Boiler cycle length

jalcoplumb_7

You have 5 zones. Why? Run the system as one zone and let the boiler modulate like it is supposed to.

How is your insulation? How are you mixing for your radiant zones.

"I did the heatloss calc and I rounded off to about 47000 btu using slatfin heatloss software."

Why a 150,000 btu boiler? Yes it modulates but on mild days with five zones you are in for some stop and go. What is the low fire on the 150?

Zoning is not all that it is cracked up to be.

Joseph A Landree
JAL Co. Plumbing & Heating.

Doug Clark

Boiler cycle lenght

This my first time posting here. I was wondering if any one could tell me why if a properly sized boiler is running under design conditions it should run 100% of the time? I understand that the load should be preportional to the boiler but wouldn't that use a lot of gas to do this. I am missing something....

Uni R_2

Why 100%?

Most conventional boilers take a while to get up to full efficiency. Every time they go through a heat cycle a portion of that cycle includes periods of inefficient operation (and this can be after firing as well as when it first fires).

In order to keep these cycling losses to a minimum, boilers are sized to minimize this cycling. In a perfect state, it should fire for 100% at design conditions.

This seldom occurs in practice because Manual J calculations typically overstate the heat loss and don't factor in the internal heat sources. Some contractors fear that they may end up with a customer who wants a 75°F internal temp at design. Some municipalities like mine insist the IBR (which is a VERY conservative rating for an INDOOR boiler) be used. Boiler sizing also creates an issue in that the jumps in capacity for any particular model tend to be quite large.

Bottom line is that a house requires "X" BTUs to stay warm. The boiler burns "Y" worth of BTUs to provide "X" BTUs for the heating system. The more the boiler can limit those cycling losses, the closer those 2 numbers will be and the less gas will be wasted in the process.

Mike T., Swampeast MO

Boilers are sized somewhat like commercial pilots add fuel. They don't want to land with an excessive amount of fuel in the event of a landing/taxying accident but they add extra for weather (like sizing such as Manual J), extra for possibly being caught in a holding pattern at the landing airport (like for people who want unusually high space heat temp) and some extra "for mother" (like Dan Holohan's famous statement of, "The drive to oversize [boilers] is stronger than the sex drive on most days."

Put all this together and even a well-sized boiler won't be running 100% of the time while maintaining indoor temperature under design conditions. From my personal experience as well as comments from people here who have FAR more experience, 50% run time under design conditions while maintaining indoor temperature is typical--if not expected.

This "normal" oversizing, more than anything, is why modulating boilers are so desirable. "Properly" sized with a maximum output near the design load and anywhere reasonable control, the modulating boiler will come at least close to matching its output to the load. The Vitodens by Viessmann is however quite special in this regard as when installed according to instructions it will come VERY close to matching output to load on a real-time basis when maintaining space temperature.

Glenn Sossin_2

Are you burning more fuel ??

Agree with all the above, but I think what your really asking is, if the boiler runs almost all the time at design conditions, arent I burning more fuel? In a simple approach, the answer would be no.

Not accounting for boiler efficiencies, specialized controls, and assuming proper radiation is installed, lets look at it in a very simplistic method, which I think will answer your question.

If the load of a house is 100,000 btus for a given hour, and the boiler delivers 200,000 btuh, what do you think will happen? Talking conventional equiptment now - its making heat 2 times faster than the house really needs it -but it won't over heat. Why, because the thermostat turns it off. In theory it will run have half the time, or 30 minutes. How much fuel did we burn? We consumed enough fuel to deliver the 100,000 btu's right?

Now lets make believe we have a boiler half the size, in the same house firing under the same conditions. If the boiler delivers 100,000 btu's into the house in an hour, it would have to run for the full hour. So if it runs the whole hour, how much fuel did it use? Enough to deliver 100,000 btus of heat into the house. The same amount of fuel, just burned/delivered at different rate. A btu of heat energy is a btu of heat energy regardless of the firing rate.

Its like filling up a gallon container with a flow of 1gpm, versus .5gpm - you still end up with the gallon container, the first way one took a minute, the 2nd took 2 minutes.

In reality, as Uni R points out there are a multitude of factors which affect this - boiler efficiencies, cycle times, types and amounts of radiaton. The point is, in a very general sense, a boiler that runs for the full hour at design conditions is probably operating very efficiently - steady state efficiency will have been reached.

Hope this answers your question.

Doug Clark

Thanks guys,
That sure answered that question!!! So I guess I have a bigger problem than I thought. I have a house that has a radiante system that is about 50% complete. I had to run slantfin baseboard upstairs (about 40 running ft.) because I could not get radiant under floor there, it is individually feed per room. I use a NTI Trinity 150c modulating/condensing/ boiler w/outdoor reset the gas is propane as I couldn't get natural. Before I installed the radiante system I was running with oil forced air, the oil cost me about $3500/year. With the new system I had a propane bill this year of 8500. I recorded the run time for the boiler one day when it was about -25* c and the boiler would run for about 35 min and shut down and then repeats after about 5-10 min. The water temp is set at 160* and the system uses tekmar 508 thermostats and has 5 zones controlled by TACO VLZ 406 zone controller. I am at a loss as to why it uses so much fuel!!! This may sound stupid but is because I have only 50% done? The unit did keep the house up to temp.

Uni R_2

Something's wrong...

Based on the wording of your original post, it seemed like you were asking about conventional boilers. The fact you are talking about a M-C boiler changes things. A Trinity shouldn't take long to warm up - the cycling efficieny losses on it should be fairly minimal. You have bigger issues.

What's the approximate price per litre that you paid for oil and propane?

Has the external envelope changed? What's the measured heatloss? How many square feet of heated space do you have?

Has the comfort level inside changed (same inside temps all winter long)?

The total gross BTUs needed to heat your house should have dropped between an oil furnace and radiant with the Trinity! Unless the propane is leaking somewhere, being used for something else or costs 2½ times or more the price of oil per BTU, there's a bigger issue here than cycling or even sizing.

Doug Clark

Hi Uni R,
I paid .76/Ltr on average for propane and did not turn on the oil furnace all season. The external envalope has not change at all. The house is an old victorian 2 story house that has gone through a few renovation in its time, I did the heatloss calc and I rounded off to about 47000 btu using slatfin heatloss software. The square ft is 2600 upper and lower floor total. The Inside temp was relatively stable at about 68 - 70 deg. But I tell I didn't have consistant temp through out the house because I didn't have enough heat surface. The other things that might be important are that the temp diff between supply manifold and return was only about 12deg and my system is Primary/ secondary setup but both pumps came on when boiler started and then turned off when it quite.
Thanks

Uni R_2

The numbers are bizarre...

Okay, I'll assume 70¢ for a litre of oil and we'll skip GST for now because the numbers are so far out that 6 or 7% mean next to nothing.

If your house was costing $3500 a year on oil at 70¢ a litre then your gross consumption was 180 million BTU for the season. On propane with radiant and a high efficiency boiler it has managed to go up to over 280 million BTU for the season.

My calculated heatloss is a bit higher than yours. The climate is the Toronto area (roughly 7000 annual HDD) and when on oil, my gross consumption was about 125 million BTUs per season and since going to NG they are now around 65 million BTUs per season.

Given that a litre of propane has 25K BTUs and a litre of oil has 36K BTUs, you'd think that at worst, your heating bill will be around $5400 and that's without taking into consideration any efficiency gains... just swapping propane BTUs for oil based BTUs.

You're paying for way more energy than you are using.

Doug Clark

Ya you are right that the numbers just don't work! what have I missed.......Leak maybe but that would have to be outside because I have leak detector inside.

heatguy

the trinity boiler is a little strange with the wiring for the primary and secondary pump if your primary and secondary pump are cycling at the same time you may want to check your wiring

Doug Clark

I was wondering about that? Seems the manual says that you can use the purple wire for secondary but I traced it back and it runs the DHW. Now saying that with this huge consumption of propane I am using I disconnected the DHW because I thought that was the problem. So could I use the disconnect the wire and use it for the secondary? But has anyone got an idea on the consumption problem? My wife is about to kill me if I don't figure this out!!!

Plumdog_2

For your headstone:

> I was wondering about that? Seems the manual says

> that you can use the purple wire for secondary

> but I traced it back and it runs the DHW. Now

> saying that with this huge consumption of propane

> I am using I disconnected the DHW because I

> thought that was the problem. So could I use the

> disconnect the wire and use it for the secondary?

> But has anyone got an idea on the consumption

> problem? My wife is about to kill me if I don't

> figure this out!!!

Plumdog_2

Your wife is going to kill you

The Ti150c model has a heat exchanger for the DHW; but no purple wire. If you have a DHW indirect tank, then you would use the purple wire to power the Indirect Circulator. So you might have the standard model rather than the Combi. Look very closely at your piping schematic in the installation manual. The DHW pump takes over the primary pumping chores when the priority call comes on. Then the Primary and Secondary(s) resume when that chore is accomplished. The check valves are a must! Pressure test the gas lines inside and out! Make sure the LP kit is installed in the boiler! Have the burner adjusted with an analyser!

Doug Clark

I have checked the Unit for leaks from the propane tank to the boiler (no leaks)Got the boiler cleaned and checked out, The guy siad the boiler was not that bad. I also phoned the manufacturer of the bioler and they said to maybe check the circulator size because it maybe to large because the differentail is only about 10* (supply and Return). How do I check that? And do you guys have anymore ideas?

Uni R_3

ΔT of 10 steady state isn't bad...

You've got a bigger issue than ΔT. First things first, was the unit tested with a combustion analyzer? If it has and it checks out, how much heat is going out the flue? If that's not it then you leaking a ton of heat and dollars out of your envelope - perhaps there's an underground water table issue or something. If you've verified that ypu're conevrting all your fuel and it isn't going out your flue in excessive amounts then it is being lost by the structure.

Uni R_3

ΔT of 10 steady state isn't bad...

You've got a bigger issue than ΔT. First things first, was the unit tested with a combustion analyzer? If it has and it checks out, how much heat is going out the flue? If that's not it then you're leaking a ton of heat and dollars out of your envelope - perhaps there's an underground water table issue or something. If you've verified that you're converting all your fuel and it isn't going out your flue in excessive amounts then it is being lost by the structure.