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# Mod-Con Boiler Pump Speeds

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• Member Posts: 3,086
edited August 2012
Gave them to you

Look a few posts above. Don't be fooled by AFUE. Condensing boiler is only that efficient when you get return water temps down below 115 degrees. The most efficient condensing boiler is the one thst produces the most condensate...
"The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
• Member Posts: 141
I understand

This calculation does not take into account infiltration, convection, size of rooms . . .
• Member Posts: 141
OK, I see them

It's just that you said 4.2 actually means 3.3 across hx, so I'm wondering if the other flow rates (5.5., 4.7), have lower rates across hx.
• Member Posts: 3,086
Not What I Said

On speed 1 a UPS15-58FRc can only move 3.3gpm at 7' of head.. .That is where the HX curve intersects with the pump curve...
"The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
• Member Posts: 141
Got it

Thanks for the clarification. This stuff is complex . . .
• Member Posts: 839
efficiency

The numbers you looking at are AFUE, annual fuel utilization efficiency , has nothing to do with boiler and system efficiency, and is a just a fanny number. What really affects efficiency are many factors, including proper installation, sizing, setup, return water temperature, load to installed radiation ratio and many more.
• Member Posts: 839
infiltrations

infiltrations might add a lot to heat loss. depending on how tight is your house.
• Member Posts: 141
Big difference between speeds 1 and 2

If pump speed 1 gives 3.3X(40X500) = 66,000 Btu and

pump speed 2 gives 4.7X(35X500) = 82,250 Btu

that's quite a jump - I guess it is a function of how those pump curves and hx curves intersect?
• Member Posts: 3,086
Speed 1

Delta is going to be larger then 40 proabably 43 or so....
"The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
• Member Posts: 141
The house is as tight as I can make it

I'd say it is very good but not great.  The roof cap is extensively air sealed and insulated.  All the windows are good.  The one big defect is that I can't get to the rim joist to seal it.  My basement has a hard plaster ceiling with metal lathe.  Getting to the rim joist would require some major destruction of the ceiling.
• Member Posts: 141
We can get down into the single digits sometimes

but most winters the coldest days are in the low teens.  Boston metro-west suburbs
• Member Posts: 141
Bumble bee

That pump looks really cool. Now that you guys have educated me a bit, I can see the advantages of a true variable speed pump with temp sensors.
• Member Posts: 4,469
Delta-T

It has always been my understanding, that the various delta-t options provided by mod/con manufacturers were provided to ensure minimum circulation through the boiler at various system delta-t's, and not that you could maintain a certain delta-t through the boiler.Some of the I&O manuals are not clear on this, and others clearly state it for selection of primary circulators.If you are shooting for this delta-t, based on system design, use this primary circulator.
• Member Posts: 10
second guessing now

Just had someone tell me the flow on the secondary pump cannot be greater than the flow on the primary pump for a Viessmann 200.  I'm planning my install and was intending on using a low loss header to decouple the primary and secondary circuits.  I'm going to use baseboad radiators on the secondary side of the system (minimum 1 gpm per radiator), using a reverse return piping arrangement.  I end up with approx. 12 gpm on the secondary side.  Max primary is like 6 gpm per literature.  Am I going to have any issues with my secondary circuit being a higher flow than the primary circuit?

From my understanding this was ideal but I just had a long conversation with someone whom says they deal with high efficiency boilers all the time and that my secondary loop cannot have a higher flow than my primary loop.
• Member Posts: 141

I am not a contractor, so I will let the pros answer your question.
• Member Posts: 839
flows

The purpose of hydrolic separator is to separate boiler and system loops. So this discussion of flow differences is irrelevant in my opinion.
• Member Posts: 3,086
edited August 2012

My friend Gennady is correct but I'll chime in.

When you use the LLH you have in essence two separate systems that have no influence over the other. The boiler will deliver the btu's and you will take them away. How the boiler delivers them is irrelevant to how you take them. In the case of Vitodens 200 we use a LLH sensor. It's purpose to is make sure the boiler is delivering the proper water temp to the LLH based on outdoor temp. In other words, the boiler is going to make sure the temp and btu's you need for that given time gets delivered no matter how yiu take them away.

If your friends theory was correct, ask him why Viessmann specs a pump for a 40 degree delta-t knowing most systems run at a 20 degree delta-t?

Bet you'll really get blown away to find there is no "TT" on the boiler to tell it there is a call for heat... Another subject for another post....
"The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
This discussion has been closed.