Modcon & Hydro Air – Big Mistake?

Hydrodux

<span style="font-size:12pt">For the sole purpose of saving energy, I recently replaced my Oil boiler with a Viessmann Vitodens 200-W WB2B60 Modcon Gas Boiler.</span>

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<span style="font-size:12pt">My new construction 5000sqft home has 4 zones and 4 Hydro-Air Handlers (3-stage) to provide heating and cooling.</span>

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<span style="font-size:12pt">The house is being heated comfortably, but my electricity bill has doubled due to significantly lower water temps supplied to the air-handlers.  The savings on gas vs oil are thus being largely offset by my higher electric bill.  I'm feeling like my investment might have made a big mistake right now…………</span>

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<span style="font-size:12pt">My oil boiler used to run between 160F-185F.  My gas boiler was factory programmed to shut off at 165F and usually runs at around 150F.  Water at the air handler inlet maxes out at ~139F.</span>

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<span style="font-size:12pt">I’m thinking I need to optimize the trade off between boiler efficiency and increased electricity consumption.  Two extremes appear to be:</span>

<span style="font-size:12pt">A) Boiler modulates at low temp, but Air Handlers run all day => low Gas, high electric, lower comfort</span>

<span style="font-size:12pt">B) Boiler cranked to the max, air handlers run occasionally => high gas, low electric, higher comfort</span>



<span style="font-size:12pt"> </span><span style="font-size:12pt">Ways I’m expecting to explore to achieve this balance are:</span>

<span style="font-size:12pt">1) Set a higher heating curve and increase max temperature on boiler</span>

<span style="font-size:12pt">2) Install aquastats on 4 airhandlers (yet another $500 investment!)</span>

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<span style="font-size:12pt">Six different heating contractors studied my system and quoted for the conversion - none of them ever mentioned this Hydro-Air/Modcon conflict. Viessman has refused to answer my calls for help.  My install contractor is struggling with this.  I’d really appreciate guidance from anyone familiar with this problem please………..</span>

Charlie from wmass

The air handling coils

Should be increased in size to let the cooler water produce the same amount of heat transfer. You can not change input without addressing output. In my area oil is cheaper per therm than natural gas.

Ironman

Charlie's Right

Nothing wrong with with the combination of a mod/con and a fan coil, but as with any system the output (fan coil) has to be equal to or large than the input (boiler).



A properly sized fan coil should not require 180 deg. water. I try to size mine to heat at design temp. with 140deg.



You might also look at installing ECM motors on the AHU's if they don't have them, that would cut their consumption below half. "EverGreen" motors by GE are readily available at most supply houses now.



If you keep boiler water temp at 140 or below, the boiler will remain in condensing and be more efficient.

Hydrodux

Thanks

Thanks Charlie & Bob - two very interesting suggestions that I'll go get pricing on for sure (and discuss with my installer).

Mark N

What?

Oil is cheaper than gas per therm. I find that hard to believe. You must be getting hosed for natural gas. In New Jersey gas keeps dropping every bill, $1.19 per therm. Bill came yesterday. Last I heard heating oil was over $3.00 a gallon. By my calculation that comes out to about $2.15 per therm for oil.

Jean-David Beyer

You must be PSE&G

My most recent bill was $1.4882/term, inclucing delivery charge and Residential Customer Charge from NJ Natural Gas. Closing date on that bill was November 19, so my next bill is due in a few days.

Mark N

PSE&G

That correct I use PSE&G. The commodity price per therm has been dropping every month. Price is down 18.5 cents since last December.

Jean-David Beyer

You must be getting large temperature drops in fan coils.

I have about 100 feet of pipe including two 14 foot pieces of fin-tube baseboard. THis loop drops only about 3F, so if I put 140F in, I would expect no less than 135F coming back. I run that loop from 110F to 135F depending on outside temperature. I realize a fan coil should drop more temperature than the same length of baseboard, though I do not know if measuring "length" of a fan coil is the way to go. I deliberately oversized my baseboard so I could run these lower temperatures. Below 110F, the heat output is so low that the boiler cycles too rapidly, hence the lower limit of 110F that I use.



Downstairs is all in-slab at grade radiant,  so I run that between 75F and 120F and it always condenses. The indirect runs at 170F input and about 155F return, so it condenses only as the boiler is warming up. It does condense then, but not enough to make much difference.



Furthermore, it is my understanding that the efficiency of condensing just starts at the dew point of the gas in the heat exchanger, and that is shown at 130F in John Siegenthaler's book. If we assume the transition from non-condensing to condensing occurs at 130F where the efficiency is 86.5%, then as we cool the return water, the efficiency goes up to 90% at around 120F, 95% at about 100F, 97.5% at 70F, and if you can get the return water down to 40F, the efficiency can be about 99%. Well, maybe if you are doing snow melting.

croydoncorgi

Condensing transition

...happens around 130 - 133 F, depending on which book you read, but it's not progressive.  So long as a reasonable area of heat-exchanger is at or below condensing temperature, then you WILL recover some latent heat.  Quality of boiler design will determine how much.  A big fog plume out of the flue does NOT necessarily indicate 'high efficiency' - a boiler condensing 100% would produce no plume at all.

seabee570

hydo-air

It will be worth the investment to install the aquastats. You may find that with the aquastats shutting the fans down,allowing boiler to catch up,should be a big benefit to you....as others said,the coils may have been sized for 180 degree water....

tim smith

RE: higher bills etc

Calculate what it is costing you to run your electric fan motors. You may be well served to install Emerson ECM fan motors (Rescue type). These are retrofit motors. Then you could look at running low speed fan during low water temp demand. This logic I think might work for you. The retro on the motors is not real cheap but may make sense for you.  Tim

Jean-David Beyer

fog plume

I was looking at the plume from my system. I had figured that the more plume, the less condensing was going on. And that may well be true.



My boiler runs at 170F when the indirect is unsatisfied. If it is cold enough, I get a noticable plume unless it is too windy out. I doubt any condensing goes on.



If I am heating my upstairs baseboard, I put between 110F and 135F water out there, and the plume is quite a lot less. But this zone requires very little heat, so the boiler fires at a lower rate (55% of maximum or less). I guess it condenses most of the time.



If I am heating downstairs (in-slab radiant), I put between 75F and 120F into the slab, and there is less plume there sometimes, and more at other times. For that zone, the boiler fires between 20% and 90%. It must condense all the time. So camparing plumes is not a precise thing.

Hydrodux

More info.....

Changing out the 4 motors certainly sounds like a logical solution. My best estimate of current daily air handler consumption is between 20 & 25KWh. If new motors reduced this by 20% I'd save $0.90 a day. Is 20% a reasonable expectation? I still need to get pricing on 4 new motors to determine break-even period, but if they're ~$400 each (my best guess) plus install costs this would take way too long........



My electric bill is already ~$90 a month higher than with oil heat, and in total the 4 air handlers state they draw 2.2KW, so at 18c/KWh that suggests that each day they're running 7.3 hours longer than they were. The Water Heating Capacity tables in my Air Handler handbook state (I think) that to generate an equivalent number of BTUH's 140F supply water requires the handler to run ~60% longer than 180F oil fired water. Doesn't sound too unreasonable..........



I'm also really tempted to invest the $500 to install the aquastats. The fans start as soon as there's a call for heat and are running for several minutes with cold air, occasionally/eventually maxing out at ~107F at the floor vents. I'd need to eliminate ~1,262 hours of "completely wasted blowing" to break even on the $500 investment at 18c/KWh. Also rather longer than I'm prepared to wager on right now unfortunately.



Given these apparently poor financial investments, perhaps I should just crank the temp on the boiler and forget about modulation efficiency. What % efficiency would I get from a Viessman Vitodens WB2B60 that's doing it's best to generate 180F water? Would this harm the boiler over time?



I hate wasting energy, and went to so much trouble (and cost) to install a high efficency boiler, so this all comes as rather a disappointment. Despite the hours of extra blowing, at least I can be very thankful that my house is still heated to a comfortable temperature, and I'm not still stressing about the $1200 monthly oil bill I suffered a couple of years back!

CMadatMe

External Demand

Here's a couple of things you can play with. Get the a/h's on external demand. You can set an external high beginning in coding adress 91. See page 64 of the service instruction manual. You can also change the room temp to trick the boiler and in turn it will boost the heating curve.



In coding address ”34” you can select the influence the External heat demand” signal should have on the connected circulation pumps. The minimum boiler water setpoint temperature in case of an external heat demand is selected in coding address ”9b”.



External High 160 degrees External Low 140 degrees.



http://www.viessmann.ca/etc/medialib/internet-ca/pdfs/doc/WB2B.Par.53083.File.tmp/Vitodens_200-WB2B_md_si.pdf



Second you could limit modulation of the heating circuit to help you in attaining higher efficiencies. The boiler would only be around 88% firing at the 176 (max temp). You may want the external demand function to get to 160 and then reduce modulation rate to 70 percent and play from there. This will get you in the 90% efficiency range. If you used a LLH and have a temp sensor you have to remember that the water temp you are looking at on the control is the temp inside the LLH not at the boiler. Once a LLH sensor is used the boiler supply sensor is disengaged.



You may want to also consider adding a Vitotrol 300 for the main space. It will provide indoor feed back to the boiler and you have control over the boiler without having to go to the basement.



I'm in NY/NJ and the only full stocking Viessmann wholesaler in the area. I generally get all the Viessmann calls so if you need some help please feel free to e-mail me. The attached chart shows the efficiencies of a Vitodens 200 at different water temps at different rates of modulation

Hydrodux

Thank you Chris!

Chris (and all the above contributors)

Thank you so much for reaching out to me on THE WALL.

Following my converastion with Chris this morning, the changes that I made to the coding and configuration of my Vitodens 200 have made a huge impact on it's ability to efficiently generate hot water for my air handlers.  This also empowered me to uncover a hardware problem with the Vitodens control panel that was causing the 300Watt boiler pump to run continuosly.

I'll continue to fine tune my heating curve to balance the competing demands of boiler vs air handler efficiency, but have a renewed confidence in the abilities of my Vitodens to get the job done.

I'll update this thread with my final conclusions in due course.