watervolume/circ pumps/bypass loops

RichardW

I have a minitherm160 boiler. The product literature indicates a need for 12.8 gpm water flow through boiler to produce a 20 degree temp rise.They also have a chart that shows with 3/4" pipes using a 1/6 hp pump a max of 72 feet and 8 elbows are all that you can get out of this unit unless a bypass is installed with its own circ pump. It does not state what size this bypass pump should be nor does it state how much additional 3/4 piping could be added after the bypass loop and bypass circ pump is installed. Does anyone have data or experience that will enable me to calculate what is necessary to maintain this flow ? Should the boiler pump be the same size as the circ pump that feeds the rads.Should this pump be smaller?Should the boiler pump be piped with 1"? Should I add a ball valve in the boiler loop to enable fine tuning of the system?
I would like two zones of about 70 feet each and according to manufacturers installation guide this would need 1/6 hp pump on each zone plus a pump on the boiler bypass loop, does this sound reasonable? It feels like a lot of pumps to me and of course I have a concern about noisy heating pipes with the water moving too fast.
Also why is the new Brute-Grundfos pump not rated in terms of hp. Could I use this unit- will it pump as much as 1/6 hp.
If anybody knows the answers to these questions, or even know where I could find information or a human being that knows this stuff I would be very gratefull.

Thanks in advance
RichardW

hr

1 question

what type of system will this be connected to? You mentioned rads? What temperature do you plan on running? Any low temperture emitters connected?

Bypass loops can and do help, in a very simple fashion.

There are other controls and piping arrangements to consider depending on how the boiler is applied.

Give us some more info on your system i.e. load at design, type of heating units, number of zones, and operating temperatures(s) needed.

hot rod

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RichardW

1 question-18 answers

Hi
This is residential application. A stone cottage 152 years old. The house is app 1500 square feet. The present state is 115 feet of 3/4" fin type baseboard rad. This is connected by a further 85' of 3/4 copper and is arranged as two loops coming of a 1" trunk. Because of the stone construction, each loop has app. 23 90degree elbows.
There is about 4-5' of head throughout system. Although there is two loops there is only one thermostat to control the entire house.
Depending on the need for auxilliary pumps I thought I could perhaps use the existing loops and set up a multi zone pump system. If there is not a requirement for that many pumps, a zone system seems to be the best. As for the load all I can say is I'm in Canada.
The factory installation literature says that bypass piping will overcome the slower flow rate of the setup I have to work with. The boiler is perhaps larger than necessary, but the homeowner is planning a large addition to be completed Sept. 05. The existing setup was driven by a vessel type boiler that, I am learning, is far more forgiving of elbows etc.
I figure there must be a way.
I appreciate you taking the time over this
RichardW

hr

In a perfect world

a heatloss calc would be done and the boiler and baseboard installed to match the calc.

You could use the Slant Fin calculator available for free here, to run some numbers. Although stone construction is a challange to get accurate numbers on. Insulation, if any, and infiltration are hard to pin down.

Does the home heat well with the current system?

The amount of baseboard you have times 600 btu/ foot output looks like @ 70,000 output. The boiler you propose is about twice the size you need at sea level. The 136,000 output would derate at above 1000 foot elevations, of course.

Plan on that boiler short cycling with that small load. A buffer tank would help that but I don't know how deep you want to get into it. Think I would use a Ergomax indirect to add some buffer and provide the DHW at 85% efficiency. Much better than a tank style water heater, gives you the buffer to smooth out the short cycling on low load days, and plenty of well insulated DHW to boot.

Keep in mind the copper tube boilers are low mass low water content. Your baseboard is also low mass. Probably see some good acceleration and I don't think a return temperature protection is needed under this condition. Especially with the gross oversizing If that boiler is available with two stage firing that would help.

Think I would pipe a primary secondary to assure good flow through the boiler. Skip the bypass piping. Use the manufactures spec to size the boiler pump. I'd run the boiler differintal at a 25° delta T. 160- 185. This will help the cycling also. The boiler spec will show the needed flow rate and pressure drop with various delta Ts. Size the primary pump to this.

Add another foot of head to their requirements to accomodate the primary loop fittings, purger, y strainer, flow switch, (you always want a flow switch in a copper tube boiler) etc.

Then you could zone with two pumps, or 1 pump and two zone valves. Depending on how you split the one zone into two or more, a small Grundfos 15-58 would be my choice for zone pumps. The 3 speed ability would let you dial in the flow rates nicely.

Leave some extra connections for the future addition heating needed.

hot rod

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RichardW

Thanks Hotrod. My connection has been down due to a switch to highspeed that was less than efficient. And speaking of less than efficient , by using an oversized boiler I figure it must not be as efficient as a properly sized boiler. What do you think is lost in terms of efficiency.
RichardW