Circulator pump sizing on converted hydronic gravity system

kfCT1

I have seen some great information on circulator pump sizing and application but figured I would confirm my assumptions for my application here.

I recently purchased a home with a converted gravity hydronic system. 250k btu cast iron boiler, around 950 SF of cast iron column radiators and long 3" uninsulated pipe runs. Most of the system is on a single zone, there was 1 additional zone added during a previous kitchen renovation. The system operates ok but runs very long cycles at low temperature. It takes approximately an hour for the main zone to heat from cold start to around 140 deg before the thermostat is satisfied.

I am considering system improvements while I consider long term options and it seems someone in the past few years replaced a red baron circulator pump with a taco 007 f5. It seems that the taco 0010 would be better suited to the large main zone according to the information I've seen on this forum - would this effectively reduce my long boiler cycle times?

There is no low temperature return water protection, the pumps are located on the return side and I suspect the SX30V expansion tank may be undersized too.

hot_rod

An hour for the supply to get to 140f, or the return temperature?
A larger pump would make  that condition worse if the boiler us not keeping or catching up

kfCT1

140f supply

jesmed1

Your system is probably working fine as is. You have so much thermal mass in the water and cast iron radiators that it takes the boiler a long time to heat all that thermal mass from a cold start. That's why the cycle time is long; it has nothing to do with the pump.

I live in a 4800 sq ft 4-unit condo building that has almost the same total boiler BTU's and total radiator square footage as you do, but our building is split in half and has two boilers, one for each half of the building. Each boiler is running at about 140 MBTU/hr, and each half of the building has about 480 square footage of cast iron radiator. I have our thermostats set to swings of 0.5-0.75 degrees, and the boilers run 45-minute cycles every 3-6 hours, depending on outdoor temps. And just like you, our supply temperature is about 140 degrees at the end of the cycle.

So when both boilers are running simultaneously, we have 280 MBTU/hr output into 960 sq ft of cast iron radiators, running 45-minute cycles from a cold start, with a final water temp of around 140 F. These are all very similar to your system.

And as I said before, your long cycle time is because it takes the boiler a long time to heat up all that thermal mass. Last month I did a thermal mass analysis of our system and posted the results here for discussion. You might find that post interesting, as you can basically double the BTU's and thermal mass of one of my systems to get appoximate results for your system. You'll see how much thermal mass you have in your water and cast iron, and your long cycle time will make more sense to you.

https://forum.heatinghelp.com/discussion/193741/understanding-heat-flows-in-high-mass-heating-systems-in-old-houses#latest

kfCT1

@jesmed1 Interesting info and thanks for forwarding that thread. The system does have a lot of mass and the whole uninsulated basement is heated by the exposed mains. Exterior walls upstairs are also largely uninsulated and attic is under insulated. Planning to address that also. 

Any issues on your end with condensing? I recently cleaned out the CI heat exchanger and there are certainly signs of rust (approx 40 years old) but I suspect the small kitchen zone that fires more regularly at higher temps (CI base and toe kicks) dries out any condensation enough that it hasn’t been damaged too badly.  

Steamhead

@kfCT1 , when the system is heating up, what is the ΔT between supply and return?

kfCT1

@Steamhead averages about 20 degrees +/-

jesmed1

kfCT1 said:

@jesmed1 Interesting info and thanks for forwarding that thread. The system does have a lot of mass and the whole uninsulated basement is heated by the exposed mains. Exterior walls upstairs are also largely uninsulated and attic is under insulated. Planning to address that also. 

Any issues on your end with condensing? I recently cleaned out the CI heat exchanger and there are certainly signs of rust (approx 40 years old) but I suspect the small kitchen zone that fires more regularly at higher temps (CI base and toe kicks) dries out any condensation enough that it hasn’t been damaged too badly.  

We have the same situation with an uninsulated basement that is heated by exposed mains. Also, our attic was under-insulated with only a few inches of old vermiculite. We blew in 10" of cellulose on top, but were surprised to find the savings were much less than expected. Based on simple R-values, I expected to reduce our oil consumption by about 20%, but our actual reduction was less than 10%.

Apparently airflow through attic insulation can reduce its effectivess, so just beware that the savings for more attic insulation based on simple R-value calculations may be less in reality than in theory. You might want to do a blower door test and seal up all your air leaks first, as that may be your best return on investment. Our blown in cellulose in the attic is looking like a 10-to-15-year payback period, much longer than I expected.

We have 25-year-old cast iron Weil-McLain WGO-5's, and have not had any problems with condensation despite the low water temperatures. My guess is that the boilers are running long enough cycles that all the internal surfaces get hot enough to burn off any condensates that might be happening at the start of the cycle. After 25 years, the pins are still in good condition.

Steamhead

kfCT1 said:

@Steamhead averages about 20 degrees +/-

That's about right. Does the system heat up evenly?

kfCT1

@Steamhead for the most part. I guess what I’m hearing (reassuringly) is that it seems to work fine. 

EBEBRATT-Ed

My take:

Insulation and windows and tighten the envelope.

Pipe insulation in the basement?

Pipe Insulation is pretty expensive not sure if the payback would be there in $$$ or in system operation. What about replacing your mains with a Pex system? The biggest advantage to this is the huge reduction in water quantity and faster system response.

You're fighting a huge "Flywheel". Back when this job was coal fired You just shoveled the coal in based on outside and inside air temp. You didn't have wide swings in water temp the system ran with either a big or small fire.

Based on 950 SF of radiation and a normal pic up factor if you had a pex system you should only need a boiler output of 163,000 BTU. You would think the system should respond faster with the size boiler you have but........

I am assuming that your boiler rated at 250,000 is the boiler input? That being said assuming 80% efficiency and with a larger than normal piping and pick up load deducted from that makes the boiler undersized based on the radiation installed even though it is probably more than enough to heat the house.


To me the most bang for the buck is to repipe in Pex and get rid of the slow response and the huge pick-up factor.

If you can live with it and do some energy upgrades you could then go with a smaller boiler

This would be a good job for a condensing boiler (or 2) (can't believe I am saying that)

kfCT1

@EBEBRATT-Ed Thanks for your thoughts. Yes, I did mean 250k btu input on natural gas. I do like the idea of replacing the large mains with pex once I decide to replace the system. I think that the pex with a mod/con would be a good setup for the home. I've done some air sealing but there isn't much else I can practically do without a large undertaking. The home is mostly solid masonry with a 1" air gap and plaster on the interior. At this point I have no plans to gut the house to add insulation. The storm windows over the original double hung wood windows work sufficiently. Living space is about 3500sf but the 400sf garage is heated via large cast iron wall hung radiator and the 1700sf basement is heated by the exposed mains so I'm essentially heating approx 5600sf

To toss in another wrench, I put in a Bosch ids 2.0 ducted heat pump this past summer. One of the previous owners added ductwork for cooling only and the 10 seer split system kicked the can. I got the heat pump to take advantage of the CT rebate programs knowing that the ductwork is not configured to effectively heat the home. Right now it is a mess of flex duct snaked around an unconditioned attic and drops down to each room on the 1st and 2nd floors with a single 24x30 return in the 2nd floor hallway. The design is bizarre and impossible to balance. With that said, the 55k btu heat pump does seem to keep the house comfortable (except for basement and garage, of course) down to about 40 degrees albeit with some noise and air velocity problems due to the poor duct configuration.

So, that leaves me with a couple of ideas for a future system. 1) repipe mains with pex (as @EBEBRATT-Ed suggested) and go to mod/con. Would need to add basement zone with baseboard or fan coils due to loss of the exposed main radiation that currently heats the space (half finished, half unfinished, 1700 sf total). 2) Stick with cast iron boiler (sized appropriately) and improve piping configuration and duct configuration for shoulder season heating via heat pump. I'm hesitant to go mod/con for reliability purposes. Although most products are made so poorly these days that I suppose its a crap chute either way.

Thanks all for the great information and ideas

leonz

kfCT1 said:

I have seen some great information on circulator pump sizing and application but figured I would confirm my assumptions for my application here.

I recently purchased a home with a converted gravity hydronic system. 250k btu cast iron boiler, around 950 SF of cast iron column radiators and long 3" uninsulated pipe runs. Most of the system is on a single zone, there was 1 additional zone added during a previous kitchen renovation. The system operates ok but runs very long cycles at low temperature. It takes approximately an hour for the main zone to heat from cold start to around 140 deg before the thermostat is satisfied.

I am considering system improvements while I consider long term options and it seems someone in the past few years replaced a red baron circulator pump with a taco 007 f5. It seems that the taco 0010 would be better suited to the large main zone according to the information I've seen on this forum - would this effectively reduce my long boiler cycle times?

There is no low temperature return water protection, the pumps are located on the return side and I suspect the SX30V expansion tank may be undersized too.

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How difficult would it be for you to have the system returned to a gravity hot water heating system and also upgrade your weatherproofing and insulation at the same time???

Adding pipe insulation where possible would be one of the first things I would do long before I would rip out those 3 inch pipes as you have all that beautiful thermal mass in the cast iron radiators and 3 inch pipe to take advantage of.

A gravity hot water system depends on the entire thermal mass in it to keep the water warm and flowing up and gradually cooling and sinking back to the boiler sump.

If you have a heat study done upgrading your weatherproofing and adding insulation will help you sooner rather than investing in a massive rip out of all the plumbing as it is an asset you already have.

Sadly, many of these old systems were not maintained and new owners make massive renovations and make the massive mistake or ripping them apart and installing forced circulation and making things worse.

If you purchase a copy of CLASSIC HYDRONICS from the Heating Help Bookstore it will help you understand these systems much more and also understand why slow even heating is better as the thermal mass is a huge asset.

Mr. Holohan makes his writing fun and easy to understand for the lay person and the journeyman plumber alike and he also adds many stories from his many decades of in the plumbing business and he talks a great deal about how and why the gravity hot water system works and works very well.
The DEAD MEN knew how to do heating right the first time.

If someone does not do a heat study with an infrared camera and a blower door with or without a smoke test you need to find another firm to do this as adding insulation and weather proofing would be the first thing that should be done and then you can have the system returned to a gravity hot water system and have a warmer home.

The other thing is that there is money available in low interest loans and through weatherproofing grants in the Heating Energy Assistance Program every year in most states if you qualify for the Heating Energy Assistance Program.

jesmed1

@kfCT1 I'd go with option #2, stick with cast iron boiler. They're dirt simple, reliable, and you already have your indirect heating system for the basement built in. Keep track of your natural gas usage this winter and use it to do a proper heat loss calculation for the house. Then when the boiler kicks the can, install a smaller one sized for your heat load. Your current boiler is probably oversized as Ed said. Here's how to do the heat load calculation using your actual gas consumption:

https://www.greenbuildingadvisor.com/article/replacing-a-furnace-or-boiler

Meanwhile you can focus on improving the ducting for your heat pump.

Dave H_2

Another option is a controlled bypass piping connecting the boiler supply to the return sending hi temp boiler water directly to the return. Not just a by-pass line with a valve but use a thermic bypass. It has an element that opens and closes based upon the desired temp settings from the supply to the return.
Like this one

Dave H.

kfCT1

Thanks, Dave.

@jesmed1 Good thinking. I tried the method prescribed in the GBA link and I get very low actual heat load (40k BTU, 395 CCF consumed in January 2023, 250k BTU input at 78% efficiency). I think my solar heat gain throws this off significantly - the large gambrel framed slate roof and masonry absorb a lot of solar energy and the lack of insulation means this partially heats the home. Couple that with the high mass of of the piping and cast iron radiators and I don't think I could practically use these numbers to size a new boiler, as @EBEBRATT-Ed previously suggested.

As much as I don't like the idea of the mod/con, the modulation would help with the sizing variable mystery. I don't see many high mass options these days but the Viessman Vitocrossal at 19-199MBH input may work well.

hot_rod

5600 sq ft of space you are heating and the load is 40,000 BTU/hr? Something is not right with those numbers.

If the boiler only reaches 140F when it satisfies the load, you have much more heat emitter than you do boiler output. The output from the radiators at 140 SWT is probably the reason why it takes so long to heat up.

jesmed1

I agree with @hot_rod that the 40,000 BTU/hr heat load you calculated seems low. That works out to about 11 BTU/hr/sq ft of living area (not counting garage/basement), which is probably lower than 99.9% of houses in the Northeast.

I'm in a 4800 sq ft well-built circa 1930 partial masonry and partial stick-framed 4-unit building with 10" cellulose in the attic and high-performance windows, and our heat load is around 18 BTU/hr/sq ft in the Boston suburbs, which is quite good for an old house in the Northeast.

Another point of comparison is @MaxMercy 's house, also in CT, with 2x6 walls, R-19 insulation in the walls, and double pane windows. His heat load works out to around 15 BTU/hr/sq ft.

https://forum.heatinghelp.com/discussion/194815/5-gallons-of-oil-a-day-normal#latest