balancing radiant and boiler setup

keyote · December 2017

radiant floors and ceilings KHN085 boiler ODR piped direct with ECM pump and zone valves to a manifold per four floors. need to get a bit more serious about this. So i want lowest RWT possible for condensing and want long runs which go together less heat over longer times.currently only one zone hooked up so i guess the ideal is the lowest swt that will meet demand with constant running, and ODR adjusting as outside temps as proxy for load change? and i should also try to get pumping to keep DT on rwt low. ( i tried to use a DT pump but fought with boilers mod, put in on speed two until add more zones when I will swap for DP pump if cant get a ECM 1-10v that boiler can control DT)

so if i'm correct about above as ideal, does that change as more zones are added. In other words is the final ideal all four manifold zones running non stop? or is it more like they alternate with say 2 of 4 running any one time?

mikeg2015 · December 2017

first challenge is whether the boiler is sized right. If new construction built to code, it’s probably too big and you’ll fight short cycling most of the year unless you add a buffer tank. 85k could heat 4000sqft or more on new construction to code especially if zoned.

You might want the outdoor reset to control to return water temp. It will compensate for load changes better that way.

Fixed pump speed is better and runthe pump based on pressure as more zone valves open or close.

keyote · December 2017

thanks heat loss should be fine did my heat loss and design in loop cad its an KHN085 piped direct with zone valves
its 3400sf of historic renovation spray foam triple pane, yes the DT pump didnt work out so its on fixed speed for now since have only changed one floor over to the new system will swap to DP pump when i add the other floors soon.
but trying to decide best way to program

mikeg2015 · December 2017

So you have a vairalbe flow primary then (piper direct).

The Lochinvar Knights have some limitations there. I think a 50F delta T maximum and a minimum flow rate on those as well. So if flow is too low, you can't even achieve design boiler capacity. Pull up the installation manual and check the heat exchanger charts. Notice the pressure drop at different flow rates. Now that it's installed you can look at the pressure drop across your pump, pull up it's pump curve charts and see where your flow is at.

I think Lochinvar engineers were concerns about a large thermal gradient on the heat exchanger that could cause metal stress and failures.

The HTP UFT in comparison has a wider turn down, no minimum flows and I emails their tech support and delta T had to practical limit. But it's a newer product, less proven and less distributor support.

Boiler size sounds right then given the size and construction. Ahtough is well sealed at the windows and spray foam, I bet a 55k would have been adequate since you have so much thermal mass. But 85k isn't too big either. Especially if you have an indirect tank for hot water.

Gordy · December 2017

The khn is a 10:1 tdr. The 55 low end is 8200, and the 85 is 8500. May as well do an 85.however the 55 is 7.5:1 tdr.

I like how loch actually calls out the min. Flow rates in their charts.

keyote · December 2017

Im confused mike, it must be how i phrased my question. When I said "So i want lowest RWT possible for condensing and want long runs which go together less heat over longer times"
I meant i was aware that condensing only happens at lower RWT, not that i was going to try and go for a DT50 or something ridiculous.
Its a radiant system that was designed with a DT 10-15 but seems to now with the temporay fixed speed pump and half arsed programing to have settled in at DT20.
But since My SWT is currently nom more than 142 I have no problem condensing and don't anticipate changes will reduce my ability to condense.

What Im really trying to understand better is a more general ideal. As i said, I think that is setting it up to run constantly or almost so, which i assume would require finding the lowest SWT to ODT curve to barely meet the load or rather constantly chase the load but never quite catching the load,thus continuing to run.
And i also assume that since this is the lowest possible SWT its going to therefore give me the lowest possible RWT.

I mean i suppose i get I could rig a system to give me a DT 50 and maybe a even lower RWT but I would guess that would come at insane short cycle or something. so thats not what i meant to say
My thesis is (and i could be totally wrong) that my ideal SWT is one that is a millisecond behind the load so the boiler keeps firing never letting the room quite satisfy but never letting it cool any further either.And that SWT will be low enough to yield a RWT that condenses well with DT 10-20.
Now if you guys say that is correct, I then follow up and ask how will that work as 3 more zones are added. I imagine those zones wont have exactly the same loads and I wonder what will happen at that point and is it better to try to maintain all four zones constantly open through some sort of balancing tricks or to try to tune it so they kind of take turns but their calls overlap so the boiler still mostly runs constantly.And how I would accomplish whatever is the ideal.

Gordy · December 2017

Your return water temps will be dictated by the supply water temps which will be dictated by the heatloss, emiitter, and the outdoor reset curve. There is no getting around that.

I believe that boiler allows for three seperate ODR curves one for each zone.

Gordy · December 2017

Mike is referring to the boilers HX delta not the system delta.

If you are piped p/s then the HX delta is controllable via the onboard pump depending on system side deltas.

If piped direct then boiler delta will mirror system delta.

The KHN wants anywhere between 20 and 35 across the HX. See pages posted.

keyote · December 2017

Thanks Gordy.
"Your return water temps will be dictated by the supply water temps which will be dictated by the heatloss, emiitter, and the outdoor reset curve. There is no getting around that." - i think you are agreeing with what i was trying to say as preface to my questions, that theres an ideal SWT to ODT curve for a given space and that will dictate a RWT (and i suppose pump speed/flow will come into play as well? but thats going to come in either as fixed up front so accounted for).

"I believe that boiler allows for three separate ODR curves one for each zone." - Hmm that could be it certainly allows for more than one SWT on separate zones, but that i think involves using mixing station programmed into the control.
I would think using different curves might be pretty easy software wise but unless the zones are plumbed/controlled to not call simultaneously not sure how that would work.

"Mike is referring to the boilers HX delta not the system delta." - oh i get this its just the HX dt extremes hes worried about didnt seem worth worrying about so unlikely I would ever see a 50DT i realized he misunderstood when i said i ideally wanted the lowest possible RWT.
you however come at it from the other side with the charts. We went into the minimum flow issues as nauseum (because of my desire to pump direct) while i was designing it and in the end figured it wasnt really a problem since no pump actually pumps that low of a flow, which turned out to be correct it can modulate down really low and hold a DT 10, but your other chart you seem to say is calling for a minimum DT not just a minimum flow. This I dont ever recall discussing since my design was originally using dt10, i do recall though you making me understand that having a DT as low as 10 or 15 was not as critical as I had been led to believe. I was led to be concerned that without tight DT i would get uneven heat across the panel.So i havnt been so concerned that as its currently programed the dt is more like 20, in fact im more pleased about it from the condensing perspective.
but now you have me a bit worried that i have a DT20 minimum for the boiler i was unaware of and need be mindful of.so looking at the manual i dont really see it saying that those are minimum HX DT more like assumed minimum design DTs
Ok looking a bit closer i see (well I cant actually read those fine prints charts but i surmise)they are showing how dt and flow are connected and and are showing likely flow dt and how that might impact the minimum flow across HX which is 1gpm. which brings us back to last years question, which i thought was answered that there is no pump thats going to pump less than 1gpm (even that DT pump)and i should stop worrying.
That said i am no longer using the dt pump idea and back then i realized both the dt and dp pumps cant pump that slow, so if i understand anything at all it comes down to can the boiler mod precise enough to maintain a lower than dt 20 if it can great (if thats what you want) if not well it just wont, but the pump will pump above the min flow no matter what. I would think you wont know until you have the setting dialed in to the ideal SWT/ODT and you see what the RWT DT is.Let me know if i am not understanding this.

But gordy can you answer the questions about what the ideal settings i am looking for, this is something i don't think i understand well at all. I get the ODR is trying to anticipate the load change and stay ahead of it.I kind of get theres a low and a high and the curve can not necessarily be curved but programmed with a plateau.I think i get Lower swt is better mor cond and long runs are better for several reasons.
But Im not confident in my conclusion that my ideal is constant running- the coolest swt for the longest run time. in my mind i see i can run it hotter and hotter for shorter and shorter runs till im short cycling like crazy or go the other way and lower the swt until the runs are almost continuous which would be the way to go because its easier on the boiler and more efficient for several reasons to run continuous at low fire than intermittently at higher fires.
if this is correct then i wonder what happens when say i dial that in today on the one zone i have hooked up, then in a couple months i add three more zones i will throw the balance off does the ideal change?

Gordy · December 2017

What you need to do is set the ODR curve up for design day. Let it run see how things act for a week. If cycling becomes an issue. Drop the curve a little at a time see how system reacts. Until you find a happy medium of long low end modulation burns, and the ability to hold setpoint desired.

Ultimately raise the thermostat 5 degrees higher than desired set point. Let the ODR drive the system making shuttle adjustments to the curve until set point is maintained solely by the ODR. Now the thermostat becomes high limit switch, only intervening with the system operation due to other gains such as solar, cooking, large number of occupants, fireplace etc.

mikeg2015 · December 2017

Gordy said:
The khn is a 10:1 tdr. The 55 low end is 8200, and the 85 is 8500. May as well do an 85.however the 55 is 7.5:1 tdr.

I like how loch actually calls out the min. Flow rates in their charts.

Thanks for the correct info. I'm a little behind on the newer residential boilers. Knights used to be 5:1 but must have been updated the last couple years just following the commercial FTXL.

I agree, in that case 85 is better, plus economy improves at low fire slightly, so the 85 at maybe 20% firing rate uses less fuel that the same output on the 55, which might be 30% firing rate

keyote · December 2017

Mike yes gordy actually gave me some excellent data on the efficiencies at various modulation and basically the lower the modulation the better efficiency it seemed to me oversizing a modcon is a good thing, however i didnt over do it if i recall my design was 65k and i think real life is more though cant be sure top floor is only zone on and its def a heavier load up there.

@Gordy Thanks
so Im setting tstat extra high with the intent of getting the ODR too low to ever satisfy it and so keep the boiler running. except i will tune the ODr to be able to keep up with my real intended setpoint,But no higher. and should a holiday meal bring the zone up to the Tsts setpt then it will shut the boiler down. So i do want ideally continuous runs- and thats a clever way to get them.If i do all the zones tstats like that they will mostly all call for heat all the time. If the loads and tubes etc are balanced it will work fine if not? I mess with the flow valves on the manifolds to tweak?

balancing radiant and boiler setup

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