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in Gas Heating

Greetings. I am the new owner of a Viessmann Vitodens-100 mod-con system for my heat (single zone baseboard) and DHW. The system has a primary-secondary loop configuration, with a low-loss header connecting the loops. Three Grundfos 3-speed pumps: on the primary loop, the radiation loop, and the DHW.

Currently, all three pumps are set to 10 GPM.

I'm wondering how to know if this is optimal. For example, would a slower pump speed to the radiators result in a larger delta-T? If so, should the primary-loop pump be slowed down also? The Vitodens HX has a max flow rate of 6.6 GPM.

I'm afraid I know just enough about these systems to be dangerous, so any advice would be appreciated.

Currently, all three pumps are set to 10 GPM.

I'm wondering how to know if this is optimal. For example, would a slower pump speed to the radiators result in a larger delta-T? If so, should the primary-loop pump be slowed down also? The Vitodens HX has a max flow rate of 6.6 GPM.

I'm afraid I know just enough about these systems to be dangerous, so any advice would be appreciated.

This discussion has been closed.

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## Comments

7,356Is determined by the maximum boiler output and delta-T. With proper hydraulic separation from the secondary circuit(s) the two pumps will not interact. P.15 of the Vitodens 100W install manual has the numbers you need for this.

You can adjust the secondary circ flow to increase of decrease delta-T on your emitters or the indirect. A bit of experimentation here is probably in order.

141Thanks for the information. I will check out the manual in more detail.

Incidentally, I read an interesting article from 2009 on low-loss headers. The article indicated that if there are non-equal flow rates in primary and secondary loops, mixing will occur in the header. For example, if the secondary loop flow rate is higher than the primary lopp flow rate, some of the return water will be mixed into the outgoing supply water from the boiler. Am I understanding this correctly?

7,356Mixing is good.

Boiler will modulate up and down, adding heat to maintain the setpoint, while your distribution loop will lose heat at a rate depending on the delta-T between the supply temp and the space being heated. The LLH allows the two circuits to function without weird interactions.

5,923I think you may be misunderstanding the speeds on the circulators. I am assuming you have grundfos 15-58s? The gpm produced by the circulators is determined by the point at which the pump curve intersects the system curve (page 27 of the manual) The 15-58 on the boiler set on high will produce about 5.5 gpm (assuming the near boiler piping is short). To figure out the system curve and gpm for the rest of the circulators you would need the pipe lengths and resistance of those components. It would be easier to measure the delta t (supply temp-return temp) of the pipes and adjust the circulators accordingly.

The only way to increase boiler efficiency is to reduce the return water temp. Slowing the boiler circ. will do this (at some point performance will suffer). Slowing the secondary circs may not have an effect on the boiler return temps. Doubling the secondary delta t will have no effect on the primary return temp if you cut the flow rate in half to do it.

Get a gauge and check it out,

Carl

Albert Einstein

141Yes, I have the 15-58s. I’m not sure I understand how a slower primary loop circulation rate will decrease return water temperature. Is it because at slower flow rates the primary loop is not delivering Btus to the secondary loop as quickly? At a certain point, will slowing the boiler flow rate interfere with condensing, or trigger short cycling of the boiler?

So I guess the bottom line question is, how do I determine the optimal speed setting for the primary pump? The installer set it to “Medium.” The near boiler piping is short.

Thanks for your patience!

7,356Watch http://www.youtube.com/watch?v=cDTuG3j04VM first.

If the boiler is firing to meet a setpoint, and lower GPM = greater ∆T, the only thing that can change is is the return temp (since the supply temp is being held constant by the boiler.) Lower return temps = more time condensing = greater boiler efficiency. Throw in some reduced pumping costs and (potentially) smaller diameter piping and you get less cost.

The optimum speed for the boiler circ is the lowest speed during full fire where delta-T stays within the manufacturer's specs (40F for Viessmann IIRC.)

4,492the primary side is circulating 5.5 gpm, and the secondary side is drawing 4 gpm, 1.5 gpm of heated water is just being recycled to the boiler from the low-loss header, and never going to the system.

5,923I would suggest taking some time to get your head around the "hydronic formula".

Check out this gauge http://www.firedragonent.com/DM6802B.htm .

With the clamps you can quickly see how circulator speeds are effecting your system performance.

It looks like Medium is a decent starting point for the primary (I did not see a minimum flow in the manual).

If you get the secondary speeds to low you may get poor performance on the return side radiators.

I would start with a 20 degree delta t on the heat and see how it goes.

Carl

Albert Einstein

141Thanks for all the patient advice. I'll check out the you-tube link and the gauge.

4,492I understand the complexity of it, but wouldn't it be great if someone invented a simple, affordable clamp-on flow meter.Just thinking out loud.

141My manual shows three "Typical System Flow Rates", which I assume relate to the three Grundfos pump speed settings, and their corresponding temp increases:

5.5 GPM = 30-degree temp increase

4.7 GPM = 35-degree temp increase

4.2 GPM = 40-degree temp increase

141This one is equally as naive as my previous ones! Assuming the primary and secondary pumps are both set at the same speed, will the flow rate on the secondary loop typically be higher or lower than the flow of the primary loop? The secondary loop has larger diameter pipes, but also far more pipe length.

3,066Viessmann recommened pump sizing for the entire Vitodens family is base on a boiler side 40 degree delta. Max flow rate for a Vitodens 100 is 6.2gpm. The wide delta keeps the pressure drop across the HX down allowing for smaller pumps as well as getting the btu's out of the boiler. Minimum delta is 35 for a WB1B10-35

Let's look at the WB1B10-35, 108,000 btu/hr output.

108,000/(40x500) = [email protected] 12' head

108,000/(35x500) = 6.2gpm @ 16'head

108'000/(20x500) = 10.8gpm

You can't get 10.8gpm cross the hx . Now find the pump curve for a Grundy 15-58 and you will see Speed 2 for a 40 delta and Speed 3 for the 35 delta.

A btu is a btu. When using the LLH just think of the boiler side and the system side as systems. Your just movig btu's to the LLH and how you want to take them for a ride around the system has no bearing on the boiler side. You can have a 40 delta on the boiler side and a 20 delta on the system side.

What I find interesting is how engineers can't grasp this concept. I see more specs that call for a 20 degree boiler delta that it makes me wonder if they even open a manual or understand it. I just recently had one enginer state the system would be unbalanced and not deliver the proper btu's if both side didn't run on a 20.. Can't make this stuff up...

3,066Digital multi temp meter and a calculator. I have a nice little Cooper that I can get three temps at the same time. System Supply, System Return, Boiler side return. The boiler gives you supply temp out. Put the boiler into high fire and run the numbers, then in low fire. You have the given min and max btu/hr output of the boiler..

141Thanks, Chris. So if I'm reading my WB1B-26 manual correctly, pump speed setting 1 will give a 40-degree delta-T on the boiler side, and pump speed 2 (which is the current setting) will give a 35-degree delta-T Is that right? If so, does that mean the pump should be on setting 1 instead?

3,066A WB1B10-26 has a btu/hr output of 83,000 btu/hr.

83,000/(30x500)= 5.5gpm @ 12' Head = Pump Speed 3

83,000/(35x500)= 4.7gpm @ 10' Head = Pump Speed 2

83,000/(40x500)= 4.2gpm @ 7' Head = Pump Speed 2

Your going to see closer to a 35 delta. You can run on Speed 1 your just not going to get the full btu/hr output that the boiler can deliver. May not matter depending on the heat loss. You'd only get about 3.3 gpm across the hx on this speed. Please note that these numbers are only good for a system running on 100 percent water and no system antifreeze.

141Thanks. Yes, the system is running on 100% water. So my next question is, with the boiler pump set on 1 (or "low"), I assume the way to determine the proper pump speed for the radiator loop is to measure return water temp and aim for a 20-degree delta?

141Chris, it sounds like you are saying that with the WB1B-26, pump speed 1 may in fact be too slow (3.3 GPM), and that pump speed 2 may be better. Do I understand you correctly?

3,066If these are cast iron rads I'd be trying to get out to a 30.. Also wouldn't be using a fixed speed pump which is what this is. I'd put money pennies away for the Taco Bumble Bee coming out this month.

http://www.taco-hvac.com/en/products/products.html?current_category=405

5,923You could add up the pipe sizes and fittings on the secondary then draw out the system curve for the loop. You would then calculate the heat emitted by the radiators on this loop being sure to derate each for the loss in temperature from the previous. This will give you your target GPM. You would then compare this info to the pump curve at the various settings.

My head hurts just thinking about it!

I would use a gauge

Carl

Albert Einstein

3,066Depending on the heat loss.

3.3 x (40x500) = 66,000 btu/hr

Is 66 enough? Delta may get out to 43 which will increase the btu output.

800you have to compare load at design temperature to radiators capability to emit heat at lower water temperature, because bigger delta t across radiator lowers median radiator temperature and lowers heat emitting capacity of radiator.

Absolute Mechanical Co. Inc.

www.AbsoluteMechanicalCoInc.com

141Well, one room has baseboards, and the rest of the house has "Convectors" which as you probably know are basically baseboard units.

3,066That's why we like constant circulation my friend. Did you get your Vitodens today?

800probably. i did not get report yet

Absolute Mechanical Co. Inc.

www.AbsoluteMechanicalCoInc.com

141Obviously, just doing DHW for now.

Total heat loss (radiation plus 15% piping loss) estimated at 60,030

3,066Better stay with that 20 then...What number setting is the right hand dial on the boiler control at?

800without TRV we started installing delta T VFD circulators

Absolute Mechanical Co. Inc.

www.AbsoluteMechanicalCoInc.com

141The "dot" on the dial

141by mistake, the total heat loss (radiation plus 15% piping loss) is calculated at 60,030. It is a small, 4-bedroom house.

3,066Is not 140. You'll hit 176 degree water temp at 14 degree outdoor temp..Setting 4 will get you there at zero degrees. What's the coldest temp you see in the winter?

141that's what Viessmann told me. I'm not saying the online tech. was correct, just that he said "the dot is 140." My understanding is that the dot is just the factory default setting.

My system has an outdoor temp sensor. According to the curves in the manual, the boiler water temp to the rads will be 140 degrees at about 40 degrees outside temp, and as you said, 176 at about 14 degrees.

800you can measure finned tubes and check output at the lower temperature to see if you have enough output to cover heat loss

Absolute Mechanical Co. Inc.

www.AbsoluteMechanicalCoInc.com

3,066Be concerned with it until winter hits. Wouldn't worry about pump speeds either. Think of this winter as the experiment in finding the sweat spot that works best for your home. Play all winter at different outdoor temps and find what works best for you. You can 't fine tune until you get the weather to do it in.

141As you can tell, I'm brand new to all this, and am climbing the learning curve. I have a lot of respect for you guys who do this for a living. It is an art and a science. My installer did a great job, but I want to learn to optimize the system myself. Thanks again!

3,066Pics of the install. We love pictures...

141but then you guys will point out all the ways it could be improved!

By the way, how did you come up with 3.3 gpm across the hx at pump speed 1?

3,066I took the pump curve chart for the Grundfos UPS15-58FRC and the Taco 00R and plotted the HX curves over them for both Vitodens 100 and 200. Did the same for the Grundfos UPS26-99 that we use on Vitodens 200 for the WB2B-45 through 105. Don't refer back to them though, there stuck in the memory banks...

I play in the Vitodens world on a daily basis. In my opinion best condensing boiler on the market..The 100 is meant for applications such as yours but the 200 is a different animal.

141I did a lot of reading before I selected Viessmann over Buderus. The Buderus is slightly more efficient (96% vs 95.2%) but I was nervous about the aluminum hx.

What do you recall as the flow rate across hx at pump speeds 2 and 3?

3,066Capable radiation output doesn't constitute a heat loss. Only means that at a given gpm flow rate and water temp it has the capability to emmitt x amount of btu/hr.. If the flow rate is unknown a 1gpm flow rate is used...Flow rate is calculated by the total heat loss of the entire zone...