Confirming mod-con install plans.

rconkling · October 2022

Sadly my attempt to keep my old Dunkirk safely fired has neared its end. Due to the prohibitive cost to keep it in service.

So I'm back to plan A, adding a mod-con to a converted gravity system with TRVs.

But Id like to confirm some things.

I met with the installer yesterday to discuss the situation and set expectations. When I brought up boiler piping and requested full flow piping, he hesitated. Stating he always does P/S loops, because not doing so causes the systems to not work well. But I insisted on what I wanted. It has since been confirmed that they can fulfill my request.

Ive been reading about this for about a year. Digging through this forum and others, as well as watching Coffee with Caleffi and TACO Tuesday. Ive read through relevant sections of Caleffi idronics and the Lochinvar I&O manual. It seems as though my requested approach is a sound one. As I would like to maximize efficiency, while minimizing cost and complexity.

The contractor has done a heat loss calc and spec'd out a Knight KHB085N. I calculated a total EDR of 634.

Looking at the I&O, the piping diagram that is closest to my system is this one sans DHW:

My TRVs will represent the zone valves, with the addition of the optional PBV.

My understanding is that it has to be piped this way for the warranty to be valid. So the "pumping away" as a best practice is out the window, what affect will this have on system performance?

This boiler comes with a variable ECM Grundfos pump, UPML 25-104. Is the pump adequate? Is there a better unit available? Am I stuck with it because of the internal pump controls of the boiler?

https://files.danfoss.com/download/Drives/UPMXL - USA 115V_.pdf

The sales literature exclusively mentions ▲T as the circulation method, but my reading suggests ▲P as the preferred method for my situation. Which is best?

Lastly, I have heard there are instances where the Knight boilers can be noisy. Id like to know of any first hand experiences with this boiler and if noise has been an issue?

Are there any other things I should be aware of?

Sorry for the ramble and I appreciate your advice and knowledge.

hot_rod · October 2022

If you don’t plan on the indirect, I would be tempted to connect the expansion tank right above the circulator, where it shows the Y strainer

Actually I would rather see a dirt sep instead of a y strainer, less pressure drop and not prone to plugging up like strainers can on old steel pipe systems.

Im almost certain the circ comes with a check in stalled in it, so skip the flow check also. If it has to have one. Use a spring check 12” downstream of the circ

Yes that boiler is supposed to use the delta T circ, although I don’t agree it is always the best option. The are trying to control the temperature across the boiler, I understand the theory but it limits the optimization of distribution piping

On mine I used a Sep 4 hydraulic separator. Let the boiler side do its thing, with the supplied delta T circ. Tthen the delta P can be used in the distribution. I think that is approved by. Lochinvar. It eliminates the dirt Sep, since the Sep 4 does air, dirt, magnetic and hydraulic

As shown with the delta T circ the bypass valve works similar to a delta P circ logic

The drawing you show above, there actually is a location the expansion tank could connect and allow the boiler and indirect to pump away. I don’t know who is doing the piping schematics for Lochinvar these days ? I might reach out to them with some thoughts on that drawing

rconkling · October 2022

Thanks Bob,

On a basic level doesn't the hydraulic separator function similarly to the closely spaced T's? Aside from the drop in velocity, dirt/air separation, and negating the necessity for a PBV?

This is their diagram for a system similar to yours. Low loss header is approved and shows secondary loop pumping away.

hot_rod · October 2022

Yes a Sep and P/S provide the same end result

The bypass valve needs to be in place to adjust flow as zone valves open and close with a fixed speed circ
OR use a delta P circ in place of that

mattmia2 · October 2022

If you are connecting a modcon to an old gravity system with cast iron emitters, a magnetic dirt separator is a great idea. The all in one dirt separator air elimination and hydraulic separator will save its cost in labor and the cost of a bypass valve and the separate components.

Since the boiler has an internal pump it is recommended to do primary secondary(such as with the hydraulic separator combo or closely spaced tees or the manufacturer's hydraulic separator assembly). If you used a boiler without a built in circulator you could direct connect the gravity system because the flow resistance of the system is so low you can size the circulator to maintain the minimum flow through the boiler. If you use trvs with the direct connect scheme you would need the pressure bypass valve to maintain the flow through the boiler when most of the valves are closed.

You should size the boiler based on the heat loss of the house(and possibly dhw needs). Using the connected radiation to size the boiler is for steam boilers only.

rconkling · October 2022

The boiler was sized with a local calc. I was giving EDR for reference.

The KHB, floor mount, uses an external circulator. It’s included in the box. Though I’m curious if its up to the task for direct plumbing.

I like the idea of an AIO separator but depending on placement, wouldn’t one of the functions be installed in a less than ideal location? dirt mag just prior to the pump, air separator near the boiler output.

Seems like and AIO is good for a new clean system and separate components for an old dirty one?

I couldn’t believe the amount of crud that came out when I drained it last year.

Hot_water_fan · October 2022

The KHB, floor mount, uses an external circulator. It’s included in the box. Though I’m curious if its up to the task for direct plumbing.

The KHB itself has extremely low head loss. You’d have to have a very high head loss system to run into problems with that circulator. What’s the heat loss? You could probably meet it with ~2 GPM if you really wanted to.

hot_rod · October 2022

If you direct pipe, you will be limited by the delta T circ that comes with the boiler. Doesn’t the manual show available pumping capacity?

use either separation method so you have more distribution pumping and temperature differential options

The vS boiler circ slows down to maintain the delta in the boiler, but not always at an ideal time

pedmec · October 2022

lochinvar's are not noisy and neither are any mod/cons. The only noise you hear is when it goes to high fire and you have to have a very quiet house to hear it then. my mod/con is below my living room. If your hearing a lot of noise something else is wrong.

I never had a noise complaint. I have had complaints that they don't think its working because they don't hear it. I have to ask them if they are cold. When they reply they have to take a step back and realize what they just asked me. They just realize its real quiet. especially compared to an oil burner.

Remember only 5-10% of the winter time if its size correctly are you gonna need it 100% of the time. Most of the year its going to be running less than full value. And with an old gravity system with all the water in the now oversized radiators, you probably gonna heat that house with condensing temperatures 100% of the time.

Zman · October 2022

Why not pipe it primary/secondary and use the manufacture provided circulator? Then you can use the delta p circ on the system side.

rconkling · October 2022

hot_rod said:
Doesn’t the manual show available pumping capacity?

It does, max head is indicated in the model number, 104. 4' seems low but 104' appears way too high.
The docs also mention internal control principles with proportional pressure and constant pressure settings as well as external control principles.

Im just trying to understand.

The more I dig, the deeper it gets.

GGross · November 2022

Why would you insist on direct piping that boiler instead of P/S ? Just curious on what your thought process is on that.

hot_rod · November 2022

Any proportional pumping function would be on the pump control, not the boiler control. And it could only be accomplished with P/S or a hydrosep, a two pump setup.

The boiler control can only control via temperature inputs to regulate its circ pump.

rconkling · November 2022

GGross said:
Why would you insist on direct piping that boiler instead of P/S ? Just curious on what your thought process is on that.

From what I have read, multiple posts on multiple forums, its the simplest and most efficient piping for a converted gravity system with TRVs. As P/S tends to yield higher return temps.

Looking at the efficiency chart of a mod-con, there is a significant drop off as the return temp rises. So it seems every degree counts.

Unless best practices have changed, letting the boiler do its thing while secondary does its thing. As I told the installer, im flexible. If my research is not up to date then so be it. For reference, they have never installed a mod-con with full flow before.

mattmia2 · November 2022

Your TRVs throw a wrench in to the works. If it is just replacing a boiler on a single zone gravity system or even splitting it in to a couple zones it is easy to direct connect it and keep the minimum flow through the boiler. When there is a varying piping path because of the TRVs you have to be a lot more careful.

Confirming mod-con install plans.

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