Do I need a primary loop? (Lochinvar Knight)

[dwassner]

We just had our Lochinvar Knight WH-055 installed. The pump is a Grundfos UPS15-58FC, its a 3 speed and is currently on medium. The contractor (a good friend of mine) did a very clean install. He is not up to date on the newest mod cons and will admit it, which is why he called Lochinvar around 8 times during the install to clarify terminology and verify the install would be correct. I don't have a camera so I can't provide a picture yet. He approached me a day ago saying that some people in the field he knows were shocked that he did not do a primary/secondary loop. To describe it, he ran 1" copper and tee'd off of it with each zone (currently 2) and it terminated into a zone. The zones all return into a 1" copper tube that returns to the boiler. The 1" supply and connected through the zones only (I hope this is understandable.) Each zone is roughly 120 feet long and it is currently 3/4" copper to baseboards. I planned on switching to pex.

My understanding was that if the supply from the boiler immediately returned then this would act like a wide open mixing valve, eliminating any ability to have condensation. I thought that you wanted as much delta t as possible (to a point). The people he spoke to are saying that the pump cant pump the water fast enough and the boiler is going to destroy itself. Currently, the boiler has been running fine for the past few weeks, and the delta t safety shutdown has not come on.(i know it works, long story)

My biggest concern is that Lochinvar verified that this would be acceptable as a set up...

What do I do if anything?

[BadgerBoilerMN]

One of the reasons we design, install and service the Lochinvar Knight condensing boiler is the companies' liberal policy on the need for primary/secondary piping. Lochinvar will allow the use of direct piping to the distribution system as long as you provide the minimum flow required by all low-mass (not cast iron) heat exchangers. Like the use of anti-freeze, there is little to be gained beyond this the critical need to take the heat away from the heat exchanger and much to lose, such as the added cost of a redundant pump and the electricity is burns and as you astutely noted, the possibility of elevated return water temperature and resultant loss of potential combustion efficiency.

All condensing boilers now have differential temperature sensors that help monitor the critical minimum flow required and unlike the early condensing boilers will allow a higher differential without sustaining damage but the installer must assure this minimum by carefully calculating all potential flow rates. Multi-zone systems-especially those with small load or narrow long loops such as a radiant floor for the master bath for example-must be used with caution. We will often tie zones together and the call for heat only allowed on our minimum design load.

We also use differential by-pass valve on many systems. The DB is closed when all zones call allowing full flow through boiler and system and opens (by-passes back through the boiler) incrementally as zones are satisfied.

The people who sell condensing boilers should be helping a legitimate contractor with the near piping design before he starts the job.

We installed the first series of Knight boilers sporting the early and ubiquitous Ginanni "water-tube" boilers, which were much more restrictive than the new fire-tube models, but even so, rarely used primary/secondary piping. The WH-55 needs a modest minimum flow of 3gpm (found in the charts provided in the "Installation Manual" provided with every unit). With one-120' copper loop at 3gpm and approximately 1.5psi/3.5 fthd the up1558 will easily handle the load. The small condensing boilers tend to be easier to pipe for gas and water and as such very forgiving.

For others that may be watching, it pays to do the math before you hang a high efficiency condensing boiler on the wall.

[dwassner]

Thanks for the reply. Is there a max delta-t that I can reach without damage (not to shoot for, but to be cautious of)? Like I said the pump is on the medium setting and I'm curious if it should be on the higher one. I did a great deal of research and up to the time of running the unit thought I would be able to program it myself. I still believe I can but there is terminology I am not familiar with, and a few concepts. Is there anywhere on-line that explains how to set the unit up? For instance, what to set the set-point or outdoor reset parameters at and why? I would consider pursuing a professional who deals with these, but I have a difficult time placing faith in contractors that I don't know, and I'm still convinced I can do it.

[BadgerBoilerMN]

Read and understand the installation manual, which shows a chart at 35° delta T. When in doubt, call someone without any.

[smihaila]

Hi BadgerBoilerMN,

I am planning to have the same Lochinvar model WHN055 installed at my house. The Manual J calculation shows a heat loss of 34 kBTU, so a boiler with max 55 kBTU and featuring a 5:1 downturn conversion factor (i.e able to fire for heat demands of at least 11 kBTU - which would be about 1/3 of my house, that is one floor out of total 3 floors if we include the basement), I guess it would be fine. Also, considering the Lochinvar Knight WH firetube design (warranting very low pressure drop/low hydraulic resistance), we could pipe it in a direct/full flow configuration - without a P/S circuit that is.

Basically, we would go with the same piping example from the Lochivar's Installation & Operation manual as the one that the original poster has indicated: Figure 6-10, page 45 (in the latest version of the manual / revision K).

And now the question: how were you able to deduce that the REQUIRED minimum flow rate for the WHN055 boiler model is 3 gpm? I tried looking for that info all over the manual, and it was never mentioned explicitly.
All I could find were those "pressure drop vs. flow" charts, shown at page 36. Plus the "Table 6A" located on the same page, below the charts. You were stating that the min flow would be 3gpm, infered from the suggested flow that was indicated for WHN055 @ Delta-T = 35F. To me those values look rather like maximum gpm values, not min values.
They match the universal hydronic formula very well. For example, for a max firing rate of 55,000 BTU, the flow would be 5.5gpm @ Delta-T = 20F, 4.4gpm @ Delta-T = 25F, and 3.14gpm @ Delta-T = 35F respectively. I'm thinking that those table are considering the worst case scenario - for pump sizing - i.e. the max pressure drop/feet of head that the pump would be required to satisfy.

I am puzzled - 3gpm as min flow rate would look way too much for a boiler like this. It's very interesting that even for other boiler brands, such as Viessmann Vitodens 200 WB2B-19 (rated for max 67,000BTU), I am able to see only the max flow rates, as follows:

20 Rise 6.1gpm @ 16' Head 6.1 x 20 x 500 = 61,000 Btu/hr
25 Rise 4.9gpm @ 12' Head 4.9 x 25 x 500 = 61,250 Btu/hr
30 Rise 4.1gpm @ 8' Head 4.1 x 30 x 500 = 61,500 Btu/hr
35 Rise 3.5gpm @ 4' Head 3.5 x 35 x 500 = 61,250 Btu/hr
40 Rise 3.1gpm @ 3.5' Head 3.1 x 40 x 500 = 62,000 Btu/hr

I'm curious, is this min flow rate something that someone has to infer from a max gpm flow value that corresponds to the highest delta-T indicated by the manufacturer?

If that's the case, then both Lochinvar Knight WHN055 and the Viessmann WB2B-19 show approx the same min flow rates - 3gpm.

Do you think that WHN055 would be appropriate for my heat load ? (11 kBTU per floor, max heat loss of 33kBTU in total).
The emitters will be steel panel rads, sized (over sized) for a supply sys temp of 130F and return temp of 110F. And room temp of 76F degrees.

I am thinking that @ 33kBTU and a system Delta-T design of 20F, the required flow rate would be 3-3.3gpm - which is very close to that min required flow.
If I were to go with higher delta-Ts, the situation would get even worse, right?

What do you think? Is that boiler ok for my load and also would I be able to afford NOT installing a P/S circuit?
The other option would be Viessmann WB2B-19, which starts firing at 31kBTU and maxes out at 67kBTU (that's the smallest model that they have for North-America). But it doesn't look too good: I would need P/S for sure, and have to use a Low Loss Header, and besides won't be able to modulate down enough so with high probability will short-cycle, right?

Thank you.

[BadgerBoilerMN]

Primary/Secondary piping on boilers was originally designed to keep conventional boilers hot so they did not suffer sustained stack condensation. Just the opposite of the goal we seek in condensing boiler system design.

Having started 25 years ago with the first high-head, low-mass, non-modulating condensing boilers; the new modulating water-tube boilers such as Burnham Alpine, IBC and Viessmann are a picnic and the newest fire-tube boilers like the Lochinvar Knight, NTI and Triangle Tube with even less pressure drop are a picnic.

Careful that the proper use of a single pump does not void your warranty as there are several mistakes to be made that will not manifest themselves for many years. The addition of a forbidden piping arrangement may leave you without help on a legitimate claim. Buderus, Weil McLain Slant-Fin come to mind as they mandate P/S, professional installation and annual maintenance.

It is good to do the math, but pump placement, load sizing, air elimination, controls, all play an important part when you decide to used advanced design techniques generally discouraged or forbidden by the manufacturers. We can help but would have to see the number directly. We design and install 16 different brands of condensing boilers here in Sunny Minneapolis and find they all have their niche and their foibles.

Keep in mind that many manufacturers caution about minimum AND maximum flow both in the stainless steel e.g. Viessmann and aluminum e.g. Crown Bimini. There are systems that will benefit from P/S, but most will not.

I wish all the professional installers would read and understand the installations manuals as well as you fellas have.

[smihaila]

Thank you for having the time to answer.

In terms of calculations and math, the data is quite simple in our case:
-33 kBTU max heat load
-only one heating circuit (no need for DHW)
-system design temp of 20F, this making it 3.0 GPM needed flow rate
-the max flow rate for Lochinvar Knight WHN055 @ 20F boiler temp rise and full fire (of 55 kBTU) is 5 GPM. Since in our case we need only 33 kBTU, then our flow of 3GPM is well within the range.
-direct flow connection (no P/S or LLH)
-preferably, we'd like to use a variable speed pump (only one for the whole circuit / no separate boiler pump), and preferably an ECM pump. WHN055 features a "0-10V BLR PUMP OUT" signal, which can modulate the speed of an external varspeed pump - with the objective for the boiler's internal control to achieve a "const Delta-T" algorithm. With the Delta-T range between 15F and 25F (I understood it can be configured via a separate PC software).
-14 steel panel rads installed using a combination of manifold and parallel reverse/return piping
-ALL piping would be copper, type L.
-the piping near the boiler will be 1''.
-the piping going into the rads would be 3/4'' for the parallel reverse/return (the main route) and 1/2'' for manifold loops and the Tee-ed connections from the main parallel reverse/return route to each of the rads.
-there will be about 120-150 feet of copper piping I estimate
-no zoning
-no zone valves
-of those 14 rads, 3 will be bath towel rads
-of those14 rads, 5 will have TRVs (thermostatic heads).
-I estimate about 11kBTU heat demand per each floor.

I have asked a Lochinvar representative about the min flow rate that WHN055 can tolerate without getting the water flashing into steam. And he wasn't able to respond. It's nowhere in the manual.

Also, the manual states that there is 2nd plug in the board, labeled "0-10V SYS PUMP IN", which would be able to read the output from a var speed pump. And with the main objective being a correlation between the the pump in the primary circuit (if that's also a var speed pump) and the pump from the secondary/system circuit. Which could help the boiler in understanding what's going on in the secondary circuit, and match the delta-Ts and the flow rates between the primary and secondary.

In other words, it works only with hydraulic separation, and ONLY if you can get a more sophisticated var speed pump which is able to deliver a 0-10V OUTPUT signal. I have tried to find such pump, but in vain.

Let's say that, if we go with direct piping, that 2nd pump is out of question. But even for the case of a simpler varspeed pump, such as the one accepting 0-10V INPUT (fed by boiler), it is almost impossible to find an ECM pump, small enough with such input interface. Only the Wilo Stratos commercial models or the Grundfos Magna series have such type of interface. And it's optional.

What's your opinion? What ECM pump would suit our needs the best? How much feet of head loss do you think I will have? Would a direct piping work fine?
The WHN055 head loss itself is about 0.27 feet of head for 5gpm flow @ 20 temp rise and full fire.

Lochinvar's manual is ok with direct piping, so no warranty issues.

Thank you.

[BadgerBoilerMN]

We design and install all kinds of hydronic systems here in Minneapolis and use Lochinvar Knight boilers to drive panel radiators in new construction and renovation. As far as flow rates are concerned one has to pay attention to minimum and maximum flow rates to protect boiler and those flow rates will affect output as well.

The pressure drop through any low-mass condensing boiler is a known quantity and can be used to size minimum circulator size or speed. Naturally, if you intend to use a single pump, as we often do, you must have experience or help from someone with advanced hydronic design skills.

Beyond the pressure drop through the boiler you must consider zone load, flow and controls.

We perform heat loads, size boilers, pumps and provide drawing for owners and contractors everywhere but do not design-on-line with data we did not generate. You have the right ideas but should get professional help before investing in expensive materials. If you have it all right, good on you, but if you make simple mistakes, warranty, longevity, reliability, efficiency and safety all come into question.