small radiant slab source

Lynne

We're doing a small building, 24 x 36, about a 6" slab, about 16,780 Btu's, no other zones or hot water in the building. Wanting to control the temperature of the radiant in conjuction with outdoor temperatures. Wanting a hot water source, seems to small for a boiler. Any thoughts

Justin Gavin

Electric boiler

There are electric boilers that will do that for you. Otherwise use a water heater (if code allows) Or you could try a Bradford White Combicor (domestic hot water + space heating) kind of expensive for what you want. Or a real small boiler with a buffer tank.

Good Luck,
Justin

JacktheShack

Radiant Heat

I question your btu estimate; depending on your location, & assuming 8' ceilings, a house with that square footage would require closer to 55,000 btu/hr to heat; have you done a heat loss calculation??

tom_25

electric boiler

here is a nice wall hung electric boilerhttp://www.efmheating.com/electthermboilers.html

Lynne

Yeah we did a heat load calculation, using Wirsbo's radiant heat loss program, what did you use?

steve l

small radiant

I would use a 5-6kw electric boiler for a project this size, here in chilly manitoba we do lots of projects of this size and would only need 9kw. I would not advise using an electric hot water tank, the standard elements are not suitable for use with glycol, as they absorb it and lose efficiency over time.

Unknown

I think maybe the previous poster forgot to add walls!

24*36=864 sq ft. His estimate would give you 63 BTUs/sq ft. That's a little high. If "strong enough to broil steak" qualifies as a little high, that is

I see no reason why you should use anything other than a standard water heater here. small heat load = not much benefit trading up for efficiency. Put a simple, cheap outdoor reset controller on it for mixing, set it to an appropriate storage temp and go.

Lynne

What's a cheap outdoor reset controller controlling, a motorized mix valve? What make do you use? And wouldn't I need just one circulator in this instance if I use a water heater? Thanks.

Unknown

Well, you said you wanted to control using outdoor reset. If the heat source itself does not have this feature (like most models), then you can use a Tekmar outdoor reset controller (or other brand perhaps) and use one additional pump to run variable speed injection. That would be the best and probably the cheapest way to implement outdoor reset control in this kind of system.


Northeast Radiant Technology, LLC

Robert Brown, Co-Owner, RPA certified Radiant Designer

207.899.2328
NRT@maine.rr.com

Joe_13

Gas/LP option

How about the Takagi tankless heater. A small one might cost a bit more then a WH and it's Ok'd by manufacturer for radiant up to 125 degrees. I guess flow rate is the only issue.
www.takagi.com

Lynne

NRT,outdoor reset is what I'm really after; I'm thinking that that slab will just be too warm in the fall and winter. I was thinking injection pumping,but it seems to be getting too complicated for such a small system. Granted, I don't think I need a primary loop, no concern for cool return water with a water heater. But I'd still have to have two circulators for this one small system, which I suppose I could live with. So I was wondering about using a motorized mix valve controlled by outdoor reset and then I would only have one circulator. Right? I really appreciate your input.

Lynne

Really, it's approved for radiant? We used an Aquastar here in our shop, but the manufacturer doesn't allow it, works pretty well though.

Unknown

I think you'll find the price of an additional pump compares quite favorably to a motorized mixing valve. Additional electricity usage is less than a light bulb.

You would probably be best off using "reverse injection" here as well; same as regular injection but the pump is pulling from the radiant instead of pushing into it. This allows you to use the full range of the pump speed better in systems like this (i.e., greater precision for no extra cost).

You could also use a zone valve instead, and use on-off injection. This would take a little balancing to get the flow right but it works. You'll be back up to the price of the pump after you replace the valve a couple times though (on-off injection = a lot of cycling). We usually stick with pumps for reliability as much as for performance.

_______________________________
Northeast Radiant Technology, LLC

Robert Brown, Co-Owner, RPA certified Radiant Designer

207.899.2328
NRT@maine.rr.com

Mark_25

this sounds like a garage. Garages typically have fewer windows. Well insulated walls and ceiling with no windows gets you low BTU requirements.
for instance, my 2500 ft shop with only two double pane windows calc'd out at 35,000 Btu. R19 walls, R30 ceiling.

Lynne

OK, thanks Robert, slab temperature is really important in my mind, I can just see those people in the office end getting too warm in spring and fall, that doesn't give radiant a good name.

doug_10

takagi

when using a takagi it's best to use a tk-2 or tkjr. the aquastar isn't set up for constant circulation.

ALH

Electricity

Why dont we use fuel for heat and leave the electricity for reading The Wall? I'd use a gas water heater and a Tekmar 356 v/s mixing control with Tekmar 507 thermostats.