New Knight boiler customer wants it adjusted.

Eric_32

Hi guys,

Just installed a new Knight KBN boiler for a customer recently. They have BB heat and a heat loss was done on the home before the install. House is from the 70's insulated with replacement windows.

I currently have the boiler reset running with a low temp of 135°F and 180°F as a hi-limit with their fin BB.

The problem is they are using a setback tstat and are dropping it back 6° at night, and the house is climbing at a rate of 1° per hr, according to them, while the burner is firing at a rate of 20% I believe. Which is taking too long for them, and they feel the boiler running for 6-hours is going to be very expensive and want it to satisify faster.

The boiler is set up to increase temp by 10° every 20-minutes if the zone isn't satisified. So I would think there would not be an issue.

Should the low temp of the reset be bumped up or have them forget about setback, or the least a couple of degrees the most?

Thanks for the input,

Eric 

kcopp

They Have...

to change their mindset. Do away w/ the set back if they have a ModCon boiler w/ outdoor reset. The boiler willnot work well that way. I have a tekmar 260 Control and I leave my tstat @ 66F all the time.

Ross_7

Boost

Can you change the "boost" setting to a quicker ramp up time?

Mark Eatherton

Can the stat...

You can have one means of energy conservation, but not both. They don't speak the same language. One doesn't know the other even exists, and could care less.



I had both in my house, and the programmable stat was the intelligent ramping type. It finally got to the point that it would set back for one hour (with maybe a 1 degree F drop) and then start back up again. No savings.



If their bedrooms are on a separate zone, have them set that one at a lower temperature, and leave it alone. Set it and forget it.



Keep the other living spaces at a constant temperature, unless they are leaving the house for an extended period of time, and then have someone turn it back up a could of days before their return.



Were it not for your using the boost feature, their problems would be even more obvious.



PS, Have them get a blower door test and plug all the leaks they can find (within reason) and their energy use will go down and their comfort will go up. It is worth the couple hundred bucks it is going to cost them.



ME

SpeyFitter

setback

So you said they have a setback for 6 hours. So is the END of this setback period the time they wake up roughly, i.e. say they start the setback period at 12 am midnight and have it set to come on again at 6 am. Do they wake up at 6 am, or wake up at 7 or 8 am? Does this boiler have any real time to get the pipes hot, start those convective currents from the baseboards, and actually see them make a difference. This IS a low mass faster response system in theory, however the fuel saving outdoor reset kind of conflicts with the setback feature in my opinion.

The reduction in efficiency from the boost feature bumping up repeatedly to satisfy a the call for heat, repeatedly, might negate their setback features supposed savings.

My suggestion is go to a reduced setback regime for less time. Say 4 degrees cooler and for only 4-5 hours. OR you're going to have to figure out a way to CRANK the temp to the baseboards as soon as the night setback is over, which negates, again, in my opinion, any savings as a result of the setback due to the boiler running at significantly lower efficiency.

rlaggren

Question from the bleachers

What range are modulating boilers most efficient at? That would seem a significant point in a case like this.



If the boiler runs efficiently at high output then boosting for an hour after setback shouldn't reduce overall savings much - the heat all goes toward the same target temp in the morning and once that is approached the boiler reduces to "normal" temps to maintain it as the heating mass coasts up to the desired day time temps.



Also, the set back at midnight sounds somewhat late - at least assuming they hit the sack about 11pm. The heated mass will maintain for at least 1/2 hour, probably more so they could set back about an hour before bed to realize more savings w/no perceptible comfort hit.



Maybe I'm not understanding something here, but I don't see where there is necessarily a conflict between ODR (lower radiator temps) and overnight setback provided (1) the overnight setback was for a significant period of time to achieve real savings and (2) the system could "boost" effectively to bring the needed zones out of setback in the morning w/out a big "overrun" raising temps too high after coming up to temp. Enquiring minds, here.



Rufus

rlaggren

Question from the bleachers

What range are modulating boilers most efficient at? That would seem a significant point in a case like this.



If the boiler runs efficiently at high output then boosting for an hour after setback shouldn't reduce overall savings much - the heat all goes toward the same target temp in the morning and once that is approached the boiler reduces to "normal" temps to maintain it as the heating mass coasts up to the desired day time temps.



Also, the set back at midnight sounds somewhat late - at least assuming they hit the sack about 11pm. The heated mass will maintain for at least 1/2 hour, probably more so they could set back about an hour before bed to realize more savings w/no perceptible comfort hit.



Maybe I'm not understanding something here, but I don't see where there is necessarily a conflict between ODR (lower radiator temps) and overnight setback provided (1) the overnight setback was for a significant period of time to achieve real savings and (2) the system could "boost" effectively to bring the needed zones out of setback in the morning w/out a big "overrun" raising temps too high after coming up to temp. Enquiring minds, here.



Rufus

icesailor

Setbacks:

Back in the '70's, studies showed that nothing was more effective at saving energy than set back thermostats. The greater the set back, the greater the savings. Most don't even understand the idea or function of them. It is to manipulate the outside temperature against the inside temperature. If you keep the inside temperature at 70, and it is 20 outside, there is a 50 degree differential. If you turn the inside temperature to 55, then it is the same as the outside temperature rising to 35 outside. The water temperature needed to heat the house to 70 when it is 20 degrees out is hotter than when it is 35 degrees. When you get up in the morning and start using domestic hot water, the controller will go into bypass/priority and the system goes to high limit and shuts the heat zones off. What's the big deal? I want my water HOT. Life is bad if my wife doesn't have her shower HOTTER.

My setback thermostats go off at 10:00 PM and go to 55. They come on at 4:00 AM. I get up at 4:30 AM. They go up to 68. At 5:00 AM, they are getting to 68. They shut off at 8:00 AM. They come back on at 4:00 PM. For 10 hours a day, the outside temperature is whatever it is outside. For 14 hours a day, it 13 degrees warmer outside compared to what I designed it for, 70/0. If you want setback thermostats to work, you need to set the system for the colder outside temperature or higher boiler temperature.

I built a house in 1972 and put indoor/outdoor reset on the boiler. I had a terrible time with the DHW and I never saw a dime in savings When I added setback thermostats, I saw every dime that Honeywell promised on the box. And I installed a lot of them for customers and never had a single complaint about them. Except one. And that is that when it gets REALLY cold out, you must turn the thermostats up during the off cycle because in reality, if it is 10 degrees outside, and it is set back 15 degrees, you have now made the outside temperature, 5 degrees below zero. And while the system circulators are off, and the temperature is falling, you may freeze up.

I tell my customers to leave their thermostats at whatever they are comfortable with. But when it is 10 degrees outside and blowing 30 or 40 MPH, to turn up the heat. Especially at night. It's when the water isn't moving that it freezes up.

SpeyFitter

Depends on the heat exchanger

Temperature is the driving factor behind efficiency of mod-cons. The colder the water they need to produce, the more efficient they run. If you can get water temps that are 135 degrees or less coming out of the supply side of the boiler (if that's all you need) then

Depending on the manufacturer and the heat exchanger, will determine whether a boiler is more efficient and low or high fire for a given water temperature. Generally speaking, most mod-con heat exchangers tend to run slighlty less efficiently at higher firing rates than lower firing rates for a given water temperature. This would apply to Viessmann's Heat Exchanger and the Giannoni designed exchangers which seem to drop off a few percent efficieny wise at their higher firing rates.

Mod-cons with more mass with multiple layers of water tubes like the IBC heat exchanger, or fire tube designs, both of which have a high water content and significant heat transfer surfaces tend to see the same efficiency at high fire as at low fire due to the mass of the exchanger having the ability to soak up all the heat.

SpeyFitter

Setbacks versus efficiency.

I think you make a valid point worth noting towards the discussion however my question to you is, will 6 degrees difference for 6 hours make THAT much of a dent when you consider that in order to recover that 6 degrees when the setback period ends, you need to run the boiler at a less efficient water temperature? (the colder the mod-cons water, the more efficiently it runs....)

Jean-David Beyer

Setbacks.

"Back in the '70's, studies showed that nothing was more effective at saving energy than set back thermostats." I am not a heating contractor, but I think one has to be careful how one understands those studies. I am gairly confident they apply to forced hot air systems with adequately sized furnaces, where recovery from setback is reasonably quick. With forced hot water systems, if they are baseboard with hot (180F) water in them, this probably works too. For radiant heat in a slab, especially with outdoor reset, setback does not work worth an expletive. To drop 5 degrees in my house by 10 PM, I would have to start the setback around noon. To get it back up, I would have to end the setback about 6PM. Possible, but very confusing. And it would be too cold in the afternoon and too warm in the evening and part of the night. Instead, I employ outdoor reset and run the circulating hot water as low as I can get it and still make up the heat loss of the building. I judge that by noticing how long the circulators run. The longer they run, the better, provided they can maintain the indoor temperature I want. If they run any less, the water is too hot. If they never stop, I probably have them too cold (or exactly right, but that would not provide the margin needed in case of high winds).



Since my downstairs zone is radiant slab at grade, setbacks do next to nothing for me. It takes way too long (e.g., 12 hours) to recover from a setback.



So I guess I do not need to worry about freezing unless the power or the gas or the boiler fail. Provided everything works, my boiler resists freezing. If the boiler water drops below 40F, it turns on the boiler circulator, the circulator to the slab, and goes into low fire until the water temperature goes up to 45F.



The coldest it has gotten outside so far this heating season has been 17.4F, and if I remember correctly, it got down to 9.6F for a bit last year. Design day here is 14F. But since my thermostats are set to keep the place at 69F inside most of the time, I have never had a chance to see if the freeze protection works. I do do 2F setback upstairs at night where there is baseboard heat, but even there it takes several hours to recover because I run far less than 180F water in the baseboards.



As far as domestic hot water is concerned, the controller goes up to a pre-specified temperature (in my case, 170F, though they recommend 190F) to heat the hot water. This can be at any time the aquastat feels dissatisfied. So I will always have a tankful of hot water if I want it. And the recovery rate is whatever I get with 170F water. If the recovery rate were too low, I would raise the boiler water temperature to whatever I needed. The boiler is nominally 80,000 BTU/hr (input) and nominally that would all go into the hot water heater if needed. The system is domestic hot water priority.

Jamie Hall

I do so love

general discussions on setback.  They are such fun.  And almost all of them -- including the studies of the savings from setback -- completely ignore one major factor: heat capacity.



Including the heat capacity of the structure into the equation makes things more complicated, which is why it is left out.  It does, however, make a difference, because if it is included one finds that the amount of setback and its duration which will create a benefit becomes dependent on it.  Let us suppose, for a moment, two extreme situations which will illustrate the point.  First, let us suppose a structure with an overall heat capacity of zero.  Now in this case, any setback can be shown to create a benefit.  You reduce the temperature which is being held at the beginning of the setback to the setback temperature, and the space temperature immediately drops to the setback temperature.  Then, for the entire duration of the setback, you are losing heat (and thus running the boiler or whatever) at the setback rate, which is less.  Then, when the setback ends, you raise the setback temperature and, because there is no heat capacity, any firing of the boiler in addition to the setback temperature loss rate will immediately raise the space temperature to the new set temperature.  Presto.  You have saved a lot of energy.  On the other hand, let us suppose we have a structure with a very large heat capacity.  You chenge the temperature setting to the setback but, because of the very large heat capacity, the temperature drops very slowly (it would not drop at all if the heat capacity were infinite, but we'll try to be reasonable here).  True, the boiler is not firing and will not until the setback temperature is reached.  If the heat capacity is very large, this may not be reached at all in the time the setback is set for -- the boiler won't fire during the setback.  Now you set the temperature back up.  The boiler must now fire at a rate sufficiently in excess of the heat loss at the upper temperature to bring the entire structure up to the upper temperature.  If we make the further assumption that the boiler is designed to fire, at 100% duty, enough to hold the upper temperature and no more (design conditions, with no safety factor) what happens?  The space temperature will never reach the upper set temperature, and the boiler will fire continuously.



There are many variables in between; enough to make modelling of this behaviour interesting (but not interesting enough for me to do it right now).  The bottom line, however, is that there will be, for evey combination of structure heat loss and heat capacity characteristics and boiler efficiency, efficiency variation with firing rate, and capacity, plus every combination of outside and inside air temperature, a setback temperature and duration which will save no energy at all.  In general, a longer setback will save energy, and a longer setback will allow a lower setback temperature for the breakeven point.  But there is a maximum length of time for the setback for residential spaces -- around 6 to 8 hours, when folks are asleep.  Thus for each installation there will be a maximum setback amount which is optimal.



And it varies for each individual situation!!!



I might point out that for structures which are unoccupied for longer periods of time, such as schools over the weekend, or some churches during the week, much deeper setbacks during those times can, and should, be considered, all else being equal.  Other structures, such as those which house temperature sensitive objects which don't like relatively quick temperature swings (such objects include musical instruments, paintings, valuable books) any economic gain from a setback may be offset by the damage to the objects.



So... there is no general rule.

Eric_32

Thanks for everyones thoughts on this...

I've enjoyed everyone's thoughts on this post. Just got off the phone with the customer. She said they have tried to raise the set back temp from 60° to 62 or 63°F and found it extremely warm to sleep at night. Since they've been doing it that way for so many years now. Had to throw off the blankets and was very unconfortable however found the boiler had heated the home quicker than it had done at 60°.



Yeah I know, 63° is brutal? What are ya going to do?  Just trying to keep the customer happy.



So at this point I told her I am waiting to hear back from Knight on the subject, and will get back to her and get it resolved in a day or two. At this point it is looking like I might increase the time the boiler runs on it's normal reset temp "before ramping"  from 20-minutes to maybe 30, and increase the temp that is "ramp up" from 10°F to 20° or 30°F. Hopefully it will only ramp up after the night setback, and not during the daytime, which she says has been extremely comfortable...... of coarse it has.... right? Thats what it's suppose to do ;)



Hey thanks again and look forward to any further recommendations or comments on this.



Happy Holidays to ALL

Eric