Direct vs P/S

DC123

For the KHN-085 or the UFT80, are there are any set-ups where they could not be piped direct? Would like to avoid P/S if possible due to added expense and energy use of the extra pump, but I don't want to compromise the overall functionality. I have two zones with about 75 feet of fin tube per zone (total 150 ft). Piping seems complicated and some of the baseboard seems to branch off as 3/4" from a main loop of 1" pipe. Currently, the existing cast iron boiler is pumped by a series 100. Please let me know if any pictures or additional details would be helpful.

EBEBRATT-Ed

This is just my own opinion and I have no experience with either of those boilers.

I prefer primary secondary for almost any application. I like the idea of having the correct full flow through the boiler at all times regardless of the load, how many of zones are calling or the position of any zone valves.

I feel this is better for the heat exchanger on most any boiler due to the low water content of the modern boilers

Gordy

System side must be more gpm than primary side. With that said in a direct piped setup. You must be able to accommodate the min. Flow rate the KHN requires.

So you must think about the smallest zone if zoning, and it's flow rate requirements. The KHN, and the uft are suppose to be able to go as low as 1 gpm. That's a large flow rate for a small zone depending on the requirements.

Solid_Fuel_Man

I like to be able to direct pipe a mod con if the system can flow the required min flow. It will keep return Temps as low as possible therefore keep it condensing as much as possible. That said, pri/sec provides complete hydraulic separation so no attention needs to be paid to the system flow rates. That adds some insurance and simplifies an existing install if you look at it that way.

Most if not all installs I have done with pri/sec have had higher return Temps to the boiler than the system return temp. In fact that is the main reason some are done pri/sec is for low return protection on non condensing equipment. That reason alone makes me think it would hinder mod-con efficiency where boiler return Temps are directly related to efficency.



Taylor

Solid_Fuel_Man

So are you agreeing with me Hat? I do not care for pri/sec on a boiler which can handle system flow rates directly. Isn't that one of the biggest advantages to a fire tube heat exchanger mod-con anyway?

Taylor

DC123

Ok, thanks. So no red flags about piping either of those two boilers direct with what I've described of my baseboard system and using, say, a VR1816 on one of the constant pressure settings? I'm not sure how best to measure the resistance in the zones/system as I don't know the exact length given how much is in the walls. How would one guarantee the minimum flow? Just by making sure the delta-T on the boiler isn't too high on the various permutations of open/closed zones and adjusting the pump accordingly?

DC123

I know that you had difficulty with the UFT-80W and the VT2218 pump, but has the KHN085 been tried with this pump in a direct-piped set-up as yet?

In a P/S set-up, is there any benefit to using the VT2218 either in the primary or secondary loops?

DC123

Ok, thanks. Not sure I'd want to be the guinea pig on that one, so sounds like sticking with the VR1816 for all the aforementioned possible uses may be better.

Out of curiosity though...the manual of the VT2218 leaves me a bit confused about the operation. It states:
In Delta-T mode, the VT2218 will start at the lowest speed. If it reads a temperature differential (∆T) LESS than its target, it will
gradually increase speed through a 3 minute “Start Cycle” to its highest speed, until target ∆T is reached. “Start Cycle” will display
on the LCD screen in place of “Watts”. If ∆T is GREATER than target, the pump will immediately go into Variable Speed
operation and vary speed to maintain the target ∆T.

Wouldn't a slower speed increase the delta T as the return water has more time to cool? Why would the pump start slow and then speed up as opposed to vice versa?

DC123

One more hypothetical question - assuming you use the VT2218 as the primary loop pump, how would you know where to set the desired delta T? I assume higher is better and the high limit is determined by the minimum flow required by the boiler...but how would you know what flow is being put out by the pump at a given delta T?

DC123

Do you think that the VT2218 would work better with the CU3A due to the increased mass? Would that smooth the temperature fluctuations out enough to prevent the issues you noted with the response speed difference with the UFT-80? Trying to figure out if it's crazy to put in a boiler with a minimum of 19k in a house with a heat load at 15F design day of 45-50k versus a low or midmass boiler that modulated lower, but if the Veissmann is easy to work with, can work well piped direct with a VT pump, and is reliable, maybe not crazy.

DC123

What about if you marry it with some modest setbacks (assuming that the recovery temperatures are still within condensing range)?

NY_Rob

Setbacks will only increase the number of times you'll short cycle because now it has to burn longer to make up for the setback.

DC123

Maybe I'm misunderstanding, but I would have thought a longer burn, assuming you're not exceeding the capacity of the emitters, would result in less cycling?

NY_Rob

Look at it this way....
Just to get the numbers right- you have 75' of fintube per zone right?
At 120F SWT that gives you 110F AWT (Average Water Temp), according to the Slant/fin chart 110F water can radiate 140 BTU's/ft. So your 75' of fintube will radiate 10.5K BTU's.
If your emitters can radiate 10.5K BTU's and your boiler can only fire down to 19K BTU's output... it will hit high limit and stop firing till the water hits the CH differential setpoint and causes it to fire up again... and this 'cycle' will continue till the t-stat is satisfied... which will happen sooner if it's only making up for one deg vs. several deg of setback.

NY_Rob

^ that is also assuming you're not over-pumping your emitters in which case they would shed even less BTU's.

I agree with Hat... we need lower firing Mod-Cons.

DC123

Sure, but in reality, the zones should probably be treated as one (they're both 2-floor zones and the thermostats are both on the first floor - piped like that due to an addition). On re-measuring, it's probably closer to 125 ft. So that puts it at about 19k BTU. In that context, I'm wondering whether a boiler with high mass and a min fire at 19k would still do ok if modest setbacks were used - and potentially could make up for the oversizing given that the design day heatloss is only 50k and the heatloss at 40F might only be 15-20k.

DC123

Re setback - it seemed to me that if the boiler needed to come up, say, 3 degrees over a few hours, it could do that by raising the water temp from 120 to 140 and still be in the condensing range on the RWT with only a small efficiency hit. And I assumed it would cycle less given that the load to raise the house 3 degrees rather than maintain a set 68 during that time period would put it above its minimum modulation (assuming an outdoor temp of 45F). I may be thinking about this incorrectly though.

DC123

I guess that assumes that the boost feature can be set with a maximum SWT - not sure if that's true.

hot_rod

NY_Rob said:

^ that is also assuming you're not over-pumping your emitters in which case they would shed even less BTU's.

Now be careful with this statement.


Increasing flow rates, in most emitters, fin tube, radiant loops, fan coils, gives you higher outputs, as that graph clearly shows. The additional flow rate increases the average emitter temperature.

Granted, pumping power to get you there may not be acceptable and staying under 5 fps velocity is important assures noise free operation.

Excessive flow in the boiler can lead to short cycling and loss of efficiency related to that.

I think we all agree on a 20K or lower mod con.

I know a few contractors installing these small "boilers" on oil fired (diesel) RV hydronics. These German built versions look to be high quality.

I agree with Hat... we need lower firing Mod-Cons.

DC123

So, my read from the manual is that it's the latter. What do you think?