Home Run/Pex Piping

I have a 2,400 ft 1914 colonial over three floors that has been completely gutted. Ran the SlantFin heatloss and am getting circa 70,000 btu/hr. I'm keeping my old radiators and installing baseboard/convectors in other areas. I think I'm going to go with a Buderus G124X(90,000 net) with Logamatic controls. Is this sized adequately? Also, we originally planned a traditional piping/zone configuration but the supply house is recommending a pex/home run system with manifold. I was wondering if anyone has thoughts on this? Also, supply house says I don't need any mixing valves with Buderus. Is that true? Thanks...

 

With the current and planned radiation what water supply temps will you need for a mild day, a typical winter and a design day?

Homerun with TRVs would be my ideal... but I'm parked firmly in camp modcon if you're burning gas and not oil.

 

I've looked at TRVs. Do the $30 ones do the trick or am I looking at $100 per? Would they obviate the need for multiple circ pumps? Not sure what the different temps are for my region but NJ Trooper told me I should run the heat loss at 4 deg for my location(Worcester, MA).

 

Yeah I have gas and I've looked long and hard at mod-cons but I'm not sure how long I'll be in the house so I have a hard time reconciling the extra cost(Can't get the rebate either). I figured the next best would be to use a good CI and pipe it correctly with 4 way valve/outdoor reset. Supply house says Logamatic accomplishes same thing with electronics. Can't seem to get any independent verification though..

 

So when you run your heatloss for that temperature and you know how many BTUs you need you can derive what water temperature you will need on the supply side to give the supply to match the demand kind of deal...

I would think that with a gutjob and good insulation you'll be looking at some fairly low water temperatures if those emitters were already generously sized. My favorite boiler is the Prestige for its simplicity. You could use the just the internal circ piped directly to the manifold. If the rads near the t-stat can handle the flow when all of the other TRVs might be closed then nothing else is needed. Otherwise, a differential pressure valve is needed. It doesn't get any simpler.

I'd get good TRVs like Danfoss... then can be added later as well...

 

I think the G124X/32 (96k IBR) is oversized. Particularly since you are building back from a gut. Your insulation should/will be good, and air sealing as well. At a minimum you should consider the /25; I would probably go with the /18. There is a fair bit of padding in the Manual J heat loss (what slantfin-ware does). From what I've read lately, it appears a lot of knowledgeable installers are using DOE output sizing to the heat loss. Some even to the gross input. Insulation/sealing have come a long way since Manual J was promulgated.

You pay for oversizing every time the boiler fires. You pay for undersizing maybe two days a year when the space temp is 67 instead of 69. Emphasis on the maybe.

4F is good for Worcester.

If you calculate out the radiator heating capacity, I'd bet that you could do a design day with <180F water, which is a further argument for the /18. You won't need it to run full-out.

I haven't read up on the Logamatic stuff in a long time, but my recollection is that Buderus built a lot of logic into that control to work with their boilers. Makes sense to get that package with the boiler. I think there's a temperature protection scheme built in, but might not allow full reset, which would be desirable with cast iron rads. You could also pipe for that.

 

I called the supply house today and the Prestige is a lot more reasonable than I thought. Do you think I can get by with the 110 or would it make more sense to go with the 175 if I want to add indirect DHW in the future? Also, not sure I follow you regarding the TRVs. Can you explain? Thanks..

 

TRVs are little valves that regulate the temperature of a single rad. There's a dial on them and the 3 in the middle tends to be about 68°F/20°C. You can dial it up or down, although it can only get as warm in the room as your water supply. The valve is purely mechanical (was expands and closes the flow to the radiator when the room reaches its temperature). TRVs are common throughout Europe.

A 110 should be more than enough for CH and DHW.

 

I'm familiar with how a TRV works but I couldn't understand what you mean that the "rads near the tstat can handle the flow"? If the boiler is producing cooler water- say 150 deg- any TRV will only let the radiator get as hot as the water the boiler is producing- correct? So you could get colder but not warmer in this example. Seems like for TRVs to work well you would need the boiler running hot constantly and then you restrict the hot water at the radiator. (I remeber this when I was in Paris 3 years ago). But a mod-con gets its efficiency from working less hard when it doesn't need to if I understand the principle. Thoughts??

 

I see where I wasn't clear enough. There are certain rules about how fast or how slow water should flow in a pipe and a rule that thou shall not cavitate the circulator.

If you have all TRVs then you don't really need a t-stat. My preference is to have one. So whereever it is that you decide to have the t-stat, the rads in that area would be controlled by the t-stat and not trvs. If all of the TRVs have closed because perhaps someone has cranked up the temperature in that one room then all of the flow from the pump will be there. If it isn't too much beyond 4 fps I wouldn't worry, otherwise you should use a differential bypass valve. Special pressure sensing pumps are coming on the market but their prices will be really steep for a while.