature it is a simple turn of the heat temperature control dial on the face of the unit.
Have a look at p. 34 it shows the face of the boiler and controls, setting both the domestic H20 temp and the delivered heat temp is done from these points.
P. 37 has a description of the sequence of operation.
Here is a link to the manual, http://www.blueridgecompany.com/documents/LaarsHT330.pdf
We have some design schematics that make the pipe lay out very easy.
Dan

Dan has it right. I don't design "open" systems as they are not approved anywhere to my knowledge. They are promoted to the unwitting novice and work until the warranty expires.

The Polaris is a high efficiency water heater but lacks outdoor reset (a key component for efficiency and comfort).

Stay with the Mod/Con and pay a professional to set it up for you.

Dan,
thanks, I'll contact you through your website to get those schematics.

To Joe, thanks for the advice regarding possible power company savings.  Unfortunately it doesn't appear that Duke power offers such a great deal and at $0.1043 per KWH the equation changes.  It DEFINITELY changes in Maine where the electricity costs are $0.1627 per KWH!

We have a 3300 SF home with tall ceiling pitched to 14 Ft. in moderate southern California climate with 6” SIP walls.

 

We’ve had several bids in excess of $40K for a turn key radiant hydronic floor system which is beyond our budget (and most small 3^rd world countries! L.O.L.)

 

Questions:

 

Can a tankless do the job? Hear Takagi is the best for radiant hydronic  {?}

 

If we omit all closits under cabinets and two rarely used {doorless} rooms and plumb about ½ (1600Sf) and have 3 High BTU fire places will this be sufficient in a SIP house with moderate (hi 40'sF) tempatures? If we omit PEX coils under the beds ??

 

I was thinking we could futurize by leaving inner wall stub outs to add a few base board radiators in the non plumbed rooms if it was a bit lack?

 

Anyone currently living in a SIP house with Radiant heat or all other thoughts?

 

Thanks

 

John

 

John,  I'm hardly an expert, but it looks like you are not making good choices if you elect to leave the area under the beds, etc, unheated.   Anyone who chooses to rearrange the furniture will not be happy (and the heat likely will not work well).  I have a similarly built small house in NY (coldest day of the year is typically about 0 F) with a Takagi, heating < 1000 srq ft.   I think the experts on this forum will object to using a tankless, and while mine works great, your house is big enough that I suspect you should go with the types of boilers they will recommend.

All other points aside, however, $6-7 per sqr ft is not an unreasonable estimate in NY for radiant heat (without the heating source), in my experience.  Again, the people who do this for a living may not agree.  This said, your quote is about $12/sqr ft, which seems high, but may not be- I don't know what the costs would be in your area.

I don't know why folks jump from turn key to DIY without careful consideration as to how they will DIY.

Tankless water heaters are not boilers.

Hire an RPA certified designer before wasting serious time and money.

Once you have a heat load analysis, the experienced designer can give you options and sound advice on what part of your project is DIY and which you should wisely let out to others.

Beware website distributors that offer "free" design service. I find them to be the greatest advocates for "cheap" tankless heat sources. Frankly conventional "dedicated" water heaters make a better heat source (at half the cost) for many radiant floors than a tankless not rated or designed for space heating.

I design systems but also work in the field and have fixed many DIY tankless systems. The cost is usually more than the the tankless.

Jeez, hold the freaking phone guys. I love mod/cons as much as anyway, but the lovefest has to have limits.

Toms workshop has a 13kBTU/hr load. That is tiny. He is never, ever, ever going to save enough in fuel in his climate to pay for a mod/con boiler or a polaris or anything else more expensive than a tank water heater.. Puttin gin a mod/con would be a total waste of money for him.

Tom, put in the cheapest water heater you can find for the radiant with the output you need. I recommend derating the output to 75% of listed to account for the fact that your water will be hotter than ground temp coming in so heat transfer will be lower. Then, put in one of those 2.5 gallon point of use heaters for your sink. No open system, cheap, effective. If you really want to, a tankless water heater would work here too (single high mass zone) but I don't know why you would spend the extra bucks for the marginal benefit you'd get.

Not to flog a dead horse but the 13K load makes an electric storage tank fairly attractive against propane anything.
J

The key questions;
Where in California, elevation/location
Platform or slab on grade, rambler or multi story, these will effect design. Sips is very tight, good choice.
Thinking in terms of where not to place pipe is not the right approach. Pipe is the least of the expense. Bigger issue is understanding what you will be doing and how to do it best for your budget and right. Therefore the whole house. Installing pipe is not a big issue if you have a plan. We distribute prefabricated mechanical packages, If you have a plumber and boiler technician available it is a doable task to do it right.
Water heaters are wrong as a heat plant, low efficiency and they do not last. You are building a home think about a condensing boiler or a traditional boiler.
So California has great solar exposure and a friendly tax incentive program.
Consider a design that may utilize a solar aspect.
We design proper systems, and provide packages and support.
Glad to help if you are interested.
Dan

So Rob,
I like the Idea of small 10-20 gallon electric H20 for a small demand shop closed loop. What about a relay that would turn on the heater with demand for heat thus off when not in use. Ever used something like that? Programmable stat, relay to pump and water heater, seems trick would be amperage. I have never wired something like that and am not familiar with the required parts. Ideas? Badger ideas?
Dan

I wouldn't even worry about it, but I'm told water heaters are not happy to sit cold. I suppose if you did the system with a bronze pump you could drain in spring, and refill every fall, but if you insulate the tank standby losses are pretty controllable, and small.

and I would say the same to John: even with a 3300 square foot house, the loads I see in california could easily make a SIP home in southern cal a tiny heat load. the degree day count is even more modest, making a moderate load during peak still a 'tiny load' in terms of yearly energy usage. the idea that a mod/con boiler is necessary or even helpful, really, is a very big assumption I would not make without first seeing or doing a heat load calculation. No one can say whether they make sense or not with a load calc and a degree day count.

blind boiler recommendations are not really helpful.

I am with Rob on this one. Though mod/cons or more than high efficiency (sealed combustion, outdoor reset, low NOx, smallest foot print) you know I could go on.

Compared to a Polaris, a conventional tank style hot water heater is a good choice. They last decades when dedicated to heating (not silly "open" radiant systems), tolerate low water temperatures, can easily be reset for comfort and economy, are quiet and cheap. (Did I say cheap?) Must be losing it. ROI on the tank water heater is a no-where low heat loads are served.

Tankless water heaters are more expensive to buy, more complicated to service and have flow and control issues beyond the skill-set of many.

Water heater size is less important than the KW meeting demand, I know "standby loss" is a dirty phrase these days, but with a 13 K heat load and no nat. gas available, a $300 storage tank that never breaks (save the occasional heating element) has it's own economy (over a multi-thousand dollar option). When heating the tank, is 100% efficient, no gas fired can do that. In many areas utilities have discounts for electric space heating which could very easily make this cheaper than propane anything.
Use the breaker to shut it off in the summer and the thermostat to activate a pump in the winter.
Do not de energize the tank during the heating season, do not drain it in the summer.
Cheap-easy-low maintenance.
We heat guys are brought up with the "electricity is expensive heat" mind set. For large structures with natural gas available that is often true. For small structures that only have propane available, it is not.
(just sayin')