Gravity Conversion

Jim_168

A few years ago, I upgraded a 1950's gas upgrade (165,000 Btu) to a coal fired gravity hydronic system. I have an Hydrotherm ID-125 (125,000 Btu) and 125 gallons in a single pipe radiator system on 2.5 floors. The system has enough heat. I did a heat calc and came up with 102,000 Btu's required. I just want to get the flow rates correct.

The primary loop has a B&G Series 100 and the secondary has a B&G NRF-95 (this can be replaced with a B&G NRF-22 or similar Taco pump if needed). The boiler loop also has a 6 gal tank in addition to the boiler capacity.

I have balancers on each loop. The boiler has a Thermoflo Balancer where I can read the loop flow. I am using a B&G NRF-95 pump on 3/4" pipe for injection between the primary and secondary. I had figured the needed flow to be 7-8 gal. I have attempted to use Taco HHS software to determine the best flow for the three different pumps, but as a novice I have reached the limit of my understanding.

What should the delta T for the boiler loop and primary loop be? And what are the best flow rates for the primary, secondary (boiler) and injection pumps?

Thanks for your assistance.

Brad White_201

Flow and Delta-T

There is no real magic to using one delta-T over another, Jim. With multiple circuits all have to work together of course.

What I do not know are your emitters- presumably cast iron, what their ratio of output to the heat loss of the rooms they serve.

If the old system had indeed 165 MBH worth of radiator (at 170 F average water temperature or AWT) and the heat loss is indeed 102 MBH, the radiators may well serve you at 130 to 140 F AWT.

This system could well use the conventional 20 degree drop (makes the math easy) but can also use 30 or even 40 degrees, so long as your average water temperature meets your heat loss on the coldest day. Make sense? Say you do use 20 degrees, fine, a 10.2 GPM flow rate, but at maybe 140 degree supply and 120 degree return.


I am not familiar with a Hydrotherm ID-125 but if a conventional non-condensing boiler it will have to have return water temperatures kept above 135 degrees.

If that is the case, that will set your boiler GPM. If your loss is indeed 102 MBH and the boiler input is 125 MBH, that seems a good selection if 82% efficiency. Your gross output, say it is 102 MBH, a conventional 20 degree delta-T, even a 30 or 40 degree can work well.

Thus your return water is held to no less than 135 F., your supply water temperature would leave the boiler at 155, 165 or 175 F. with flow rates of 10.2, 6.8 or 5.1 GPM respectively. You can also run the boiler at 175F supply and 155F return with the 10.2 call it 10 GPM. Very flexible.

The boiler is higher to avoid low-end condensation but also you will need less of that water to make your radiation loop the correct temperature. Less flow in the bypass too of course. Keeping your boiler hotter than the radiation loop keeps that in control.

So, you can see that your two loops might have the same flow, 10.2 GPM, call it 10 even for easier math.

Your radiation loop will accept say 4 GPM at 175 F boiler supply mixed with 6 GPM at 120F radiation return to make 141.0 degree supply water.

(6x120) + (4x175) = 1410 / 10 GPM = 141.0 degrees to the radiators, close enough.

Thus your 3/4" injection line would work, with that flow rate.

On the boiler side, your 4 GPM of 120F radiation return water mixes with the 6 GPM recirculating 175F boiler water (10 GPM total on the boiler side, remember). That mixes back to the 155F return water you would expect for that boiler.

(4x120) + (6x175) = 153F close enough.

Hope this helps. Any number of flows and combinations can make any desired temperature within and between these numbers.

Jim_168

Granvity Conversion Flow Rate

Brad,

Thanks for all the input. What you laid out makes sense with my limited background. I forgot to mention the system control is a Tekmar 356. Emitters are cast iron radiator. House was built in 1930.

I understand how adjusting the boiler delta T will affect required boiler flow rate. For cost efficiency, is it better to run the boiler at a lower temp and flow rate or a higher one?

I realize the Tekmar control will select the best injection settings no matter what you throw at it. Even though the system may be poorly tuned and not balanced, the house is warm. However, I know I am using too much fuel.

Just need to know if the boiler flow rate would be better at 10.2, 6.8 or 5.1 for economy on natural gas consumption? I understand about staying away from flue condensation - direct experience. The boiler is a class I, 81% efficiency, venting through a lined chimney. Venting to chimney is class III stainless steel Z-vent. With current boiler side pump (B&G NRF-9F/LW), the flow rate is 6.4 gpm registered by B&G Thermoflow balancer. I can switch this pump out for a higher flow rate as well as the one used on for injection if I need higher flows.

I have attached a photo of the layout. [Venting in attached photo was only temporary. Old photo.] The challenge was using the existing pipe layout wrapped with asbestos. Didn't want to disturb any of the insulation.

Thanks for all your assistance. I realize my post sounds like a complete amateur. I am a tradesman, just not the right one for this application.

Jim