Should I convert cast-iron radiators to Runtal wall panels or Slant Fin baseboard?
Climate zone 5A, Massachusetts
Backstory
Our 1200 sq/ft home is a single story ranch with a basement. The existing system is a Kenmore boiler and 7 cast iron radiators. 8 originally but the previous owner installed a sliding door and removed one of the 8 radiators.
We began replacing the existing insulation with Rockwool comfortbatt and will continue through the remainder of the exterior walls. We’re also planning to replace the exterior cedar shingles with rigid insulation and fiber-cement siding.
Present
We’re sanding and refinishing the red-oak hardwood flooring and to make things easy we remove the radiators. With the radiators temporarily removed should we re-install them, or plan to replace with Runtal wall panels or Slant Fin baseboard?
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Replies
The last thing you want to do is get rid of likely oversized moderate mass radiators- they are your ticket to maximizing heating comfort!
Neither Runtal and DEFINITELY Slanfin has the thermal mass of high volume radiators, which generally means less steady heat & lower comfort. Depending on the output of the boiler relative to the EDR (equivalante direct radiation) square feet of the rads, without the combined thermal mass of the water volume and cast iron it might even short cycle the boiler into lower efficiency when operating at a lower more comfortable water temperature.
Estimate the total EDR of the rads using this guide:
https://us.v-cdn.net/5021738/uploads/editor/6r/h97cadrtsyzq.pdf
Find the DOE BTU/hr output rating on the nameplate of the boiler, and calculate the BTU/EDR ratio.
With that ratio the nomograph on page 2 of that guide will tell you the average water temp at which the radiators will emit the full boiler output. You can also use it to figure out the water temp needed to keep up at any given heat load.
While the rads aren't hooked up it might be a good time to clean them up and paint them to match your decor. It looks like they may have been painted with a silvery metallic paint (?), which would be an indication that they are oversized for running with high temperature water. The low-E paints knock 15-20% off the output, and was done to avoid a roast/freeze cycle when running with steam or high temp water, but it's worth it on a comfort basis to find a lower more appropriate temperature and run them there.
If you have a winter's worth of fuel bills, estimate the design heat load of the house based on fuel use, as explained in this bit o' bloggery:
https://www.greenbuildingadvisor.com/blogs/dept/guest-blogs/out-old-new
If you will share the fuel use numbers and ZIP code I can run that napkin-math for you. I'd need the input BTU rate as well as the DOE output BTU rate to adjust it for boiler efficiency.
If you just have to have a more modern look, flat panel radiators can get you there, and with enough panel rad it can be at least as comfortable as cast iron rads, but it's not cheap.
Thanks Dana,
The radiators in this room are 5 columns x 20" tall by 14 sections. 5 x 14 x 170 BTU = 11,900 EDR.
Our Kenmore Dunkirk boiler is a 4EW with a 1.1 nozzle. It appears the boiler is rated for 0.9, 1.25 and 1.5 GPH. I'm guessing the previous technician increased the nozzle size from 0.9 or decreased from 1.25 GPH. I'm not sure what the BTU/hr output is.
Yes I think they were previously painted metallic gray. If we decided to keep them which I think we're considering we will pressure wash/media blast and re-paint. I recently read the paint type should be oil based, could you recommend which aresol paint is best?
We have a 275 gallon oil tank but we're conservative with the temperature. Our thermostat during winter is set to 55 during the day time and 62 in the evening. Our zip code is 01756. I think we fill the tank 3 times a year.
We only wanted the slant-fin/base board system if it was more efficient. At first I thought the systems were more efficient but maybe this isn't true. I then thought maybe the Runtal wall radiators would be a possible replacement. Wall style radiators from Runtal would cost approx. $600-$800 each. I don't think we'd want to sacrifice efficiency for the style and cost of the Runtal radiators.
We don't mind continuing to use the existing system. I'm now wondering if the two existing radiators might be overkill for the bedroom. I think we might remove one of the two radiators so we can place the bed and mattress against the northern wall. And, place the unused radiator back in the living room where a radiator once was.
>"...4EW with a 1.1 nozzle"
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>"...not sure what the BTU/hr output is."
At 138,000 BTU/gallon and 1.1 gph the input BTU is ~152,000 BTU/hr, and at ~85% steady state efficiency the output is ~129,000 BTU/hr which is a LOT for a 1200' house (even if it weren't insulated.)
>" ...columns x 20" tall by 14 sections. 5 x 14 x 170 BTU = 11,900 EDR "
If it's 5 tubes per section, 20" tall, taking a look at the chart on page 5 it's 2.67 EDR' per section, times 14 sections is 37.38 EDR'. At a water temperature of 180F that would be 170 BTU/hr x 37.83 EDR'= 6355 BTU/hr per radiator.
If you had only 8 (now 7) of them it could originally emit in total 8 x 6355 BTU/hr = 50,840 BTU/hr at an AWT of 170F, so with the 1.1 gph nozzle it was ~2.5x oversized for the radiation. Now that one has been yarded out the rads can emit 7 x 6355 BTU/hr = 44,485 BTU/hr, and the boiler is ~2.9x oversized for the radiation.
The thermal mass of the water volume and cast iron in the rads are the only thing keeping the boiler from short cycling. If you replaced it with low-mass heat emitters such as fin tube it's likely to short cycle quite a bit, lowering the net efficiency and adding wear & tear to the boiler.
There's not much I can do with the gallons per year number without exact fill-up dates, which are necessary for looking up the heating degree days between fill-ups. If I had to take a WAG at it, your design heat load at ~8F (approximately your 99% outside design temperature in Mendon) is between 15-20,000 BTU/hr @ 68F indoors (code min interior design temp, even though you're keeping it cooler than that.) So even with just the 7 radiators the radiation is more than 2x oversized for the likely load, closing in on 3x oversized. That means you COULD heat the place comfortably with 130-140F water. Running an oil boiler that cold creates corrosive condensation issues, so you'd have to do it with a thermostatic mixing valve on a system-bypass or boiler-bypass branch in the near-boiler plumbing, keeping the boiler at some warmer temperature. Ideally the return water entering the boiler would never be below 140F, which can be done only by mixing direct boiler output water with the return water from radiation.
Given that the boiler is something like 5x oversized for the design load the duty cycle is miniscule, and even if it's not short-cycling it's running well below it's rated efficiency. Installing a retrofit heat purging economizer such as a Hydrostat 3250 Plus or an Intellicon HW+ would likely cut your oil consumption by more than 15%, possibly more than 20%. With a tuned up burner and the system tweaked for efficiency there is no way you should be burning through 800 gallons/year in a house that size- a lot of that is currently going into standby losses, which will be dramatically reduced with a heat-purging economizer control.
Take a look at Table 3 in this document: https://www.bnl.gov/isd/documents/41399.pdf
Note that ONLY system #3 had heat purge control. Compare it's annual efficiency relative to it's steady state efficiency at 3x oversizing, and compare that to the other systems tested. Then consider that yours is more like 5x oversized, with a nameplate steady state efficiency of 86%. If yours were "only" 3x oversized your as-used AFUE might be as high as 75%, but at 5x oversizing you're WAY over the edge of the efficiency cliff, but can recover most of that by adding heat purge. (Most new oil boilers come with heat purging controls these days.) It's possible to add the controls for well under $200 as a DIY if you have any electrician skills and can follow the schematics for the existing aquasat controls.
>"We don't mind continuing to use the existing system. I'm now wondering if the two existing radiators might be overkill for the bedroom. I think we might remove one of the two radiators so we can place the bed and mattress against the northern wall. And, place the unused radiator back in the living room where a radiator once was."
If the room to room temperature balance is pretty good as-is, taking one of the bedroom rads out is going to lower the temperature in the bedroom. If you added TRVs you could tweak it all back into whatever room temperature balance you like, but it's true that you have WAY more radiation than needed to heat the place with 180F water, even though it's not enough to balance with the boiler output.
I should have submitted this in the last post. I thought the Beckett boiler models and Delevan oil nozzle sizing was the same value but it's not.
Here's a chart of the Beckett boiler models and Delevan oil nozzle sizing. Our boiler is a 4EW.
That chart agrees with others from the manufacturer. It looks like it's legit to install the 0.90 gph nozzle on the 4EW with a Beckett burner provided the tech is also making the necessary air adjustments.
See page 7:
https://dunkirk.com/sites/default/files/3022_0.pdf
A competent tech could also down-fire it further, but probably not enough to matter at your very low load. A heat purge control is more expensive, but really the way to go if you're going to keep using it.
Like most oil boilers it won't be difficult to retrofit a heat purging economizer, but the manual doesn't have any helpful control schematics.
https://dunkirk.com/sites/default/files/2895_0.pdf
Can I assume, since you are thinking about replacing them, that they use hot water and not steam? If you decide to replace them, which I would not, re-piping will have to be done in order to install the proper amount of baseboard for each room. The cost may not cover the benefits of just keeping the cast iron.
Hi Tom,
Yes hot water. Right, if we replaced the system with slant fin baseboard we would remove all existing 1" copper pipe and replace with 3/4". I'm beginning to think we should keep the existing system as is. The Runtal wall units look nicer but similarly sized radiators would cost between $600 and $800 per radiator.
Yup, especially if your home is two stories, or if the system is piped in a mono-flo configuration or in series. You would also possibly have to add zone controls, additional pumps and wiring which would add to the cost. With the radiators, heat can be more easily controlled in each room by slightly closing the radiator valves.
You could keep the existing piping and just reduce to 3/4 above the floor, but you would have to loop back in each section of baseboard to avoid making more holes in your flooring and totally re-piping. It may work out better in some rooms depending on how your remodel is designed or if a radiator starts to leak.
I would keep them, but keep in mind you want to typically structure your hydronic system with similar water temperature requirements. You can mix low mass and high mass if they have similar water temp requirements. Also, you typically want to home run emitters in high efficiency applications, rather than loop them and rely on high temps at the start of the loop.
Low mass can be very comfortable with constant circulation (think TRVs and ECM delta P pumping, and a super tight outdoor reset). This is the euro way, the North American way seems to be to cycle zone pumps on and off based on thermostats. If you are cycling, then a bit of thermal mass is your friend to mitigate the swings between cycles, but with constant circ low mass is actually helpful.
I have a bunch of panel rads with constant circ off a buffer tank, each rad(with TRV) is just hot enough to keep the respective room warm. with no cycles or fluctuates in room temperature.
Here is a nice read:
https://www.phcppros.com/articles/7804-when-old-tech-meets-new-tech
Hi Josh,
Our existing valves are quarter turn valves.
Should we consider retro-fitting with more advanced valves?
By the way, what software did you use to create the system diagram for the new plan?
> Should I convert
Depends on your goals. Aesthetics? Efficiency? Better temperature control?
Hi Jon,
My fiance and I are both interested in reducing our carbon foot prints and avoiding unnecessarily high fuel bills. The only smart device we have in the home is a Nest smart thermostat. I think efficiency is our most important goal.
Justin- just for yuks, run a freebie/cheapie online Manual-J on your house for the "after air sealing and insulation upgrades" version of your house, using loadcalc.net or coolcalc.com.
The odds are pretty good you could heat & cool your house more comfortably and at a lower operating cost than the oil boiler + window AC with a ducted 1.5-2 ton mini-split. That would be a better investment in comfort than spending several grand on panel radiators and a more efficient boiler. Right now there is some state subsidy money for heat pump retrofits, but with the bill currently working it's way through the state house that could become an even more attractive proposition in a year or two. See:
https://energynews.us/2019/07/12/northeast/massachusetts-bills-aim-to-promote-efficient-appliances-net-zero-housing/
https://www.masssave.com/en/saving/residential-rebates/electric-heating-and-cooling/
Since the existing boiler can legitimately be down-fired to 0.9 gph, next time you have a tech tune it up you should ask for the smaller nozzle. Even at 0.9 gph it would still be crazy-oversized for both the radiation and your heat load (even the smallest oil boilers would be), and a retrofit heat purging economizer control would still pay for itself in less than 2 heating seasons, but every little bit helps.