Heat Loss Calcs Question

J.C.A._3

Radiators being over sized IS a good thing. Do the math and find a way to heat the joint with 140° water at design temp.....and you'll be in condensing mode almost permanently.The call for hot water will get it out of condensing, but again, that ain't a bad thing for recovery. (rule of thumb here: if momma ain't happy, no one is happy. Keep her in hot water and the world is yours!) The best of all worlds, and the way to save the most dough!!!

For a place that small,with new and great insulation... the boiler seems to be a bit big...even with the theory provided. Chris

Victoria Homebuilders

How much can I SAFELY reduce

the existing BTU load when superinsulating?
Hello All,
I have a 1920's house in Northern NJ (outside temp design 10 deg, inside 70 deg) with ZERO insulation and 28 drafty single pane windows. Had gas/steam & cast iron rads, but after wasting $450/month on heating the neighborhood (and the huge crack that just appeared in the boiler), I am doing a total gut which will allow me to bring entire structure and systems to super efficient standards. Old system seemed to be sized properly for existing structure conditions, so I used that as my jump off point. Replacement is Buderus GB 142-45 (158K BTU) and Buderus ST 150 water heater (119K BTU) I thought the boiler was a bit oversized but Buderus said this unit performs better at something like 80% of max rated output and the boiler/ heater combo was sized for me by Buderus sales tech. I already performed sizings for replacement rads but my question is how much can I safely reduce the BTU load calcs assuming I will have an incredibly tight and well insulated structure? Existing uninsulated and very drafty house is 1245 SF and using standard sizing rule of thumb in my area (SF X 45= BTU load) I get 56,385 BTU. House had CortoTube rads totalling 240.57 SF radiation putting out 57,456 BTU at 215 deg. So whole house was only 1071 BTU oversized and seemed to work although not very efficiently due to house never being insulated or weatherproofed. However I will be replacing all windows with LowE triple glazed units & sprayed in Icynene on all walls, ceilings, floors & window weight pockets. I expect to have a minimun of R19-21 in walls and minimun R38-44 in ceilings and floors when finished. I will be using whole house air quality system, I follow the build tight & vent right guidelines, so I do not expect huge ACH/HR numbers and am leaving them out of the equation for now but the house is so drafty now I can see the shades move in every window on a breezy day. So, how oversized would the current radiators in this system be if I replace them matching the current BTU output, after I insulate the entire envelope and seal up drafts?
I am using a combo of radiant over tile floors in bath & kitchen, cast iron wall block rads (for appearance in downstairs rooms) and Runtal VLX 42/42 wall panels in bedrooms. System designed at 160 deg, outdoor reset came with boiler. Each rad is run in a parallel circuit loop so there is temp drop at end of line, and have thermostatic valves. There are 4 separate pumped zones: DHW, floor 1, floor 2 and radiant (basement slab, kitchen, bath). I guess I could install the largest BTU rad each room using the calcs and keep them from getting room too hot with the thermovalve but that seems like a waste. I ran the numbers with free online sizing software (supposed to be similar to Manual J calcs) and in one room it showed I could drop from the current 5040 BTU to a MAX of 2688 BTU and a MIN of 1344 BTU. This sounds way off to me. Any thoughts?

Brad White

Slow down just a wee little bit....

You need to perform a calculated heat loss based on what the final product will be. Any shortcut we can give will be only order of magnitude and there are a lot of variables outside of our control.

The radiators should be assigned to each room proportional to the heat loss of that room. If the existing radiators are re-used and wind up being 50% over-sized (a good thing by the way), just make sure that all radiators have the same proportion of over-sizing.

The biggest single piece of your heat loss puzzle is infiltration. Your air barriers have to be well-sealed. Can we presume you will be stripping the outside and installing Tyvek or other moisture-breathable air stopper?

Instead of fiberglass, have you considered Corbond or Tiger Foam? Icynene is OK too but has half the R value of the first two. All have excellent air-sealing capabilities.

I also recommend, while you are at it, performing a blower door test, even while the insulators are still on-site, if foam application. This way you can find and fix any missed spots before they leave.

If you do use fiberglass batts, the "fit" issue becomes apparent. A few gaps here and there, and your R value drops a good deal. The integrity of the R value is better than "more R value".

Given your opportunity to take a drafty, uninsulated structure and improve it, you could expect with normal insulation practices to drop the heat loss by about 30 percent. If you use high-integrity foam systems and scrupulous air sealing, you could get that down to 50 percent in my experience. On top of this, a properly-sized boiler, especially one that modulates and condenses, will do wonders.

That "80% loaded is best" rule? Never heard of it. Perhaps the person who gave that meant that a lower firing rate would be more efficient given all of the heat transfer area? I have no idea.

To me the ideal boiler is one that runs flat out on the coldest day and does not shut off and when the weather warms a few degrees, the boiler modulates downward.

You will find that your avarage winter temperature is a lot warmer than you think. Size the boiler tightly.

Victoria Homebuilders

follow up info

Brad,
Thanks for the info. But just to be clear, I am a residential restoration contractor in business since 1993. I'm not one of those guys that says I've been doing it this way for years; I have been doing it the RIGHT way for years. Thats why I've come here; to get the most up to date perspective from pros who pride themselves in doing it right. I can't tell you how many jobs I have been called in to investigate problems with newly remodeled work only to find out fiberglass insulation was improperly installed, not installed to proper density or left out all together. And they were supposed to be "pros" doing the job. I've seen butchery you can only appreciate if you know the difference between good and shoddy work. I do all my work myself and closely supervise my laborers and spec out everything my subs are to do down to the last detail. I have done several whole house remodels where I was able to superinsulate (I was green when being green wasn't cool) but I either don't get feedback data on how the system is performing or didn't have pre-remodel data to get a baseline comparison. I pride myself on a zero callback record, but maybe that's a bad thing since I never know how much better the job could have been even if it is working trouble free. Trouble free doesn't always mean 100% as good as it can get.
Anyway, I have heard that Manual J calcs have generated BTU loads 1/3 to 1/4 those of rule of thumb or as built calcs. The 80% I referred to was given to me by a tech at Buderus. I have installed several complex systems before but no condensing system yet. This is my house and I can take the time to work out all the operational bugs if need be, but I want to have the hardware right from the get go. Local suppliers I deal with have been sizing the boiler to the DHW demand and I haven't had a problem with that yet. Using that as a guide, I could have shaved 25K BTU and went down a size in the boiler, but when I called Buderus for some specs (I'm the ONLY one of these boilers my suplier has ever sold so they are as new to this as I am) they asked me a few questions: house BTU load, DHW BTU load and total radiator BTU output. House SF is not changing from existing conditions, DHW is fixed at 119K BTU, and ALL rads are to be ordered new. That's why I'm asking the question and CHRIS (J.C.A.) touched on it in his post: My rad supplier wants to size the replacements the exact same BTU output as the old ones, which I think without doing any calcs would be way oversized since I am installing heavy duty insulation and vapor barrier. Old school construction here: 3/4" cedar siding over 30# asphalt felt. Can't use Tyvek due to cedar tannins. I say the existing rads were sized for an uninsulated, drafty house even when it was new and there was inherently more heat loss and air changes but the rad guy says existing radiation surface are the only calcs that matter. He is basing the replacement rad BTU output on the existing rad output. I don't want to have rads 4X the size I actually need. I also believe in oversizing slightly since system is designed for 10 deg but we may have a week of 0 deg. I would have liked to run at 140 deg, but in order to match the NEW hot water rad BTU to the OLD steam rad BTU, and still get them to fit under windows without blocking them, I had to boost temps to 160 deg. I could do it at 140 but the rads wouldnt fit the walls. If I can reduce the BTU output of the new rads to ? % less than the existing, I will use 140 deg because the new ones the rad supplier specd physically fit perfectly AND match existing BTU at 160. I will calc what dropping the temp by 20 deg will give in BTU. Rather than use less rad surface area, I would much rather drop to 140. Buderus said the GB145 is most efficient when running at 80% of MAX BTU. It was months ago when I spoke to them, so I could be mistaken about the actual number it could have been 85%, but they wanted me to oversize the boiler by approx 20% more than the highest demand, which was the DHW. So a 119K BTU DHW needs a 158K BTU boiler according to the Buderus tech I spoke to. As I said I have no previous installations of condensing units so this is somewhat new to me. I understand the physics behind air changes, heat loss and the other variables in construction but is it actually possible that a 66% to 75% reduction in BTU load is achieveable IF I were to build the ideally sealed structure? I took into account outside wall area, floor materials, sun exposure, window shades, glass U values, glass surface area, door areas, etc on a room by room basis. The numbers I am getting are ranging from 10K to 29K BTU needed LESS than what was existing. If they were 5K to 10K off I wouldn't be so worried, but replacing 57,456 BTU worth of rads with 30K seems a bit too small.

J.C.A._3

V.H.

Consider that the radiators were ORIGINALLY designed to keep a room at 70°, with the windows open a bit....for the Spanish Flu of the late 19-teens , until (probably) the 1950's....

Then consider that the "Manual J" has a bit of fudge factor built into it...(and IT DOES!!) Why would you want to have a boiler that is already set at a "conservative" 1/3 over the needed firing requirement?

Trust your instincts on this one. The needed load for the hot water IS a big consideration...but not THAT big...with a mod con boiler. If you've sized the tank properly for the dwelling, that particular boiler will "prioritize" the load for that situation. The heat is the most important concern in MOST cases, because you aren't calling for hot water at any given moment, where the heat could be needed, except for MAYBE a few 10 minute periods a day.

Consider it a wash. Have enough hot water standing by, and worry about the heat load. Unless you're trying to fill a "sin bin" every few hours.... a 60 to 119 gallon tank won't tax the system that much.

The lower you can get the "operating temperature", the better any mod con will perform, hands down. Chris

Victoria Homebuilders

Chris, so what's your opinion

on the sizing for the new rads? It's too late for the boiler; it's bought & paid for, and partially installed. Too late to turn back now. Buderus sized it and I guess I have to trust the manufacturer even though I would have went with a unit one size smaller. DHW tank is only 32G but heats 160 Gal/Hr. It is the next to smallest tank they make and IF I decide to add a bath (house currently has only one), it can easily handle the future demands. It is on priority and should only call for heat 2 times a day. My problem is the sizes of the rads? Is it your opinion that leaving the rads the same BTU size is overkill? Example: I have one room 7X15 all exterior wall. Sizing by 105 SF X45= 4725 BTU load, but doesnt account for all that glass. Manual J sizes it 7520 BTU, around 30% smaller than the current rads put out 10,214 BTU. I feel comfortable with 2 rads on opposite walls each approx 4,000 BTU. You think the Manual J isn't that far off and I can rely on it for all the rooms? It just seems too little BTU when the whole house its totalled up.

J.C.A._3

V.H.

With the insulation you're proposing, and the load of a 1245 2/ft home you're proposing...if that boiler isn't condensing MOST of the time...I'd be REAL surprised.

I would have gone with a smaller size, based on the info you gave me, personally.Even at 30 btu/sq, ft, (like Colin Hay sang..."It's just overkill" Men At Work)My numbers say around 50,000, and that's being generous,and assuming little to minimal insulative quality.

Overkill is the norm....in most situations (apparently) but wouldn't be MY choice.
Shoot for 130° temps. and see what you come up with. I bet you'll come closer to the low end of the modulation from that boiler BTU/HR wise. Chris

Keith_8

Domestic hot water

Shouldn't cause you to oversize the boiler. We install larger indirect hot water heaters or storage tanks to keep the size of the boiler in line with the actual heat loss of the house.

Believe in the heat loss #'s you are arriving at. My guess is that your actual heat loss will be closer to 40 MBH and you have a boiler that is 3x larger than what you need. What is the total BTU's that you arrived at for the whole house?

Keith

Victoria Homebuilders

Keith, the total house BTU

was calclated several ways: 1) existing rad output= 57,456. 2)SF X 45= 56,385. 3) Manual J= 29,178.71
The ONLY reason I went with the boiler I did was BUderus told me that they sized the boiler to the LARGEST BUT load placed on it, in my case the DHW tank. I questioned them and said the boiler I was replacing was only 80,000 BTU and they said THAT was oversized for the load of the house. The theroy that was explained to me it because it is a modulating boiler, it is MOST efficient when on the lower flame therefore by oversizing it slightly, it will not need to run flat out except for the coldest days or when calling for priority DHW.
I originally called them for specs on the size smaller, but when they asked for my calcs, they said it would be too small. Now I'm worried it is not just too big by one size, but possibly 3.
Mike

[Deleted User]

Interesting thread

If I can just say something here ....

IMHO sizing the boiler to meet a DHW demand rather than a heat load would ... bear a lot of consideration. Generally I would not recommend that unless there is a specific reason to absolutely insure thr DHW load. That is almost always not the case in a residence. Size the boiler for the heat on the coldest day - determined with a heat loss calc - and let the "extra" BTUs on a warmer day - or the higher firing range of a mod. boiler - heat as 'properly sized' DHW tank.

When you look at the Manual J load of approx 30k BTU it looks scary I'll bet - but if the calc is done correctly then you will be all set. If the tank is close to correct for the first hour draw, then the modulation of the boiler, or the increased size of the tank (due to meeting the draw rate with less btu's) - should suffice. A well insulated larger storage tank is the answer I think - not an oversized boiler.... IMO

Mike

Keith_8

Not heard that theory before

Seems to me that would be like saying install a 600 HP engine in your car. That way you will only touch the petal slightly and go 60 miles per hour.

If the requirements of the house are minimul then why bring all the extra BTU's into the basement? IMHO your boiler will rarely exceed the 50% firing rate. Sounds like your paying for something you don't need.

You would be better served in the long run with a properly sized boiler and a larger, well insulated indirect.


Keith

Victoria Homebuilders

I'm going to check

with Buderus tech rep I spoke with when researching the boiler. I agree with you 100% and was totally confused when he told me I needed to size it to the demands of the DHW even though there is only one bath in the house. If I was running a rent by the hour motel I could see sizing it to keep up with the call for DHW. I'll let you know what info I get from Buderus today.
Thanks,
Mike

tim smith

Victoria builder

.02 worth, regarding original question, if you look at the load calcs for uninsulated totally to fully insulated, your loads drop down to almost 20% of uninsulated #s. That means if your existing radiators were say appx 60mbh and heated house when uninsulated, they would would be almost 4 to 5 times oversized, which as stated above is not harmful and even good with a mod/con boiler albeit way oversized. I would not worry too much about sizing boiler to indirect coil size but more to load of bldg w/ indirect in back of mind when doing so. I would be more likely to upsize storage if my heat load for structure was so low that sizing to indirect would grossly oversize my boiler. Size your radiators to 140 max if you can and keep closer to condensing pretty much all the time and you will get better fuel savings. Tim maybe .04 worth

Victoria Homebuilders

Buderus tech just said

this boiler is NOT oversized. He gave a few reasons with his largest concern to address is the DHW BTU demand and it's recovery time. With the modestly sized (30 Gal) indirect heater having a 119 MBTU load, the next size smaller GB142 (@ 102 MBTU) would not have the BTU to handle, resulting in much longer run times for recovery. This is not a concern for me now, but it's only me & my wife in a 3 bedroom, 1 bath house; 1 shower a day each. Someday it may be occupied by a family of 5 adding another bedroom or two and two more full baths (typical trend in my area) so recovery is a slight design concern to consider for future renovations. What Buderus tech just said was the biggest feature in the GB142 is the ability to reduce the gas (flame) output to as low as 30% rated max BTU output. They said I can comfortably run water temps as low as 90 and as high as 180, so if I wanted to add several huge radiant loops, I have the ability to boost flame or increase temps or a combination of the two to get anywhere between 89% AFUE running "oversized" with 100% flame and 160 deg temps in non condensing/non modulating mode but not short-cycling (which I could have simply went with a typical boiler in that case) OR up to 95% AFUE with some "fine tuning" after the system is up and running. He said it was going to take a little bit of experimenting with flame/temp combo on my part, but he said this is ABSOLUTELY NOT oversized. I asked the questions/comments which were posted (and ones I had asked when I originally called for them to size it) and the tech said they were a non-issue because of the controllability features the system has built in and again said it is NOT oversized. I said it seemed to me I was putting a huge V8 engine in a Mini Cooper that only needed a 4 Cyl. He said the engine I bought could be converted to a 4 6 or 8 cylinder through the built in controls. This way if I eventually expand the house, I have the expansion capacity already there on stand by but not being used. He agreed, I could go down ONE boiler size (102 MBTU) but my recovery times for DHW would be unsatisfactory unless I installed a much larger tank capacity. He said that would be the less preferred way to do it because I would be heating more water than normally needed. He said dping it that wasn't wrong, just not as efficient with the GB142 series and not the preferred way of Buderus. He reminded me that ALL GB142 units were rated with TWO BTU outputs: one is the MAX (my unit is 158 MBTU) and the MIN (my unit is 42,500) and those ratings are dependent on flame and water temp adjustments that I have to make once in operation. Right now I need almost 70MBTU on the cold and windy nights, but when I am superinsulated, the unit will be just slightly oversized typically only calling for 38MBTU with an outside design temp of 10 deg, I will also see some 0 deg days where I will just be at the minimum 42,500 and can run it flat out in mod/con mode. The DHW is the variable I will need to address, but Buderus thinks the size I have for my house loads is absolutely perfect.
I GREATLY APPRECIATE ALL OF YOU who have posted a comment. I am looking at another two weeks for all the compnents to arrive and will post an update once system is operational. I welcome anyone with any experience with fine tuning these units to let me know any tricks they found, and anyone who still wants to weigh in on this PLEASE post. Thanks! Mike

Rock Solid Restoration_3

So, now that we're halfway through winter, how is the new system working?

Victoria Homebuilders

Thanks for asking

The system is performing very well so far. I have half of the radiators up and running (just the Runtal wall panels) the cast iron rads are still being assembled and should be here in a few more weeks. I still run one t-stat controlled radiant electric space heater to keep the chill out overnight, but as soon as the sun comes out it shuts off. The icynene insulation will get applied to the second floor rooms in Feb and I will see how much of a difference that makes. I'm living here during the work so its getting done piecemeal. Right now I'm letting the boiler run itself with the water temp setpoint at 160 deg, as this Buderus allows it to control itself without a wall thermostat all from the control panel in the unit. It has return water sensor, outdoor reset and DHW sensor installed. Sometimes the supply water hits 180 when the DHW calls, but only does so for 10 minutes or less. This Buderus is an UNBELIEVEABLY QUIET operation; the only way I know its firing is I hear the gas meter flowing. Even running at only 160 deg, I haven't been able to run out of DHW and I tried; two 20 min showers back to back, with dishwasher and laundry running during it all. I only have a 30 gal Buderus tank; standby loss is virtually nonexistant.
There are a few things I have learned about the efficiency of these MOD/CON units which most of you already know, and I'll pass along actual performance data as I get it since I'm living with this system 24/7. On jobs, I don't get call backs so I never get feedback good or bad. First thing is OVERSIZING the radiators is CRITICAL for efficiency. Right now I haven't altered the structure from what I've started with (NO insulation, terrible airtightness and single pane windows)and all things equal it is more comfortable than it was for the past 5 years with steam heat. Dec 06 gas bill was $450, Dec 07 $230. I only have half of the rads installed, the unused rooms are closed off and unheated. Keep in mind until the construction work is done I am losing 3 to 5 times the heat (depending on the room) according to my current and my final-built Manual J calc comparisons: design water temp is 130 when house is completed & I am building to EXCEED all codes (building and energy). Walls will be R19 & roof R40. Although I could have went with a much smaller rad in each room, I went with one that fit the proportions (smaller looked odd) and I am running 20 deg LESS than I anticipated I would need to compensate for heat loss this year. IF I went with the smaller rads I would most likely be at 180 now to make up the BTU. Who knows I may get down to 120 when all is done. In order to keep wiring and power consumption to a minimum, I installed a TRV on each panel (each rad is home run) so they are modulating independently as well. I am using pumps instead of zone valves via TACO 3 zone control relay and 4 GRUNDFOS super brute 3-speed pumps with 1 pump for each: 1 high temp (160deg) DHW zone, 8 med temp (130deg) rad zones & 10 low temp (110deg) radiant zones, all set on LOW speed at 80 watts 0.5 amp each. ONE 15 amp circuit is more than enough power for everything so the electrical supply didn't need any changes from the old boiler. For now, I wired the rad circuit pump to stay on constantly since it is set to untimately be controlled by a room thermostat: the boiler overrides the thermo in its current setup and when finished one master thermostat for all rad loops and a floor sensor in each radiant loop will control pumps. The pumps are so quiet I have to touch the body to see if they're running. I always used TACO 007's but they cant compare to the GRUNDFOS which I got for a steal at $60 each! The pumps also came with removable flow check valves so that was 4 less parts I needed to buy and install, saving me time and about $100. I also found that by building a small "mechanicals" wall off the foundation wall I was able to install the builer, expansion tank, fill and purge valves, zone controls, etc on one side and install the radiant manifolds, radiator loop manifolds tempering valves and zone pumps on the back side making for a clean easy to service package that takes up only 3 feet of wall space. I have seen similar setups easily gobble up 10 feet of wall space, this uses a 3'X4' footprint not counting the DHW tank. I have pics if youre interested...
I will keep updating as progress continues.
Mike

Al Corelli

Pictures???

YES. I am interested. Show me the pictures!!!