Mono-flo or Parallel piping is small home?

Gordy

Red flag

Did you do a heat loss calculation room by room for your home?



Seems odd that all rooms would require the same size radiators? Not impossible.

Gordy

My answer

Direct return with TRVs.



Reverse return with out TRVs.



With out knowing the heat loss, and radiator sizes can't say on pipe sizes.



But larger is better to reduce head loss, which reduces circulator sizing, which reduces electrical consumption.



3/4" copper pipe can deliver 42000 btus in the 2-4 FPS velocity range. You want the circulating water to move fast enough to keep air in solution, and slow enough to avoid velocity noise, and pipe erosion.

Scott_Mountain_View_CA

Yes, I did a heat loss calculation

...in every room...lol. The rooms are approximately the same size and so are the radiators. Good that you asked, though

RJ_4

pipe

pipe

Gordy

Heat source

What are you using for a boiler Scott?



What is your heat loss?

SWEI

So what is your total heat loss?

You can size panel rads at 25-30F ∆T, which will reduce flow requirements.

Gordy

Pipe

Pipe??

Scott_Mountain_View_CA

Boiler is an older AO Smith Hydronic Copper

1960's or 70's era, 80,000 BTU output. It originally heated a in-slab radiant heating system in a 1,200 sq ft home. The in-slab system failed many years ago and the house was retrofitted with a fin-tube baseboard system in the 1970's, which was old, rusty, and generally crappy looking when I purchased the house. I did a major remodel on the house: added 350 sq ft (now 1,550 sq ft tot.), completely insulated (walls/roof/dual pane windows), and removed the crappy old baseboards from all rooms. I'm now planning to replace with an all Runtal radiator system. The boiler has been checked out and is functional (it pumps hot water).



The home is a slab on grade with a flat tar and gravel roof (yup, no attic or crawl space).

Scott_Mountain_View_CA

Total heat loss response

I initially used the SlantFin Corp heat loss software Hydronic Explorer 2 to calculate heat loss.. The calculated result was about 20 BTUH/SqFt which was a bit lower than I anticipated. I live in Mountain View, in the San Francisco Bay Area. I've read several "rule of thumb" heat loss articles and the suggested number for San Francisco is 25 BTUH/SqFt. I live about 40 miles south of San Francisco where the average temps are higher than SF so maybe 20-25 is the right number?



I talked to a Hydronics installer awhile back and he said 25 BTUH/SqFt was about right for my area but that he uses 30 BTUH/SqFt for his jobs.....he said it's better to plan for those rare uber-cold days.



I'd appreciate comments on this....is it better to overshoot BTUH/SqFt to account for rare cold days? What is the downside to sizing radiators a bit on the high side? I want to avoid overheating...we like a warm but not hot house.

SWEI

BTUs per hour per square foot

depends on both your location and the construction of the house.  We're building now at 12 BTUs/sq ft with an outdoor design temp of 10ºF.



DO NOT OVERSIZE A HEATING BOILER.  Really.



What was the calculated heat loss for the whole house from the Slant/Fin app?

Scott_Mountain_View_CA

Response to SWEI

I'm in the process of re-entering my data into the Slant/Fin app...it's been over a year since I did it and I want to do it again to make sure it was done correctly. I'll post the numbers in a bit.



What do you mean when you say: "DO NOT OVERSIZE A HEATING BOILER"? Are you saying that the boiler shouldn't have excessive BTU output? Or are you saying not to over estimate BTUH/SqFt when sizing radiators? Please be specific....I want to fully understand your concern.



thanks-

SWEI

Oversizing

means installing a boiler which is larger than your design day heating requirement.  There is already a safety factor in the heat loss calculated by the software, so in most cases a boiler that is 5-10% smaller than that number will work just fine.



More specifically in the case of a modulating/condensing boiler, pay attention to the minimum modulation rate of the boiler.  Aim for 1/3rd of your design day heat loss or less on that.  The larger this ratio, the more time the boiler will spend at the bottom portion of its modulation curve, which improves both efficiency and comfort.



On the radiator sizing, be aware that most manufacturers rate their products at 180ºF or 190ºF.  Radiators are sized using the average of supply and return water temps, so a 180ºF rated radiator running at a 30ºF ∆T would need 195ºF boiler water to deliver that rated amount of BTUs.  Size them for 120ºF - 140ºF using http://www.runtalnorthamerica.com/bisque/calculating_btuh.html, make sure they will fit the available space, and you will end up with a quiet, comfortable, efficient system.

Gordy

Good advice from SWEI there

I was kind a wondering, and leading up to SWEI's questions. That program is closer to 15% padded.

Scott_Mountain_View_CA

Boiler size

SWEI, I'm still working on the SlantFin app....but....my boiler has an 80,000 BTU output and my whole house requirement will be 45,000 BTU or less. Are you saying my boiler is too large?



In general, I'm having a difficult time with the jargon you are using...remember, I'm a neophyte and terms such as "design day heating requirement", "modulation rate", etc. etc. have no meaning for me. I guess I have a homework assignment...or perhaps you wouldn't mind putting some color on your suggestions....I think you're making important points, I'm just not following.

Gordy

all most twice

the size needed.



Modulation rate: is like the accelerator on a car in the modulating condensing boiler category. They have a turn down ratio usually 5:1. They folow the load with supply sensor to try and match it thats what makes them efficient.



A regular cast iron boiler is not modulating, and you get full output every time it fires.



Design day is the coldest day of an average heating season. Usually occuring 1% of the heating season. Thats why its important NOT to oversize.



You must plug in the correct numbers in the heat loss program or it will be off. especially design day.



IF you did an envelope upgrade while renovatings the boiler will really be oversized even though you added 350 sf.

I hope you are ditching the boiler.

Scott_Mountain_View_CA

SlantFin heat loss program

I have an issue with the SlantFin Heat Loss calculator app: The "Ceiling Factor" drop-down choices do not match what I have. I have a tar & gravel built-up roof with 4" of rigid foam sheathing beneath the tar and gravel. Total R-factor is 25. There are no selections under "Ceiling Factor" that come close. I've concluded the app is useless without a correct ceiling factor for my home. Dead end.

Scott_Mountain_View_CA

Am I ditching the boiler?

Hadn't planned to. What are the ramifications of using an oversize boiler other than added annual fuel cost? A new boiler is what, $4-5K installed? When I balance that against a couple hundred/year added fuel cost, the math says to wait until the old boiler breaks down before replacing.

SWEI

Keeping the boiler

while maximizing comfort and efficiency would involve adding a buffer tank (smart plumbers know how to repurpose an electric water heater for this) and a motorized mixing valve running on Outdoor Reset Control.  The delta in materials cost for this versus materials cost for a small mod/con boiler (which removes the need for the foregoing bits) is less than the labor cost for either solution.



Translation:  Have a properly sized, quality mod/con boiler installed by a competent radiant professional.  Sit back, enjoy the outrageous comfort, and brag to all your friends about how little you spend heating your house.

Scott_Mountain_View_CA

I'd rather brag to my friends...

...about the smart economic decision of keeping a perfectly good boiler rather than blow several thousand on a new boiler for which the payback is maybe 10 years.



Seriously, though, other than fuel cost are there other significant downsides to keeping the old boiler in service?

Gordy

yeah

like maybe lasting this year or 5 years, and installing a new one plus the xtra fuel consumption.

Gordy

Rvalue

Not the assembly will work for what you have.

Scott_Mountain_View_CA

Gordy

Could you elaborate? Are you saying the piping and/or radiators could fail in 1-5 years?



thx

Scott_Mountain_View_CA

Gordy re: Ceiling R-factor in Slant/Fin app

Gordy, could you clarify?  I would need to know the decimal factor for R25 and it's not among the menu choices.  How  would I determine the decimal factor for R25?

Gordy

Boiler

The boiler is quite old 40 plus years minimum is pushing the limit.

Scott_Mountain_View_CA

SlantFin heat loss calculator question

I'm stuck at "ceiling factor".



The input field calls for a decimal factor to be chosen from a drop down menu. Problem is that my ceiling is at tar/gravel built up over foam boards that amounts to R=25 and that is not among the choices. Any thoughts on how to convert R=25 into their equiv. decimal factor?

SWEI

Keeping the old boiler in service

will require replacing enough of your current system that (if it were my house) I'd be quite leery of making the investment.  Given that the existing boiler puts out twice the heat you need on the coldest day of the year, it's going to be short-cycling the overwhelming majority of the time.



Assuming you still want to take this approach, I would strongly suggest adding a buffer tank.  If you size the radiators for a low water temperature (please do) and set the boiler aquastat for 150F/130F, TRVs will throttle back the flow and deliver some comfort.

Scott_Mountain_View_CA

Boiler questions

Could someone explain to me the relationship between boiler water temp, boiler input BTU, boiler output BTU and radiator sizing.



My current older AO Smith is a 80,000 BTU output @ 180 degrees. I had planned to size my Runtal radiators based on 180 degree water temp.



I notice in the Runtal literature a table that shows different temps and the required adjustments in radiator output based on those different temps.



It seems obvious to me that if I size my radiators for 180 degree water and then later replace the boiler with a new one that produces a lower water temp, my radiators could become undersized (for the new, lower water temp). Could one of the pros comment on this? Is this a valid concern? What is a typical water temp in a new boiler?

Scott_Mountain_View_CA

Questions for SWEI

"Keeping the old boiler in service will require replacing enough of your current system"



Not sure what you mean by "old system". I have no system today, just the boiler. I'm building the system from the ground up with new radiators.



What will it cost to have a technician install a buffer tank on my old boiler? And are you saying that it is possible to lower the temp output of my boiler? To what temp?





See my new post down below...I'm concerned about sizing radiators to 180 degree water than having to install a new boiler later that has a lower water temp...and then i'd have undersized radiators. Do you have any idea what temp my old AO Smith be adjusted down to?



What is the most common/typical water temp in the new 40,000-50,000 BTU output boilers?



This is a very valuable discussion....I'm beginning to see some potential flaws in my planning/thinking.



Thank you

Gordy

The Basics

http://www.masterplumbers.com/plumbviews/2002/boilerratings.asp





And size your rads so in the future when your boiler dies you can get a more efficient one, and use lower temp water to do it.

SWEI

Boiler water temps

The lower the temperature of returning boiler water, the cooler the flue gas -- and the greater the combustion efficiency.



In a conventional cast-iron boiler, that returning water temperature must be kept above 130ºF - 140ºF (depending on the specific fuel in use, the humidity) in order to prevent flue gas condensation from rotting out the flue pipe and boiler heat exchanger.  To maintain a reasonable degree of system efficiency, the supply temperature should be at least 20ºF hotter than this.



A modulating condensing boiler has no lower limit on its return water temperature (OK, perhaps 34F might be a good idea.)  This allows efficiencies over 98% when paired with a properly designed emitter system.