I got my heating system online and it's been running over a week. It's running off of a wall thermostat set at 60 deg and is maintaining the temp between 58 and 63. It's powered by a 10kw tank-less water heater that is putting out 120 degF and returning 70 deg. The building is 940 sq ft with 4 loops of 1/2" pex aprox 200' to 230' each. It's a 4" concrete slap with 2" xps arount the sides and full bottom. The pump is a Grundfos 1/25 hp. I've read I should be getting a 20 deg delta T if the pump/flow is optimal. Is this correct? Is my pump too small? The heater has about a 3/16" restrictor and there is a spring loaded check valve after the pump. Could these be issues? Any help or ideas would be appreciated. Ron C

At first glance the tankless WH seems to be your greatest restriction. What are the flow specifications for that heater?

The head loss of the system has to be calculated and the pump is sized accordingly. I typically aim for a 10F deltaT for more even temps across the floor.

From your figures I am calculating your current system flow at 1.37gpm.

I would further suggest if your system is maintaining temp at that deltaT your actual avg system temp is 95F. When you get the right size pump and flow you can turn your supply temps to around that figure.

So I should be looking at upping my pump instead of eliminating the restrictions? I did find out the heater is rated at 3 gpm for rated temp rise with a max of 124 deg.

what do your flow meters say your loops are getting?

I will have to go look at the actual numbers, but the fluid is about half way in the tubes.

I looked at the flow meters and I'm getting about .3 gpm on all 4 loops.

generally those meters are also balancing valves. can you "open" them? they may be the source of your restriction.

I'd remove the temperature/flow control valve from the heater and consider a bigger pump - that's what I did....

Are you talking about the restrictor in the heater? Or the check valve? Or both?

I may need more pump, but I don't know how to calculate how much pump I need and I don't want to over pump.

I'm going to try opening the flow meters valves today.

Thanks

I removed anything that caused restriction in the heater. But no matter what you do, the heater will have resistance caused by its small tubing.

My heater is electric. There is a restritor in the outlet side. I was told I could get by without it, but I figured would start with everything "as designed". I should be able to get 3ghm through the heater. I figure I'm getting less than half of that now.

Ok, now that I've learned how to read my flow meters, I find I have so little flow, that the meters don't pick it up with all 4 zones open. If I close all but one, I get about . 3 gpm on the open zone.

What information do I need to determine the correct size pump?

This document from Taco is very informative-

http://www.taco-hvac.com/uploads/FileLibrary/SelectingCirculators.pdf

Your biggest obstacle will be figuring the head loss of the Tankless.

Is your current pump a Grundfos a UP15-10 or UP15-18? I am assuming it is bronze by the color. Is that correct?

I'm pretty sure it's a UP 18-42 SF and yes, it's bronze.

I'll check out the pdf. Thanks

I'm sorry, it's a 15-42 SF. I've had it for a while.

I would pipe the heat source with it's own pump, another system pump for the loops and probably get rid of the mixing valve. Use closely spaced tees for Primary/Secondary piping. I used this for my similar type tankless heater system. I'll try to post a pic later.

Why is there a mixing valve if your water heater is set to deliver 120F water?

The heater is supose to put out 125. I didn't want to put more than 120 into the slab. Plus, if this heater didn't workout, I wanted to be prepared for something more powerful.

I looked over the Taco document (thanks G.O. Joe) and figured there is about 18' of head in the loop alone. If I was even close on my calculations. That puts my pump undersized from the start.

I remvoed the restrictions from the heater panel. The spring loaded check valve and the restrictor in the heater. Not much difference when I removed the check valve, but when I remover it and the heater restriction, I got about .2 gpm on the 4 loops. That's more than before. The heater output dropped by 10 deg. The heater restrictor was only a .175"hole, I'll run it like this for a while and see how it does.

Just how critical is that check valve?