I'm in the process of redesigning the heating panel for my 4 loop, single zone shop.  I'm replacing the point-of-use water heater with one designed for radiant heating systems.  Since I'm starting over, I'm wondering if it is better to use a primary/secondary two pump system. 

My building is 960 sq ft, one room, 4 loops, 180' to 220' per loop, and one thermostat to control everything. 

Would there be any benefit in a system this small?  One of my goals is to keep this system as low cost as possible, but not if the extra expense is of benefit.

Thanks,

If you're using a water heater and it's doing domestic, you'll want a heat exchanger. that will have two pumps. technically primary/secondary, I suppose.

otherwise, only if your flow rates for the system exceed what you can push through the heat source, or are less than a minimum flow rate through the heat source would you need primary/secondary.

so to answer that, you'd need to know what heat source you are using and what your flow rate expectations are. I'd guess 2 to 4 GPM max given your 4 loops.

It's a closed loop with no DHW, heating only. Right now I'm considering a either a Hydro-Shark 3 SH-3-7 (7 kw) or a Thermolec B-6TMB (6 kw). The Hydro-Shark claims there is no min flow required and the Thermolec requires 2 gpm. My loops are 1/2" pex hooked to a 1.5" 4 port manifold. All of the panel plumbing is 3/4" copper. No check valves or mixing valves. The pump I'm using is a Grunfoss with 15' head. Does it sound like I'm in the ballpark?

I'm not positive of the pressure drop off the top of my head through those units but I don't believe you'll run into any problems if you check the numbers.

The Thermlec has 1.25" inlet and outlet and their manual states that the internal restrictions are negligable. The Shark has 1/2" npt and the manual has no info on internal restrictions.

The thermolec is a boiler, good product will last years. It needs flow but has no restriction in your case. a 3 spped grundfoss or Taco on low will have plenty of flow. You are looking for I think about 5 gpm over the boiler, check. You will not need primary /secondary. set the floor temp on the boiler and go. You will want an air scrubber ( I like B&G EASB-Jr Enhanced Air Separator) ,external low water cut out or high temp cut out as back up, code may require both, expansion tank and perhaps a back flow pressure reducing auto feed.
Hydro shark is a stibeleltron water heater with a shark sticker.
Dan

I was just told the the drop accross the Hydro- Shark is 5 to 7 psi.

wow. If that's true, use the thermolec.

The very sparse installation manual for the HS shows several examples, all with primary/secondary setups. If I have to build a new panel with a second pump, the price will be real close to the Thermolec alone.

Hydro shark has a huge pressiure drop, 5-7 is accurate, same is true with a Rinnai. You need ether a 009 Taco or primary secondary.
This next week are launching a heat approved version of a American electric tankless waterheater. It has a host of UL certifications specific to heating,full flow, capability (no restrictions) and an air eliminator on the high point. A well thought out design, fully modulating, and will be priced in the 650.00 range. A step between Hydro shark and Thermolec. And a huge step ahead of the Hydro unit.
Dan

I would be very intrested in hearing about this new unit. When will it be avalible?

I couldn't wait, I went with the Thermolec. My panel design was much simpler that my first try. So far it's working just fine. My only concern is that at night, it will run for about 15 minutes, shut down for 10, then repeat pretty much all night. Temps have been high teens at night and mid twenties during the day. I'm using a simple Honeywell heating stat, wall mounted. I was wondering if a slab stat would be better. The slab is low mass (4" of concrete over 2" of foam board with 2" around the perimiter). Any sugestions would be appreciated.

Your slab is high-mass.

Electric boiler come with outdoor reset built in.

I check the cost per therm of available fuel before I specify a boiler.

I tought a thin slab that was insulated from the ground was low mass. Thanks, I learned something. Yes, the boiler has outside rest built in. My only choice for fuel was electric or propane. My electric is 6.6 cent a kw. Propane is high here. Price depends on whether you own or rent a tank, have a contract or not, and what time of year you lock in a price. So I went the easy way.

the boiler is infinitely modulatable, so if it's cycling it has to be because your thermostat has firing logic.

If your room temperature is even your thermostat is doing its job.

Yes, the room temperature is stable. I was wondering if a slab thermostat would reduce the cycling.

I don't think you want it to. Pulse firing is appropriate high mass management technique. the cycling shouldn't hurt this boiler anyway.

Then I won't worry about it. The temp is stable and the doors are not opened very much. The little Honeywell works and I don't know if I need to spend $80 for a digital display. This boiler should hold up better than an instantanious water heater and I didn't have buy a hi-buck pump to push the water through it.

I was watching the system tonight. It will run for about 10 minutes. The temp out is 90 f and the return is 70. The little flow meters on the manifold are showing .4 gpm on each of the 4 loops. The thermostat is set for 60 and the room temp is right at 60. The data logger shows the temp min at 58 and max at 62 over a 24 hour period. Ameter shows 37 amps while the system runs. Boiler set point is 122 and the unit 9 kw. OAT has been 16 to 20 at night and 25 to 34 in the day over the past 4 days.