HTP Elite FT boiler not modulating low enough...problem?

Nom_Deplume

We got a new HTP Elite FT.  The control is pretty much the same as our late unlamented Munchkin, with a few small improvements. Among other things I can now see the modulation level (%) on the status screen.



The unit is supposed to have a 5x turndown, i.e. lowest level should be 20%. However, the displayed modulation level never goes below 30%. As a test, I set the water temperature really low, 85. The return temperature is 81. Delta-T at full fire is 23, so (85-81)/23 = 17%.  But the burner never goes below 30%. It doesn't go on or off as far as I can tell from the display; it just sits at 30-31%. This makes no sense to me.



Is it losing the extra heat somewhere?



There's no one from HTP on these forums, is that right?

Zman

delta t

The delta t in a radiant slab is not usually very consistent. It will be much higher when the zone first fires, then drop as the slab warms. It will also vary at different water temps. I don't think you can use it as a constant.

As long as your boiler is not cycling, you have to assume it is matching the load. Can you throttle some of the radiant loops to reduce the load for your test.

Not sure you have a problem.

Carl

Nom_Deplume

no radiant loops

There are no radiant loops, only cast iron radiators. I never treated deltaT as a constant. Also I am only reporting deltaT after things have settled down (become stable).



But I should have made clear that I was talking about the boiler deltaT (deltaT in the primary), not the deltaT from the radiators (secondary). So the nature of the radiation (and whether the load is being throttled)  is not relevant.



Water comes into the boiler at T1 and goes out at T2. (Both these numbers can be read at the boiler front panel.) The difference deltaT =T2-T1 depends on the BTU being delivered to the water as it goes through the boiler, and nothing else. It is not a constant, but it depends only on the modulation of the boiler.



When the boiler is running at 100%, I see that deltaT is 23F (reasonable). When the water temperature is set at 85F, deltaT is 4F (reasonable), but boiler is running at 30%. It should in fact be running at (4/23)*100% = 17%. (Roughly---of course it can't modulate below 20%.)



Another way to see the problem is that if it really requires 30% fire to heat the water from 70F (room temperature) to 85F, then at 100% fire it will only manage to heat it to 70+(85-70)*100/30 =120. That's not true, it actually gets to 180F without a problem; so why is it running at 30% when the water output is 85F and the input is 81F?

Zman

Mod/Con

Every mod/con has it's own built in algorithm that controls the output rate. My point is, if the boiler is not overshooting the target and is not cycling, you don't have an issue. You must have a load equal to the output. The energy is going somewhere.

Carl

SWEI

If it bugs you

you could always put a ∆T pump on it.



Rinnai (ATAG) does this in their Q series boilers with the onboard pump and a ∆T target of 35F.  First one I've seen do that here in the US -- will probably give them a try on the next job.

Nom_Deplume

not the issue

I can increase or decrease the primary pump speed (it's a 3-speed pump). If I do, the deltaT at full fire goes down or up (naturally).  The deltaT at 30% fire also goes down or up. Their ratio remains the same, and so the problem I described remains exactly the same. It has nothing to do with the pumping speed.

Zman

Problem

I guess you need to be more clear on the "problem".

It sounds like you have sized the boiler to the load very well and the boiler is purring along. I think the mass of the cast iron rads is pulling the heat out nicely even at low temps. Most folks come here wanting to know how to make there system stop cycling and function as yours does.

Nice Job,

Carl

SWEI

It has everything to do with the pumping speed

and a ∆T pump will manage that dynamically, which will maximize the boiler efficiency.

Rich_49

Piping

ended up being Primary / secondary ?  Sounds like the Delta T in the the primary loop is low . The energy is being taken out somewhere . I agree with the others in te fact that a nice long efficient cycle is a good thing , the only thing better is when the boiler is not firing at all .  

Nom_Deplume

I'm trying to explain

Thanks to everyone for the feedback. Clearly I am doing a lousy job explaining the problem. I realize that this is not a calculation that people do normally.



I am NOT complaining that the cycle is too long---that is of course good. It is NOT true that deltaT (difference between boiler supply and boiler return) is too low--- 3 or 4 degrees it is just about what you expect when the water temperature is holding steady at a cool 85F. (DeltaT will go up to ~20 degrees as the water temperature goes up to 180F in cold weather.)



I am saying that the boiler firing level is way too high (at 30%) in relation to the low T (85F) and low deltaT (4F). It should be about half that. I showed my work in earlier posts. I am concerned that the extra heat is going up the chimney for some reason, but probably I am wrong. Where's my mistake?



I am going to monitor the gas meter and see if it is really running at 30%.

Nom_Deplume

looked at the gas meter...

I clocked the gas meter when the boiler is running steadily (and nothing else is), It takes a half foot in 82 sec,, which works out to to 22,000 BTU/hr. For this 110,000 BTU boiler, that is exactly 20% (the minimum fire possible, since it has 5x modulation).  It's not 30% even though the display shows that...



I feel better now. The display is wrong, but I'm not losing energy up the chimney.



Many thanks to everyone for the help.