Modcon Sizing

BobAlu

I got a question for all the pros out there...how do you properly size a modulating boiler? Now I know from reading here that heat loss calculations, etc are the proper way to size a boiler. But one thing I notice about these mod/con boilers is the tremendous range that they cover. The one Embassy ONEX boiler I see modulates from 30K to 160K. That's quite a range! I think that's the only size they make so if I were to buy one, that's the size, period. I see some of the other companies make different size boilers. Now I, like most homeowners, am always worried about getting something too small, whether it be an AC or a boiler. Since these modulating boilers can modulate down to what's needed, wouldn't it be best to size a boiler larger than what's actually needed to make room for any expansion whether it be a new radiant floor or another room? The units will run at the proper modulation for what's needed...no? Perhaps I don't understand how these units work and they still must be properly sized. Thanks for any knowledge you guys may send my way! The Wall is the place to get knowledge! Happy Holidays all!

Jean-David Beyer

No, it would not.

"wouldn't it be best to size a boiler larger than what's actually needed

to make room for any expansion whether it be a new radiant floor or

another room?"



I am not a professional, but I have a mod|con that I had a disagreement with the contractor about. The contractor made the same arguement you do. He said the boiler he recommended would go down to what I calculated to be the heat loss of my house.Trouble is, you only need that amount of heat on design day. Most days you need less, far less, heat and if your boiler is oversize, you cannot get it that low and still have the boiler operating efficiently. You want the minimum size boiler larger than the heat loss you calculate. The wide modulation range you see is not all that wide; typically 5:1. My house would need 35,000 BTU/hr when it is 0F out, even though design day is 14F. On a not-so-cold day, I may need 6000 BTU/hr or less  when only one zone is calling for heat. My boiler will go down only to 16,000 BTU&/hr.

Gordy

Depends

 On your heatloss calculation which is based on the design temp for your area.



You will only see your design temp maybe 1% of the heating season the rest of the time it is warmer. So if your heat load is less than 30000 btus using the boiler you suggest, and its 45* outside then that boiler will cycle a lot even at low fire in its modulation range. This is more wear, and tear on components than a longer steady burn that matches the load, and less efficient.



You will not always find the perfect modulation range in the boiler you want, to the load of your home, but you should stay as close as possible for the best efficiency the boiler can offer.



 Things always start with a load calc. You don't know if the guy who installed the existing boiler did it right. Just because it works does not mean its the right size. Most heating appliances are over sized because at one time fuel was cheap, and no one ever complains about to much heat.



Gordy

Gordy

Depends

 On your heatloss calculation which is based on the design temp for your area.



You will only see your design temp maybe 1% of the heating season the rest of the time it is warmer. So if your heat load is less than 30000 btus using the boiler you suggest, and its 45* outside then that boiler will cycle a lot even at low fire in its modulation range. This is more wear, and tear on components than a longer steady burn that matches the load, and less efficient.



You will not always find the perfect modulation range in the boiler you want, to the load of your home, but you should stay as close as possible for the best efficiency the boiler can offer.



 Things always start with a load calc. You don't know if the guy who installed the existing boiler did it right. Just because it works does not mean its the right size. Most heating appliances are over sized because at one time fuel was cheap, and no one ever complains about to much heat.



Gordy

Robert O'Brien

Same as any other

You size a Mod/Con the same as any other boiler. What's the heat loss of the house? I don't know where you are located but on LI,160K will handle about 3 average houses!

CMadatMe

Oversizing a Mod/Con

While I agree with the rest that a heat loss and proper selection should be based on that heat loss I also feel you can oversize certain mod/cons. I would only do this if the customer had concrete plans on an addition within 5-7 year time period.



Some condensing boilers have the capability to limit their modulation rate. For instance, the Viessmann Vitodens 200 can limit the modulation rate for heat separate from the modulation rate for domestic. Since we are using Lamda Pro combustion I'm not playing with the RPM rate of the blower but rather telling the boiler when I want the gas pedal to stop and that my new high fire rate is where I want my heating curve to begin. With Lamda Pro the gas valve operates independent of air so the boiler has total control of gas input at the gas valve. Since the boiler always conducts a combustion analysis of itself on each and every fire we can be sure that the boiler operates at optimum combustion.



You need to check the installation and operating manual and speak to your contractor to make sure the piece of equipment you are choosing gives you this option and it can be done. Since no other condensing boiler has Lamda Pro combustion other than Viessmann the set up would be critical for boiler performance.

Jean-David Beyer

I did not deliberately oversize my boiler.

I bought the smallest one in the product line recommended by my contractor (W-M Ultra 3). 80 KBTU/hr. But when heating my upstairs zone that requires only 6500 BTU/hr on an extremely cold day, the boiler will not modulate down far enough (16,000 BTU/hr minimum.



I must not lower the minimum, but I can limit the maximum with the controls of this unit. The default maximum firing rate is 94%. The firing rate at ignition is about 50%, and the minimum is 20%. Now the control seems to be  a Proportional-Integral-Differential type whose gain is too high, so it moves pretty quickly up to maximum rate and when the temperature gets close to the upper limit as determined by the reset curve, it reduces the firing rate too far too late, and ultimately stops firing. Eventually it figures things out, but by then it has often satisfied the thermostat, so it has to learn it over alll over again the next time there is a heat call.



I did two things to deal with this (instead of adding a buffer tank). One is that I changed the limits from +|- 5F to +7|-8F to let it operate longer, and I set the maximum firing rate to 55% to keep the temperature from rising so fast. This has enabled the system to have an acceptable cycling rate because the reduced fiing rate gives the control time to react properly to the temperatures.



With this boiler, I can set reset curves, temperature limits, firing rates, etc., independently for each zone (because I have only two).



I am not sure what would happen if I set the maximum firing rate lower than the firing rate for ignition. There are things I would like to know about the controls that are not in the manual.

Gordy

Modulation ratios

 Are another thing to consider.

2:1, 3:1, 4:1, 5:1, 6:1........10:1



All though the boiler you are considering has a 5:1 ratio there are some boiler brands that do not modulate in so many increments.



Its like saying the accelerator on your car has 1, 2, 3, 4, 5, or up to 10 settings. All though the car accelerator is infinite think of trying to drive with the above steps in acceleration, and deacceleration, and the MPG it would get.



Now apply the anology to a boiler trying to navigate the hills, valleys, and flat ground which translate to temperature changes outside, and the boilers need to supplement the heatloss of the dwelling as best, as it can.



Gordy

BobAlu

Great Information Here!

Thanks for all your comments and knowledge on this question about sizing the boilers. I never thought of the low end going low enough to really save money when there isn't a real demand for heat. We always think of that real cold day that the boiler is struggling to keep up with the demand, fortunately that day really doesn't come around too often. We really save on the regular types of days in the winter, where the boiler can "kick back" a little and just produce the heat that's needed. I found an excellent plumber here on The Wall and I have complete confidence that he will size up my boiler correctly...I have no doubt about it! Furthermore I understand that the Vitodens is no slouch of a boiler! Talk to you soon plumber! (I know he's reading this!)

SpeyFitter

Smallest Mod-Con

I would rather pick a smaller boiler and buffer it's reduced capacity with a larger indirect than pick a larger boiler and smaller tank. I would pick the smallest mod-con you can get away with since the lower modulation ratio (assuming all boilers in the companies line up had a similar modulation ratio i.e. 5 to 1 or there abouts) will reduce short cycling when say only one zone is calling for heating on a shoulder day. 

For example, I have a house I did a design for that had about a 34,000 BTUH heat loss with 4 bathrooms, with at least a few 2 loop zones. Knowing the time Vancouver spends near it's design day temp (which is maybe 20 hours a year give or take), I elected to go with a Lochinvar WBN050 and an 80 gallon tank. This boiler still has enough juice to ramp up, but it's low 10,000 BTUH firing rate is beautiful when only one zone is calling for heat on a fall or spring day.  The 80 Gallon tank set at 135-140 degrees mixed down to say 120 degrees for the hot water lines will have lots of capacity for the bathrooms.

SpeyFitter

Viessmann Not the first...

Chris,

Viessmann isn't the only or the first company that produced a residential mod-con boiler that had a gas valve that modulated independantly of air flow, nor the first company that produced a residential mod-con boiler that analyzed or referenced combustion quality to adjust it's firing characteristics for optimum combustion.

Scott

Jean-David Beyer

Increments.

"2:1, 3:1, 4:1, 5:1, 6:1........10:1





All though the boiler you are considering has a 5:1 ratio there are some

boiler brands that do not modulate in so many increments."



I do not think I understand what you are getting at.



If I say a modulating boiler has a range of 5:1, or whatever, there is no way to tell from that how many increments are actually delivered between the highest and lowest firing rate. If the controls were all analog, there would be a near-infinite number of increments (limited by the signal to noise ratio of the control system). If the controls are digital, it could be 1 (very unlikely)  or any number. I think it would be pointless to have over 100 increments and if there are less than about 8, I think it would give very rough results, but maybe not. I tried to guess how many for my boiler, and can conclude only that it is more than 16, perhaps 32 or even more. What I do is watch the modulation rate on the front panel, but it is in the form of a bar graph, and the resolution of the LCD display is something like 16 or 32 in the area where the modulation bar is displayed. The actual number of increments is up to the designer of the control unit, and in my case, the designer of the air blower that functions something like the inducer of a non-modulating boiler. I do know that I can set the firing rate to any percentage of maximum I wish, with numbers 0 to 99 (or 1 to 100). Now whether the controller keeps all that to 1% accuracy is something I am not in a position to know. Can the speed of the blower be varied in 1% steps? I have no idea. And does it matter: i.e, does it need to be controlled to 1% accuracy?