Secondary pump selection

mokomon

Upgrading my in the slab hot water shop floor heat. Been running my system for 18 years. Not really that efficient , but works.

I have 4 -250' runs of 1/2" pex in the floor slab all connected to a common manifold to a 6 gallon hot water tank set at 120 Deg F, one circulator pump. Usually have the thermostat set at 60 deg F.

Picked up an almost new 6KW Argo AT electric boiler to replace the 18 year old hot water tank. Can I just put this boiler into the system in place of the hot water tank, or do I need to redo the plumbing to a primary / secondary system with another pump ?

I am planning on using a Grundfos 15-58 for the boiler pump. Wanting to know if I should use one of those pumps for my floor (secondary circuit). Would a Grundfos 15-55 F/LC be a choice for this type of single zone system, or better to stick with the 15-58 controlled by a relay when the thermostat calls for heat . There are no zone valves on the loops.

GroundUp

Pretty much any circ will be adequate for a 4 loop system at 250ft. Either of the options you mentioned would do great. Personally I'd stick with the 15-58 due to ease of replacement and obviously the lower cost, but if you want an Alpha that's certainly not an issue.

hot_rod

The Alpha or any ECM  will be 1/2 or less to operate. Over the life of the pump that might be worth the upgrade

GroundUp

IF it ran 24/7/365, with a power cost of $.10/kWh (obviously this varies by location but that's typical in my parts), the break-even point is still 6-8 years. Being realistic and saying it runs 20% of the year, that's a 30-40 year payback and we all know they'll never last anywhere near that long. In my neck of the woods, for a single zone system, there is zero benefit to an ECM circulator.

hot_rod

Average electricity cost in the US as of Feb Is .23/ kw  Some folks on this  list pay over .30. So it depends on where you live, and hours of pump time
The pump on my shop runs 17 watts, a 15-58 is probably 70 watts or more

There may  be rebates available also for high efficiency circulators. Some areas it was 50 bucks a pump

So many factors to consider.

mokomon

This all being said . Do I even need to convert my sytem to primary/secondary with this boiler , if I am just pumping into a single zone. Can I just use the boiler pump to run my 1000' of 1/2" pex ?

psb75

Well, this is confusing. What is: "100' of 1/2" pex" you are mentioning?

mokomon

that was meant to be 1000' of pex. I corrected that in the earlier post

neilc

is that 1k ft a radiant loop(or it doesn't matter)?
that should be 4 - 250 ft loops,
you'll have a hard time driving water thru 1k ft of 1/2 inch,
and loop performance will be poor,
Radiant Loop ?
you're gonna need a larger high head circ,

neilc

too many threads,
@Erin Holohan Haskell

GroundUp

Depending on the size of the Argo, maybe. If your 2.5-3 GPM through the loops is adequate flow for that particular boiler, there is no need for P/S piping or an additional pump

Erin Holohan Haskell

neilc said:

too many threads,
@Erin Holohan Haskell

I've merged the duplicate discussions into one here to prevent confusion. Thanks!

mokomon

The boiler is an Argo AT 6kw . The manuals says it requires minimum of 2gpm flow. So I should be able to set the boiler at 120 F and run a single 15-58 pump thru the 4 - 250' loops of pex without going P/S piping without harm to the boiler ? Will the boiler deal with the long time it takes to bring the system up to thermostat temperature (60F) each fall without a problem without a P/S piping ? Sorry for all the questions , I never had any experience with boilers before.

hot_rod

A typical 1/2 pex  loop is .50 gpm. So the 4 loops add to 2 gpm
You should be fine direct piping. Use a good air  separator 

Lots if air problems on HH this time if the year 

mokomon

Thank you.

GroundUp

Yes, you're good to go with direct piping and the 15-58. Like Bob said, just take extra care to assure air removal is prioritized.

mokomon

Thanks