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Old 01-12-2017, 11:28 AM   #1
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Calculate needed circulator HP


I've installed a heating system in my small 2 story house using a Norwitz tankless water heater. Radian heat on the 1st floor works great. The problem is on the second floor, where I have total about 4 sq ft of low temp style radiators. The water circulates but the Norwitz burner does not fire up. I was advised that the 1/25 hp pump was insufficient and to install an auxiliary 1/25 hp pump in the main hot water line to boost the flow to the required 1/2 gal/min flow to turn on the burner. Still doesn't work.

Would a bigger pump do the trick? if so, 1/35, 1/6, 1/2 hp? Can the pump be too big? Do I really need two pumps? Or one bigger one? Two bigger ones?

Thanks,
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Old 01-12-2017, 09:39 PM   #2
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Re: Calculate needed circulator HP


To clarify, you are using a water heater, designed to supply on-demand heated domestic water, as a heating system?
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Old 01-12-2017, 10:17 PM   #3
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Re: Calculate needed circulator HP


Each rad will have a "head loss" as well as the piping/ft. You look it up, and add all of it up. This is your resistance or friction that the pump must overcome. You pick the pump to suite, the motor is matched to the pump.

If you're unsure, and rather just buy then calculate, then try a grundfos 3 speed.

http://www.supplyhouse.com/Grundfos-...volt-4701000-p

That's good for 19.5ft head. I'd be surprised if you need more in a closed loop of your description. Yes, there is something called too much. It's good for 17gpm, but I'd expect more like 2 or 3gpm

Cheers!

Last edited by supers05; 01-12-2017 at 10:22 PM.
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Old 01-12-2017, 10:42 PM   #4
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Re: Calculate needed circulator HP


Of course this is overkill.... 1/2hp like you requested only as an example. Rated for 20 zones. It'll pump between 10-50gpm.... It'll kill your heater in no time. It also costs about half as much as the heater.

I DO NOT recommend this. It is for information only.

http://www.supplyhouse.com/Grundfos-...mp-115V-1-2-HP

Cheers!
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Old 01-13-2017, 06:46 AM   #5
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Re: Calculate needed circulator HP


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Originally Posted by lenaitch View Post
To clarify, you are using a water heater, designed to supply on-demand heated domestic water, as a heating system?
Yes, in a small, well insulated house a water heater can do it. The radiant system and the radiators are designed to work with 130 degree water, if you can pump it.
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Old 01-13-2017, 06:52 AM   #6
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Re: Calculate needed circulator HP


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Originally Posted by supers05 View Post
Each rad will have a "head loss" as well as the piping/ft. You look it up, and add all of it up. This is your resistance or friction that the pump must overcome. You pick the pump to suite, the motor is matched to the pump.

If you're unsure, and rather just buy then calculate, then try a grundfos 3 speed.

http://www.supplyhouse.com/Grundfos-...volt-4701000-p

That's good for 19.5ft head. I'd be surprised if you need more in a closed loop of your description. Yes, there is something called too much. It's good for 17gpm, but I'd expect more like 2 or 3gpm

Cheers!
That sounds like a plan...thanks so much!!!
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Old 01-13-2017, 08:10 PM   #7
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Re: Calculate needed circulator HP


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Originally Posted by leerube View Post
Yes, in a small, well insulated house a water heater can do it. The radiant system and the radiators are designed to work with 130 degree water, if you can pump it.
Can the heater handle the temperature of the return water? I honestly don't know what the typical return temperature would be, but I would think water heaters are designed to work with normal cold water as input.
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Old 01-13-2017, 08:31 PM   #8
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Re: Calculate needed circulator HP


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Can the heater handle the temperature of the return water? I honestly don't know what the typical return temperature would be, but I would think water heaters are designed to work with normal cold water as input.
so far, so good.
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Old 01-13-2017, 10:31 PM   #9
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Re: Calculate needed circulator HP


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Can the heater handle the temperature of the return water? I honestly don't know what the typical return temperature would be, but I would think water heaters are designed to work with normal cold water as input.
Many models are designed for dual application. You must read the manual though, as the venting material often must be changed.

Takagi and Navien are 2 brands that have units rated for this.

Cheers!
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Old 01-14-2017, 07:02 AM   #10
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Re: Calculate needed circulator HP


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Originally Posted by supers05 View Post
Many models are designed for dual application. You must read the manual though, as the venting material often must be changed.

Takagi and Navien are 2 brands that have units rated for this.

Cheers!
And Norwitz, like mine.
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