Steam EDR compromise

HenryT

Hi All, Quick question.

If I have a boiler that puts out say 400sqft of steam, and my radiators EDR is about 250, will adding bigger radiators that add up to 400 sq/ft of steam to match up the boiler output help me with the short cycling of the boiler?

Current boiler is being controlled by the pressuretrol as oppose to the tstat.

Thanks for the input.

Mark Eatherton

In short..

No. The EDR is supposed to match the heat loss requirements of the your house. Once that factor is correct, then the heat source should be sized to the load of the "system". (exposed piping, pickup factors etc)



The only time that the properly sized boiler would not theoretically short cycle is at "design outdoor heating" conditions, which for most of the US only occurs for about 2% of the time, so the boiler is technically oversized for 98% of the time. I use the term theoretical, because in actual practice, on systems that I know for a fact were properly sized, when I show up during design conditions, I see the boiler doing a 50% duty cycle, which tells me the system as a whole is 2 times larger than it really need be, but what contractor in their right mind is going to cut the heating system in HALF, based on field experience.



I didn't think so...



Others will chime in I'm sure...



ME

DanHolohan

I agree.

Adding more radiation will overheat the house, and if you operate off the t-stat, it will short-cycle anyway.

HenryT

Thanks!!

Thanks, what do you suggest short of replacing boiler?

Should i have someone come in to replace the orifice... the short cycling is very worrisome for me... unless im being OCD about it... unit will cycle on and off before it reaches set temp.. i have enough venting per the cacls..

Jamie Hall

OCD

If the 'short cycling' about which you are concerned is occurring right near the end of the cycle, don't worry about it -- assuming that your pressures are right and the vents are working and all that jazz.



First off, you are or should be losing very little efficiency because of it -- a typical boiler will not be off long enough to cool down to the point where it needs much firing to heat back up to steam.



Second, most burners aren't influenced much by turning on and off.  It's not a big issue (residential size, here!).



Third, there isn't much you can do about it.  In an ideal world, we would have fully modulating burners which fired just hard enough to maintain some sort of ideal pressure in the system until the thermostat is happy.  This is not an ideal world, and as ME notes we need the burner to be big enough to satisfy the design conditions -- which are startup on a very cold day.  So to make it work right on a warm day, it's designed to turn on and off at just the right rate to maintain the right average pressure.  It's doing what it should.

HenryT

Thanks Jamie.

Last question, so in your world, cycling on and off by way of pressuretrol is normal. Even if it cuts in and out for 30+ times in an hour to reach desirable temperature?  pressuretrol is set to cut in and out as recommended by Dan's books. i shouldnt worry about it too much huh?



Thanks everyone for easing my mind!!

Mark Eatherton

No thermostat?

Do you actually have a thermostat in the controlled sequence, or not?There is a control put out by tekmar http://www.tekmarcontrols.com/prod/279.shtml that will reset the run times of the boiler based on numerous points of feed back, but it too requires an indoor thermostat. Gotta have SOME means of indoor feed back...ME

Jamie Hall

Now that's

an interesting doo-dad, Mark.  I find the blurb on the link is a little opaque, though -- I would assume that what it does is what the digital thermostat does not do: act like a variable anticipator or cycle length timer, depending on the outside air temperature?  And possibly also vary the effective cycles per hour setting, ditto?  Might be something to look into.



I would assume that one would still have the vapourstat and the regular thermostat in the mix, too...



Can you tell me more about it and what it does?

Charlie from wmass

One question

IS the thermostat set for steam if it is a digital? Micro switches need flipped for some units to work properly with steam.

Mark Eatherton

S.O.O.

Here is a link to the sequence of operation for the control.



http://www.tekmarcontrols.com/literature/acrobat/d269.pdf



I have used this control successfully in numerous jobs and seen a minimum of 20% savings, and as much as 30% savings.



Everything else in the control chain MUST be in good working order. The control is NOT a final safety device.



In the case of the 30% drop, we lowered the operating pressure from 10 pounds down to one.



ME

nicholas bonham-carter

short-cycling

try the "open-pipe test" with, the thermostat jumpered on; and a ball valve temporarily mounted in place of your main vents, so you are sure you have the maximum[ =right] venting. if it still short-cycles after closing the valve, then you may have over-sizing.

what is your actual pressure on a good low pressure gauge?--nbc

Jamie Hall

neat gadget

I like it!  I can see some very nice applicatons for it.  While it isn't particularly applicable to the ones I've got to hand now, I'll keep it in mind.



Amazing what dropping pressure to where it belongs can do to the fuel bill, isn't it?  One wonders why people don't believe it...

ttekushan_3

orrrr. . .

You could simply run a steam line to a neighbor's house to heat their home as well. The boiler will cease to short cycle. And you could charge for your steam generating services and come out ahead.



I'm just a fount of great ideas today.



-Terry



P.S. depending on the design and burner type on your boiler, you could choose a less ambitious route and have it downfired to match the EDR. This would stop the short cycling but energy savings would be dependent on the boiler/burner design if any savings are had at all. Creative and more effective ways of downsizing a boiler exist, but you may find it difficult to find someone willing to do them.

Roland_18

TEKMAR doo-dad

Yikes! That is one pricey bit of technology. I wonder about the R.O.I.

Mark Eatherton

Shirley, you jest....

20% (minimum) reduction in annual fuel consumption. Installed cost divided by annual savings = R.O.I.



So, as usual, THE only correct answer is (drum rolllllllll) It DEPENDS!



Not a whole lot of competition on the open market.



ME

mel rowe

Downsizing

Just a homeowner with some experience that I thought might be of interest.  I struggled with my one pipe system for many years until I go Dan's book and started reading and learning a few things.  With helpful advice from Wallies, i started doing a lot of things to improve my system and try and make it more efficient and make it work better.  After many improvements I found that my main remaining roadblock was that the boiler  was over sized by about the same percentage as yours.  Last year I finally took the plunge and had my boiler downsized.  We took out four of the 16 burners and adjusted the gas properly.  Since that time my system runs more efficiently.  It also runs better, to the point that I have had to find other things to do with my time now. LOL  It had no problem handling a week of close to design day temperatures.  In conjunction with my repiping and adding a correctly sized drop header,  I don't seem to have any more wet steam problems.  Most times it will run and shut off at stat without ever getting over 8 oz of pressure on my 0-5psi gauge, so the Vaporstat is seldom needed.  Overall I wish I had done it years ago.    FWIW

ttekushan_3

Hi Mel,

Glad to see that downsizing worked out for you. A job well done. Nice, isn't it? Except for that boredom part...



Terry