Forced circ zone on grav system

Jeff Perry_3

I am still dealing with the 1-pipe gravity system some of you may have read about last month. The main house is a downfeed 1-pipe gravity system and it looks like the owner is going to leave it alone. However, sometime back when indoor plumbing was invented they built an addition on the back of the original house and added all the baths with rads. It was piped as a 1-pipe gravity system also except it is a horizontal loop that goes around the mechanical room. They connected into the unused supply and return tappings of the boiler. We come out of the boiler, run approx a 60' horizontal loop and right back into the boiler. Supply taps for the rads come off the top of the loop, returns tap into the side.

The addition has been completely torn off. They are building an all new 2 story addition. I want to turn the loop into a 2-pipe reverse return with cast rads and use a circ. Does anyone think the main gravity system will be affected? Main thing I'm worried about is loss of flow in the main house. I can't see an issue but what am I potentially not seeing.

Oh, and Dan.... these security question math problems are getting out of hand. It's bad enough doing a 1+1 after a hard days work but today's question was double digits !! I have a headache !

Mike Kusiak_2

Why not

If you use completely separate boiler tappings for the circulator loop supply and return, I don't see how that would affect the flow in the existing gravity system piping. As long as the temperature differential exists in the gravity piping supply and return, you should have flow.



How will you set up the circulator loop? As a separate zone, or with a small circulator operating continuously?  It might be more difficult as a separate zone as you cannot maintain temperature in the boiler without it being lost to the gravity zone and overheating it.

CMadatMe

How About Something

Like the attached. Although you have existing hot water would think you could do the same thing where as you could then use the boier taps  to a manifold and elimate the need to use the main. Just a thought

icesailor

Forced circ zone on Gravity System

Some of us really know how to pick them. And they don't make it any easier for us.

If the One Pipe gravity HW system you write about is an antique counter flow system, I don't think you can pump it. I don't see how you can mix the two systems unless you leave the old system as it is. If it is a normal gravity system, you could pump it. If its a counter flow gravity, you can't.

As big as an addition as they contemplate, I would suggest to them another system. Like a modern boiler and an indirect for the new and leave the old as is. Anything you try to do will most likely bite you in the butt.  I know that I speak heresy here when I don't loose my sensibilities over low temperature systems but the size of the addition, and the additional radiation required to heat this addition with low temperature water will add considerable cost. That cost could pay for a boiler.

I always try to give lots of choices so that someone doesn't get sticker shock when the prices come in. The basics to the top. I don't know where you are but I could see this easily going to scorched air with air handlers and a system that is totally screwed. In my opinion, from what you have described, its like a minefield with most of the mines already marked. It's up to you to find the missing mines.

Jeff Perry_3

Now that is an issue

I didn't consider until after I had posted the original thread. I've been thinking about it and we don't have a lot of options. I can't get into the original house piping so that has to remain the same. I could do constant circulation with TRV's and a by-pass. However, one thought is will the old system really overheat? You have an old house that they can't insulate because it's solid exterior walls. It has a downfeed gravity system that operates slow as hell with a huge water content. We have a new addition with foam insulated walls and built to be very efficient. Tube radiators and a small main loop and rad feeds. So low zone water content. For the amount of time the addition zone would run, will we really overheat the main house?

Jeff Perry_3

We gave them

all kinds of options. From just replacing the controls on the old boiler all the way to epiping and a new Viessmann wall-hung. But money is a real issue. One option they are looking at before they close all the gutted ceilings up is to leave the system as gravity with the existing boiler but repipe the mains so we end up with a 2-pie gravity system. That way, in the future the boiler could be changed and the whole thing turned into a circulated system with even TRV's without uch effort. The option of doing that is dependent on what my price comes in at to pipe up the addition zone. You know how it is, total cost a lot of time drives what options they go with.  

We won't need to worry about a scorched air system. It won't happen. That mne has been found and disarmed.

icesailor

Bigger problem:

I think that the bigger problem will be when the "new" zone is calling for high temperature and the old part of the house gets a call. Who gets priority?

You could use multiport thermostatic mixers and run the boiler at a higher temperature and control the old part. But "The laws of unintended consequences" may come into play. "No good deed goes unpunished".

I've never seen a new system mixed with an old that would balance without doing multiple zones.

Mike Kusiak_2

TRV's

TRV's and constant circulation is what I had in mind. You already have constant circulation and higher heat loss in the gravity zone, so the thermostat in the old house would set the system water temperature. With constant circulation and TRV's in the addition with far lower heat loss, the TRV's would individually zone each room, with the existing system water temperature more than adequate to meet the load.

Jeff Perry_3

Now, I like that.

Of course, with the large water content in the whole system it does take forever to heat up but with the well insulated addition I think it is not an issue, The stat is on the middle floor of the old house, We already have a top floor that is too warm, middle just right and bottom floor cool. (Sounds like the 3 bears). We can run a stat wire to the addition for when, or if, they change the system over but meanwhile just do constant. And I even have one rad that I could leave without a TRV without overheating anything and we wouldn't need a bypass.



Thanks



Jeff 

Jeff Perry_3

Man

I have to slow down and type better. You have to interpret 6 or 7 words in my post just to figure out what I'm saying.  Like my efforts trying to text. Fat thumb syndrome.

Jeff

Gordan

Yeah...

The two-pie gravity system sounds like it would be messy...

Jeff Perry_3

Now that's hilarious

You just made my morning. I'm still laughing.

Jeff

Mike Kusiak_2

Radiator sizing

Just remember to size your radiators based on the actual temperatures you see in presently with the existing system. With the high water content, the old system probably never gets to the usual hot water design temps. These old gravity systems operate with their own sort of "outdoor reset", with the high mass and water content averaging the boiler cycles and resulting in water temperatures only as high as needed to meet current heat loss. Since the existing thermostat is in the old uninsulated building, maintaining room temperatures here should result in higher than needed water temperature for the addition if the new rads are sized appropriately.

Jeff Perry_3

Good point

Very true. I will do so. A couple of the rads being used are the old ones, which will be oversized now. However, 3 or so will need to be new/used.



Jeff

icesailor

Texting with fat thumbs:

That's why I stick to the basic phones. No texting for this guy with bear paws for hands. Extra Large gloves from EMS are hard to get on until I get them stretched out.