Is There Such Thing as a SIMPLE Hydronic System?!
GBA Experts! I need help!
My wife and I recently moved to a 1000sf house in Taos NM (Zone 5B) . The old mechanical system was a gas Slantfin boiler with baseboard and a gas tank water heater. The house was a foreclosure and clearly not winterized so the old baseboard was cracked in multiple locations. I am in the process of updating the systems to make it more comfortable and energy efficient.
I had a plumber who appears to be fairly well experienced replace the old domestic water lines (a random mixture of pex, galvanized and copper) and plan to have him install a new DHW and heating system. I initially told him I wanted to do a combi boiler with panel rads. I asked him to give me room by room load calcs so that I could size the rads and he clearly used rule of thumb (surprise! surprise!) sizing. I wanted to check his numbers so I input the house specs in CoolCalc software and came up with significantly lower room loads (again no surprise there). I came up with a total heating load of approximately 22,000btuh.
Another red flag from my plumber was when I asked him what supply water temp I should use in sizing the rads and he said 180 degrees F. He also suggested using a single loop for the whole system of 7 rads.
I am confident that if I give him the equipment I want to use that he can install it (more or less) properly. I just don’t have faith that he can design such a system.
After a STEEP learning curve sizing the rads (made more difficult by the fact that I was sizing them in a single loop which meant that the incoming water temperature to each rad was lower with each rad) I decided I should try to design the whole system.
I read many posts here and on other forums and many articles by Dana Dorsett, John Sigenthaler et al. The more I read the more I realized what I didn’t know.
With winter fast approaching I am trying to get answers to the following questions to help me complete the design and get the system installed.
1) My research seemed to point me in the direction of installing a mod con boiler with an indirect hot water tank instead of a combi. Would folks on this forum generally agree that that is the better system durability wise? What about from a cost perspective?
2) Is there a relatively easy way to determine whether or not I would need a buffer tank given the volume of water in the rads and mod con boiler?
3) If I don’t use a buffer tank should I use a hydraulic separator? If it’s a whole new system do I need a dirt and magnetic separator or just a hydraulic separator if either?
4) For my relatively small house and load would a single loop circuit or a home run system make more sense (7 rads total)? While it was difficult to design for the decreasing supply temp going into each rad, if I did it correctly I was able to size the rads to meet the loads with 120-130 degree F initial supply temp and a roughly 100 degree F final return temp which I thought was a good thing from a condensing boiler efficiency perspective. It also seems like the sizing would be good for a future conversion to a air to water heat pump with maybe a couple of rads added to the largest spaces.
5) We are getting ready to have cellulose blown into the attic so I was going to pre-plumb the supply and exhaust venting prior to making a final decision on exactly which boiler I will be installing. Seems like 3″ vents would be a safe bet. Would you all agree?
6) Second guessing my rad sizing how do I determine if the delta T for my system is 20, 30 or 40 degrees F?
7) My control system plan was to use an outdoor reset with thermostatic radiator valves on each rad. A Coffee with Caleffi video I saw seemed to suggest that these should be limited to no more than 3 rads per zone. Again I was hoping to do this all as one zone. Any idea why they might have said this?
8) Would you all agree that a variable speed pump based on Delta T vs pressure is the way to go?
9) Any recommendations on boiler manufacturer for what would be a relatively small design load (22kBtu). We are at almost 7,000 feet altitude so we would need to derate the unit for altitude. Is 28% a reasonable adjustment for derating at this altitude and am I just upsizing the boiler size by 28% to get the ultimate boiler size I would need?
I told you I had a lot of questions! I have attached the floor plan and Manual J load calcs for anyone to review and comment on. Here is a link my spreadsheet for rad sizing:
https://docs.google.com/spreadsheets/d/1iHSwWyvOKqpzguf_-moOhcsJVWmHfuRwWzySQ13bh8U/edit?usp=sharing
Thanks in advance for any and all comments, suggestions and advice!
Matt
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Replies
heatinghelp.com
lots of very smart hydronic guys there
In my unprofessional opinion, oversizing radiators is not a problem, as long as they are balanced, especially with outdoor reset.
Second this. This forum was started by Dan Holohan, whose books on hydronics and steam are absolutely amazing.
If you truly want to learn about this topic, buy some of Dan's books. Not only are they incredibly informative, but they're written in a way that is engaging and makes you want to learn more.
Hydronics can be as simple as you make it. I come from New England where millions of houses have radiators, lots of people just have a single zone for the whole house.
I think for 1000 SF, 7 radiators, single story I would do just one zone. I don't see you getting big variations in temperature in a house that small. What you do want is to be able to put a balancing valve on each radiator, so you can make minor adjustments. In order for that to work the radiators have to be plumbed in parallel -- you have two big pipes, a send and a return, with a little pipe going to each radiator from the big pipes. With only 22,000 BTU, at a 20F delta that's 2.2 GPM, your "big" pipe can be 3/4" and the little ones are 1/2". I would recommend oxygen barrier PEX unless they're going to be visible. There's no need for a fancy circulator.
The most common control mechanism is the boiler is on all the time, when the water in it goes below the set point the aquastat fires the burner until it reaches the set point. The thermostat is connected to the circulator pump, if the house is calling for heat the circulator runs and pulls heat out of the house.
A system like this doesn't have to be highly engineered. Traditionally both the boiler and the radiators were oversized, there's no penalty. Then adjust the balancing valves by turning down the warmest rooms until the whole house is roughly the same temperature most of the time.
1) My research seemed to point me in the direction of installing a mod con boiler with an indirect hot water tank instead of a combi. Would folks on this forum generally agree that that is the better system durability wise? What about from a cost perspective?
Indirect will be more expensive and would be overkill in my opinion. A separate gas tank is also a fine option.
2) Is there a relatively easy way to determine whether or not I would need a buffer tank given the volume of water in the rads and mod con boiler?
Yes. You need to know the minimum boiler output, the water volume, an anticipated water delta T, and the desired minimum run time. minimum run time x minimum output / (60 x delta T x 8.34) will give you the gallons needed.
4.) a third option would be a two pipe system that supplies each radiator with the same temp.
6.) there are several delta T’s. For the radiators, that delta T is radiator average temp minus air temp. So a 70F house and a radiator with an average temp of 100 would be 30 delta T. The supply - return water temp delta T is different - if you want to eventually use something that can’t heat as high (like a heat pump or wood boiler with massive storage), a lower delta T there is probably necessary.
7) My control system plan was to use an outdoor reset with thermostatic radiator valves on each rad. A Coffee with Caleffi video I saw seemed to suggest that these should be limited to no more than 3 rads per zone. Again I was hoping to do this all as one zone. Any idea why they might have said this?
Can you link the video? That doesn’t seem right.
9) Any recommendations on boiler manufacturer for what would be a relatively small design load (22kBtu). We are at almost 7,000 feet altitude so we would need to derate the unit for altitude. Is 28% a reasonable adjustment for derating at this altitude and am I just upsizing the boiler size by 28% to get the ultimate boiler size I would need?
If you’re going with a combination boiler or a modcon boiler, sizing isn’t really relevant - they’re all about the same size and can modulate down to about the same minimum. You’ll find sizes of ~80,000 and 100,000 for modcons which have minimum outputs 1/10th that. The combis have to be bigger on the max end, but the minimums are not much higher. If you went low efficiency boiler, then sizing smaller is better and you have more options.
In general, this is probably not a job for a plumber. You want someone experienced and specialized if you can find them.
Thanks everyone for your helpful recommendations and insightful replies! Patrick and gusfhb I will definitey take a closer look at the Heating Help forum and Dan's books.
DCcontrarian the "little" pipes will definitely be partially visible. What would you recommend that I use for those? I've used black pipe in a previous install I did but then I'm introducing more easily rustable ferrous material into the equation and they don't look great. Any other suggestions?
Paul, with regards to this comment:
Yes. You need to know the minimum boiler output, the water volume, an anticipated water delta T, and the desired minimum run time. minimum run time x minimum output / (60 x delta T x 8.34) will give you the gallons needed.
Thanks it is very useful to have a formula! Are you referring to the rad delta T or the system supply/return delta T?
Here's the link to the Caleffi video. The part about limiting the # of TRV's is at about 0:30.
https://www.youtube.com/watch?v=yRPiIpm0oPA
Thanks!
Matt
I would use copper pipe and paint it if it has to be exposed. If it's just a little bit exposed you could do chrome nipples but that will add up quickly.
The delta T in that equation is the supply/return delta T. And you can include the volume of the boiler and the piping in that calculation. I've never seen a traditional boiler that had a buffer tank, usually the volume of the system is sufficient to prevent short-cycling.
I want to modify my answer a bit on the exposed pipe to say it depends on the type of radiator. Slant-fin type radiators are designed to go all the way to the floor, you can use any piping. A lot of wall-mount radiators will have a manufacturer-supplied elbow and expect a chrome nipple to get you through the floor.
I'm assuming your piping goes in the floor.
I would only use TRV's in a retrofit. If you're plumbing starting fresh I would go with zone valves and wall thermostats. It's about the same cost, it's simpler and you get better performance. Plus if you ever want to go with home automation it's easy to upgrade the thermostats. I would run at least a five conductor thermostat cable for future compatibility.
But with a house so small and compact I don't think you'll need zoning anyway.
Every time I read the title of the question, I want to scream there is no simple or low-cost hydronic systems and are totally a luxury item.
I understand the home had a hydronic system but from the sounds of it almost none of is salvageable.
Seems to me in NM you would need air conditioning and you could add heating to that system at a very low cost.
Walta
At 7000 ft in NM they may not need cooling.
I grew up in New England, where millions of houses have hydronic heating systems. (They're frugal Yankees who consider air conditioning an extravagance.) Almost all of these systems were installed without computers, without calculators even, by guys with a high school education at best using rules of thumb. They work reliably for decades and their efficiency is limited only by the choice of boiler.
Here's how they do it:
In every room on the perimeter of the house, measure the exterior walls and install half that length of Slant-Fin baseboard radiators. Count up the total feet and divide by 2, that's your boiler size in thousands of BTU/hr. Go to the supply house and get the next biggest size of boiler; get the kit that comes with the circulator, expansion tank, fill valve and air separator. Run pipe as I outlined in post #2. Put a balancing valve on each radiator. Put a thermostat in a big room on an interior wall. Read the instructions in the box and install according to them.
On a cold day, walk around the house with a thermometer. Some rooms will be hot, some will be cold. In the hot rooms turn the balancing valve down a little. Wait a few hours. Repeat until all rooms are within a degree or two of each other.
This results in a boiler that is oversized and radiators that are oversized. But it doesn't matter because oversizing doesn't affect the efficiency, baseboard radiators are cheap and extra boiler capacity is essentially free.
mcsmuts said the house is 1000 SF, looking at the picture it seems to be about 40x25. That's 65 feet of baseboard, or 32,500 BTU/hr of capacity, about a 50% oversize. The Slant-Fin baseboards run about $15/foot so that's about $1000 of radiators. It's just not worth trying to figure out the perfect radiator size for each room.
Running pipework for radiators is going to be a lot easier than running ductwork to all those rooms. Yeah, the cheapest solution would be to run a couple of minisplit heads but with that many rooms you wouldn't get the comfort level.
Agreed, if AC in the cards, a hydronic system is a complicated extravagance. If AC isn’t, maybe it’s closer.
“ I grew up in New England, where millions of houses have hydronic heating systems. (They're frugal Yankees who consider air conditioning an extravagance.) ”
If you don’t mind me asking, when was your childhood? It’s hotter now!
I was born in the mid-sixties, lived the first 21 years of my life in Massachusetts. I never lived in a house with air conditioning until I moved to DC. Never had air conditioning in school either.
I spend my summers in Rhode Island these days, I don't have air conditioning in that house.
Most people here don't have AC. While the summers are getting hotter the swings day to night are frequently 30-40 degrees F so if you close your windows during the day and open them at night AC is really not necessary.
Also --
I wouldn't be quick to get rid of the old baseboard even if it's cracked. If it's the cast iron style like Base-Ray you probably have about $10,000 worth of it. It's nicer than the SlantFin style that is the go-to, which is why it costs ten times as much. Usually when it freezes you get hairline cracks. If you're at all handy you can fix those by using a grinder to smooth out the crack and then filling it with JB Weld for plumbing.
The operating pressure of a hydronic system is really low by plumbing standards, 12 PSI might be normal pressure with the release valve set at 30 PSI. If they test out at 50 PSI I'd say that would work. That's around where tap water normally is, so if you hook them up to a garden hose and they hold you're good. You could also test them with compressed air but sometimes it can be harder to find small leaks.
You may be able to test and repair them in place, but if there are leaks on the side facing the wall you have to disconnect them to get at the leaks.
Let me add that if you're looking for simple, the system that was in there was probably simple.
The old baseboard was slantfin. Really nothing salvageable from the old system (except the copper for scrap!)
Yeah, I wouldn't salvage frozen slantfin.
Another common way to heat low load places is by tapping heat off a higher BTU regular tank water heater.
You can use a regular power vented one with a bigger burner (60000 Btu or so) and using something like a Taco X block for isolating the heating loop from potable water.
There are also water heaters with a built in hydro coil such as the Laars Combi heat, BW Combi2 TTW or Bock EZ75-76PDVN-C that are specifically meant for this application. You do have to upsize your rads a bit as most of these deliver 140F or so water to the heating loop.
Most power vented tank water heaters when used for space heat get mid to low 80% efficiency which is similar to what you'll see with a mid efficiency boiler. A properly set up modcon can get you a bit higher efficiency but in a low load place the fuel saved is pretty much noise.
The nice part about a water heater is they are low maintaince and you don't have to worry about short cycling. If you are worried about hot water capacity, you can always disable the heat for the one or two hours in the morning when there is high demand.
Simply hydronics is possible.
PEX helps a lot. If you Tee off the heat source in two directions you can likely ran 3/4 mains, and 1/2" to your rads. You can save money on manifolds and just use 3/4x1/2x3/4 tee's and ball valves on the takeoffs. Or just buy the copper manifolds.
1. A delta P pump (varies speed to maintain a constant pressure). This eliminates pump controls.
2. TRV (thermostatic radiator valves) only a room that overheat, use sparingly.
3. Outdoor reset on heat source (match the hot water temp to the load based on OAT, but don't drop it below the equilibrium point of your emitters unless you have a buffer tank). On my house I can supply a constant 42-45degC and the radiators can match the boiler at min output.
4. Don't zone, just balance, if you have some high slower gain room put a TRV on the rad. Microzoning increases the need for extra controls, buffers. TRVs are ok if necessary but set the to limit more than control so they don't shut off unless it gets too warm (more a fine tune after the primary OA reset does the heavy lifting for load matching).
5. You could put a thermostat on if you want, I control to a bathroom temp at night, and my office temp during the day. You don't want a TRV in the room with the thermostat. The thermostatic isn't really there to cycle the heat, but to shut it off if it gets too warm (solar gains, cooking, crowding). With warm weather shutdown and outdoor reset a thermostat can be optional, but I like having one.
6. Deal with your DHW separately, generally the min fire rate on combi's it too high for space heating unless you have a decent sized buffer tank. Just put a standalone hot water tank in (choose your preferred fuel).
7. Choose the boiler with a low min fire rate some go as low as 6000btu. This will allow lower temperature heating without excessive boiler cycling.
8. Regarding buffers, if you select your rads to be able to output the min capacity of the boiler at the lowest supply temp (maybe 35-40degC). You don't need a buffer (this will save space and cost even if you have to upsize the rads). But, if you're like me and like to heat just your baths well into the shoulder season you might need a buffer.
9. Oversize the rads to allow for a future ASHP (40-50degC max) swt.
10. Limit the boiler's max fire rate to just over you heat loss, most boilers have way too high of max fire rate and will just result in more cycling. Slow and steady is the way to go.
This is a good article of a simple heating system. https://www.phcppros.com/articles/7804-when-old-tech-meets-new-tech
Some good theory on heating.
Coffee with caleffi is a treasure trove. https://www.youtube.com/playlist?list=PLuuV0ELkYb5VE0I4evUZ30b5U78CRlRdg
Good heating theory but uses UK words, some interesting progressive ideas. https://www.youtube.com/@HeatGeek