Heating system design questions

rhl

Hi,

I have a 1930's tudor home, 2500 sqft, with hot water radiators on the two main floors and the basement, in one zone. The radiator main feed/return pipes in the basement are 2" galvanized/black pipe that seem very old. Based on my reading it seems like I have a two pipe system with direct return piping. We had a pump on the return oil system, and we converted to a modern combi weil mclain aqua-balance boiler (max output of 155k BTU, min output 15k btu). The plumber built a primary loop near the boiler, with a large fixed speed taco pump on the secondary loop, that makes the old piping system the secondary loop. He left stubs in for future zones. The primary loop is pumped with a grundfos three speed inside the boiler.

To save money, and the hassle of dealing with plumbers, I plan to run these pipes myself. I computed the BTU output of each radiator on each floor of the house, and I came up with just under 35k upstairs (3 bedrooms, 1 bathroom), and about 65k downstairs. I had to make some judgement calls on two radiators which are not cast iron, but are quite small, so I feel these are overestimates. I plan to run 1" pipe in either copper or pex downstairs and 3/4" for upstairs. I will use grundfos alpha 2 pumps throughout to save energy.

I have a few questions:

1) If I stick with a two pipe system with either direct or reverse returns, is there any reason to use copper over pex for the main feed/return pipes? It seems using pex will save money, and require less fittings.

2) It seems that strictly speaking I can get away with 3/4" pipe for upstairs, but, its borderline. I'm considering pushing this up to 1" to "future proof" the system, in case somehow we need more heat upstairs. Does this make any sense? Is there a significant benefit to using 3/4" pipe in terms of lowered water volume, so lowered heating costs?

3) With a two pipe system, is it necessary for the risers need to be connected with diverter tees? If it's not necessary, is it desired in some way?

Snowmelt

draw a schematic of that, it makes sense to stay with the one inch. my own opinion would be stay with copper unless you have to go with pex.
moniflo system uses divert-er tees not reverse return, that just requires more piping

rhl

I’m happy to do that. My understanding is that mono flow systems come with significant balancing issues compared with two pipe systems, using either reverse or direct return. I’m therefore interested in a two pipe system. Am I missing something?

Tinman

If it were my home, I'd run Pex-Al-Pex from the boiler room to each radiator and connect them to manifolds with flow meters and valves. 1/2" pipe would cover most radiators unless you have some really large ones. If you want to make it even better, install TRVs on the radiators, use constant flow with an ECM pump, and run the boiler off an outdoor reset control.

rhl

Excuse the crude drawing, but here is a rough idea of the existing system. I found it hard to render the primary loop, with its taco pump, but, its by the boiler. Not shown are the various feed and return risers, that come off the main loop. These pipes just dead end at the ends.

I plan to essentially follow the existing pipes, inside of the joist bays.

The boiler does have a temperature reset.

I'm not a fan of the manifold solution, due to the layout of the house, that would require much more pipe, thats like at least an extra 20 feet of pipe per radiator. A 1" pipe holds 4x the water of a 1/2" pipe. So that design would mean saving 1/4 of the energy per pipe, but, i now need 5x the pipes (5/4 more water volume), over a standard two pipe design. Also much this piping is run the wrong way across the joist bays (which run from the top of the picture to the bottom). Also, i think some of the radiators on the first floor are fairly big.

My main question is why not just use pex for the main feed and returns? what are the pro's and cons?

Zman

There is nothing wrong with running it all in PEX. Pex-AL-Pex will expand and contract less which is an advantage where that is a concern. All pex products have a smaller inner diameter that their copper counterparts. This needs to be factored into the resistance calculation.

Your existing system with large cast radiators is likely running (or should be) at low water temps. Be sure to size your new radiators to run at lower temps as well.

If the boiler was piped correctly with the new taps hydraulically separated, you can run the new system any way you want. The size of the pipe is dictated by the BTU's it needs to carry.

If you take the time to draw the whole thing out, you will get some very detailed feedback. You have given a general description, so the advise is and will be fairly general.

rhl

Thank you. I will work on a better drawing and post it, along with some pictures. I apologize as I am new to this scene. I think running everything in pex will be a cheaper and easier for me.

W.R.T the water temperature, last year was our first year with this "new" system. It is outputting 180 degree water. We didn't have the temperature reset last year (an oversight by the plumber), and we have it installed now. Im not sure how much lower you are thinking things should be. We are hoping the temperature reset will save us on our gas bills.

Zman

A high efficiency boiler running at high temps is just an overpriced conventional boiler. With an old house and cast iron rads, you could probably run 130-140 degrees on design day. The average day could be in the low 100's. That is where the savings lay.
Learn to adjust the curve yourself and see how low you can go.

rhl

I think what your saying is that the temperature reset really helps, and to be aggressive about using it. This is fair advice.

rhl

Hi, I have been a bit busy (new baby in the house), I don't have time to draw a picture of the system. However, I did take a photograph of it.

I had neglected to mention about a new zone for floor heat added on to the system by my "plumber" it was piped incorrectly, as the circulator is on the hot supply port of the mixing valve, and he is using copper + galvanized in contact with one another. He is coming monday to fix his mistakes.




I am reading Dan Holohans book on "P+S Pumping Made Easy" and people keep discussing how the feeds and returns on the system should be closely spaced < 6". All the supplies on my system are close together, and all the returns are close together, but they the distance from a zones supply to a return are far apart. Is this wrong? Does anyone have any more comments on my system? I am installing a caleffi magnetic dirt separator likely on the primary loop return pipe.

I was speaking to a local heating "expert", and he was echoing your comments about low water temps, in particular he was saying my cast iron rads likely never exceeded 160 degrees when they were coal fired back before the great depression.

Canucker

Closely spaced tee's should be no more than 4 pipe diameters apart, then depending on the pipe size within 12". Provided that your piping can support the required flow rates, the boiler would have to be decoupled and it appears to be. The rest appear to be connected in an "antler" style. That should work fine, it supplies the same temp. to every zone. I see copper press fittings and iron pipe, no galvanized, unless it's not in view. There's no need to fix that as there should be very little oxygen in your system to cause galvanic corrosion.

rhl

I see, I am a plumbing newbie. My background is in mathematics. I thought the "iron" pipe shown in the photos was galvanized pipe. I can't find any literature references to the "antler" style. Does it have another name?

Ironman

Moose antler:

Ironman

You're creating another issue by zoning up that radiant manifold: short cycling of the boiler. That will cause it to go to very early grave as well reducing efficiency.

rhl

Can you comment further, or point to a detailed explanation ? Currently operating this zone on its own is wildly inefficient as the boiler makes 15k BTU at a minimum but the (single) bathroom needs about 2k BTU. I’m planning to staple up floor heat on the first floor to give the radiant section higher heat load.

One other observation is that the boiler could produce water no hotter than the mixing valve is set for,currently it doesn’t do that. I’m wondering if there is a controller out there that does that.

Solid_Fuel_Man

What type of radiant in that single loop? Is it stale-up, have aluminum plates, or embedded in concrete?

rhl

The existing loop is in concrete. I am planning to staple up the whole first floor.

Solid_Fuel_Man

Ok, well likely that is the only "low temp" loop then. Your staple up will likely work with the same temps you are using in your cast iron.

rhl

Perhaps. I’m going to try it first on the manifold. If it doesn’t work I’ll just replace it.

DZoro

I'd prefer to see the pump at the mixing valve moved to the Mix side. Thus pulling through the valve, and not pushing into the hot side. You will get better flow and system performance.

rhl

DZoro -- that has been done, agreed it works better.