Pump Sizing

huntsaver

I have 9 runs of 1/2 pex @ 275’ each. I’ve been playing around with flow trying to get some better return temps and a lower delta T.  Currently at 90F supply running .8gpm on all loops I can get my return temp to 75F.  If I increase temp to say 105F my return stays around the same 75-80F.  I have my manifolds wide open and the max flow I can reach is .8gpm.  I have tried all the way down to .2gpm and did not see anymore efficiency between gas usage and run times to satisfy my thermostat.  At 85-90F supply temps so far this is the least gas usage I can get for the run times needed to keep up with demand.  Boiler says it is pushing 5gpm.  Would love some opinions on the pump size for my setup. System is running at 12psi Currently using: UP15-42F, Circulator Pump, 1/25 HP

Jamie Hall

First, is this a radiant floor setup? Keep in mind that it may take hours (days if it's concrete) for a radiant floor to respond to changes in heat input.

Second, you are -- at the moment -- getting about 52,000 BTUh into and out of your floors. How does that compare with how much heat the boiler is supplying, assuming that the boiler is running all the time?

And none of this has any bearing on efficiency and gas usage. If you run more flow, the temperature differential will be less once the floor stabilises, but the BTUh won't change unless the floor is made to be warmer.

Alan (California Radiant) Forbes

At .8 gpm with 1/2" PEX, you've got 6' of head per loop which is also the total resistance for all the loops together. Depending what else the pump has to flow through, 5 gpm should be no problem for the Grundfos 15-42.

hot_rod

.8 gpm is on the high side of what is commonly used on 1/2” Pex. RadPad shows .5- .75. Do you have a design for the system, it should show required flow.

Look like the perfect size circ. Is there a mixing valve in that manifold circuit? The pressure drop through it would need to come into the head calc. Or is this direct from a boiler?

Gas consumption has more to do with the heat load of the building

huntsaver

I’m not running any mixing valves.  Attached is a pic of the setup.  

hot_rod

What size is the slab? looks like a basement. Well insulated below and around it?

How long has it been running?
. 10-15 is a common design for residential slabs.
The only way to supply more heat to the slab would be more loops, more heat emitter so to say. what tube spacing?

huntsaver

It's a 40 x 60 pole building. 3IN closed cell in the walls and 4IN closed cell in the ceiling. 60mil vapor barrier then 2IN Formular under 4IN slab. 12IN tube spacing. Dug 2FT all the way around the perimeter with 2IN Formular board as well.

hot_rod

Pump is correct, delta T is fine, 52,000 btu/ hr is going into slab, or 22 btu/sq. Ft

Was there a load calc done? 22 btu/ft sounds right for a well insulated slab. Is it not warming up?

huntsaver

It warms up great I’m just trying to dial in some efficiency.  My installer set water at 120 and walked away.  It was running only a few hours per day and burning a lot of LP.  I have been playing around trying to find the best settings the building likes.  I just found it strange that if I wanted to put 100-105 degree water into the boiler the delta was 30-40.  That is when I started playing with flow and reading that guys were running 1-1.5gpm on systems.   My system is topping out at .8 got me wondering if my pump was undersized.  It is happy running 85F water.  Boiler is running 10-12 hours per day now and using 170-200ft3 gas usage. Not sure how that converts to propane gallons but it’s way better than when it was set to 120F water 

hot_rod

The boiler should have an outdoor reset function. So a sensor mounted on a north wall tells the boiler to increase and decrease its temperature. As it get colder outside, boiler supply goes up. That would be the most efficient way to run the system, let the control decide on the best temperature.

But if you try to setback the temperature the outdoor reset can slow recovery. Some boiler controls can compensate for that also, by boosting the boiler temperature if it sees a long heat call not getting satisfied.

If you are up for it, read the control setting section of the manual.

As you assume, them lower you can run the boiler temperature and still get adequate heat, the better the fuel efficiency.

huntsaver

I have an outdoor sensor but I disabled it because it was always wanting to run my temps up 105-110F.  Then I burn a ton of LP with short cycle times of 1-1.5 hour 

ratio

The outdoor sensor is a reset sensor, it's job is to apply an offset from the actual set point. If it's getting up too high, the ramp needs to be adjusted. The manual should have instructions on doing that.

Jamie Hall

huntsaver said:

I have an outdoor sensor but I disabled it because it was always wanting to run my temps up 105-110F.  Then I burn a ton of LP with short cycle times of 1-1.5 hour 

Nope. You will only burn fuel to compensate for the heat loss of the house. Short cycles, long cycles, whatever -- the difference in efficiency is small.

hot_rod

The ODR may, probably will run lower temperatures if set properly, so some fuel savings to be had. Think of it as a cruise control for a boiler, constantly finding the right amount of power input.

huntsaver

I tried low mass and high mass presets.  Could not get the boiler to come down.  It would want to run high temps very quickly and the boiler would not ramp down below 60-70% the custom curve has a min supply temp of 104F so again the boiler would never modulate down very low. If I manually set temp to say 87F like I have it now it runs sometimes below 20% and idles.  My guess is because my return temps are so low with higher heat?  I’m willing to try anything to tinker with it.