Sizing circulation pump for DHW

mostlybirch

Hello,

I am building a radiant-in-slab heating system for our 700 sq ft garage in interior Alaska. I am using a "semi on-demand" oil-fired boiler, a Toyotomi OM-128HH running the DHW for attached house and a glycol heat system with a heat exchanger on the hydronic circuit for the garage.

Based on the manual J calcs and LoopCAD, I require ~1.5 gpm in the hydronic circuit at 120F. This allows me to size the circulation pump in the hydronic circuit, but my question is: how do I size the circulation pump on the DHW side which will circulate fluid through the heat exchanger?

Thank you

Attached a sketch (valves, drain, etc. omitted for clarity) and the manufacturers plumbing example.

hot_rod

I don't see a pressure drop spec for that heater. It sort of looks like a mod con HX. Maybe the dealer could give you some spec. The circulator needs to be sized for the flow resistance, or pressure drop.

How was the plate heat exchanger sized? Usually you want some flow rates to get that sized correctly also.

I don't like their piping schematic. Usually the pump is pumping away from the expansion tank and into the boiler on the return.

The air separator should be on the top supply out of the boiler, at the hottest point in the piping. then the tank, then the circulator.

Screenshot 2024-11-15 at 8.11.48 PM.png

Screenshot 2024-11-15 at 8.08.55 PM.png

mostlybirch

Not a mod con, fixed rate 0.85 gal/hr burner.

I haven't seen pressure drop specs in the literature, but I do see flow rate:

6.2 gal./min. (40°F rise)
4.1 gal./min. (60°F rise)
3.1 gal./min. (80°F rise)

https://www.ruralenergy.com/om-128hh-toyotomi-128-hydronic-heater

Bad description on my part, the heat exchanger has not been sourced/sized yet.

hot_rod

As you can see that heater is designed to heat small flow rates across a wide delta t. The exact opposite of what you want for hydronic. So you need to size the pump to get the unit to fire to full capacity.

Either get the info on the pressure drop or trial and error it

mostlybirch

Thank you for your help. Could you explain what you mean by getting the unit to fire to full capacity? I would assume that would be a high flow rate to extract as much heat as the boiler can make while it is running.

Does it also make sense to run the glycol loop on the heat exchanger side as I have it, versus on the boiler side as the manufacturer shows in the diagram?

hot_rod

How many loops of tubing do you have in that 700 sq ft shop? Is it 1/2"?

The amount of tubing is going to dictate how much heat you can put into the slab , and what size boiler, pump, piping etc would be needed.

Let me check in with some Alaska contractors to see what they know of that boiler/heater. Or any other oil guys on the list?

Some shops use glycol in the tubing to prevent freeze up in a prolonged outage of power. How at risk is your shop to boiler, oil, or power outages? Is it left unoccupied for extended periods in the winter? Do you have a generator that could keep it going? Those are what I base glycol use on.

A thermostat with wifi capability could notify you of a no heat condition.

Glycol can be a a messy, expensive fluid to use if not necessary.

mostlybirch

3 loops of ~250' 1/2 ox barrier pex in the garage, 1 loop of 170' 1/2" in the connected utility room on separate zone. 9" spacing with 6" along walls. 2" R-10 below, and ICF block perimeter.

We currently heat the shop with Toyo oil heater, which will be the backup heat source if boiler goes down. It is ran on a smart switch which will turn the heater on if the slab temperature goes below a set point. Also have a generator with small battery backup. However we do leave for 1-3 weeks at a time in the winter which is why I was going for a glycol mixture to dummy-proof the system. I am open to the idea of running straight water if the benefits outweigh the risks with precautions.

edit: attached a photo of similar system using the same boiler, however the cold water supply looks odd to me

hot_rod

In this picture the boiler/ heater is heating the domestic water also. Cold water enters on the blue arrow, jot water out on the red. With a temperature limiting mixing valve.

Is that what you want to do? If so the boiler pump needs to be stainless. The one in this pic shows a stainless pump, the silver label on the pump is the indicator.

If you are not using the boiler for domestic water, I don't see a need for the HX? I would do a primary loop to assure the boiler gets proper flow, then tie the radiant into that loop. With those loops, you need around 2 gpm total.

If you are not doing the domestic water a basic cast pump will work. I suspect a Grundfos 15-58, or the Grundfos Alpha is a high efficiency model.

HomerJSmith

Remember, Pressure Drop is a calculation of the loss in all the components that the pump is pushing water thru @ the recommended flow in GPS. Supply Water Temperature has a lot to do with pressure loss because colder water is more dense than hotter water but, it probably won't make much difference with boiler water.

You might choose a multi-speed pump or a variable pressure pump.

mostlybirch

hot_rod, yes this boiler will be running DHW on the boiler side. I have roughly sized the circulation pump on the hydronic/heat exchanger side (~2 GPM). My question is: will the circ pump on the boiler/DHW side, which circulates fluid through the boiler side of the HX, be sized similar to the pump on the hydronic side?

DHW runs are short with a 10' elevation head.

hot_rod

I suspect this is the pump shown in that example. I’d check with the dealer, rep, or manufacturer. Some of those heaters require a high head circulator, like a 26-99 Grundfos. Those get $$ in stainless.

IMG_0831.jpeg

mostlybirch

I'll check with the dealer, but I suspect not because the DHW variant (OM-122) of the same boiler shows no pump in its plumbing example (below).

Maybe I will try a variable speed ECM pump like the Alpha 15-55 on the DHW/boiler side.

mostlybirch

Another way to phrase the question: should I match the pump on the boiler side and hydronic side? Final product would be similar to diagram below.

hot_rod

https://forum.heatinghelp.com/discussion/comment/1820717#Comment_1820717

The pump on the boiler is only there to circulate the "A" side of the HX when you are doing both DHW and heat. The Plumbing Example above does not have heat, a heat exchanger, or a pump.

The pump for your heating side can be a cast iron type.Yes a 3 speed 15- 58, or a Grundfos Alpha high efficiency model.

Get an answer on the pump for the heater side before you but that one. Too bad they don't give you the options for that pump?