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[Ridge]

We have added 3 zones of heat load to existing 4 for a total of 7, it seems like my 15-58 single speed might not have enough flow when all zones call for heat which seems all the time ( poor insul ) What would the next bigger pump be? 26-99 seems over kill, 15-55 alpha has 5gpm more but not sure if that would be enough.
Water coming into house around 170 to  40 plate w to w exchanger but water gets up to 155 after exchanger? sounds like water is going too fast to heat up? What should I expect temp to be after going thru heat exchanger when OWB has it at 170?
thanks

[slimjim]

Not more than 20 degree drop.

[Ridge]

I worded it wrong, water comes in at 170 at its highest (but goes down to 155 before blower on owb kicks in, i can change that) the water from owb after the 40 plate it is about 168 but on the other side the house boiler water never gets above 155 and mostly 140-145 and with radiant heat house stays cool. I was wondering how much heat should transfer or what should the difference be on plate exchanger from owb side output vs. indoor boiler side of plate exchanger.
It would seem if I had better transference of heat I could keep water temp higher going to house boiler system, originally the house gas boiler was set at 180 high to 160 low and we had enough heat and If I can get that temp close we would be good.

[slimjim]

Why can you get only 170 for supply temp to the heat exchanger? Is it a lack of flow or perhaps water infiltration into the underground pex?

[Ridge]

The aquastat only goes to 180 (has a stop on it) and it draws from the lower tank and when fan shuts off the front gauge says 170-172 and the line leaving in rear owb to house is 170ish and about 170ish inside house using a laser temp to the pex line, hard to tell if there is any loss to house. the laser seems to be right on to the temp gauge on the house boiler when pointed at pex and almost matches the front gauge on stove. I wish I could turn the aquastat up to maybe 185 but short of bending tab and some warranty issue if there was a problem I can't

[RSI]

The Alpha will likely pump the same or a little less.

The 26-64 is the next size up, then the 26-96, then 26-99.

I would probably go with the 26-99 since the price isn't that much different than the others. You can run it on low or medium if needed.

[lmann]

Is there anything wrong with going to big on a pump?

[slimjim]

To fast and the circ will cavitate and shorten the life of the circ as well as erode fittings, typically I use temp differential of 20 degrees to adjust speeds on circs.

[LittleJohn]

Is there anything wrong with going to big on a pump?

If your pump is DRASTICALLY oversized, you can push water too fast and erode metalic fittings. 
Over-pumping will also cause noise related issues.

[caper]

hey guys,  as I explained to slim,my secondary loop pump is a grun 1558 and I just got a 2699,got this just because I wish to heat my water a bit quicker as my temp does drop a lot when multiple zones come on at the same time,trying this just to see if it will help,so this is ok right,and when I do it,should I just keep it at med speed .

[RSI]

If the 15-58 was just a little too small then low speed may be enough.