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#356931 - 03/11/07 08:26 AM Venturi baseboard hot water system not reliable
grantnlee Offline
newbie

Registered: 10/19/04
Posts: 41
Loc: Warwick, RI
I have a large house with 5 zones of baseboard hot water heat and one zone of DHW. The Burnham V86 boiler is only a year old. Each zone has it's own circulator. Most everything works well except for one heat zone. Four of the zones are plumbed directly but the fifth zone is actually a venturi loop. This Venturi loop is the one I have trouble with.

This loop works most of the time but on occassion (at the worst times it seems) the zone does not get heat in all the drops. It seems that it only happens in the morning on the coldest of days! The main loop in the boiler room get's very hot when the zone is calling for heat, and it seems that the venturi drops that heat the addition at the same grade with the boiler always work, but the feeds off this loop that heat the room one floor above the boiler room are not always hot. Most of the time they are, but as I say at the worst times, they just decide to remain cold!

The house is 100+ years old. The venturi zone supplies an addition that is only 50 years old. The house has been in the family the whole time and this zone does not have a history of trouble which makes me feel that the venturi design is not the problem.

Any ideas what could periodically cause this to stop working? I'm about fed up with this whole Venturi design but am not sure how to replace it since it's got multiple drops off the main loop.

Thanks in advance for any advice!

Jay

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#356932 - 03/11/07 09:05 AM Re: Venturi baseboard hot water system not reliabl [Re: grantnlee]
HeatPro Offline
Carpal Tunnel

Registered: 07/30/04
Posts: 28022
Loc: South New Jersey
A venturi circuit operates by having sufficient gpm through the resistances in the tees of the main to drive some water to the radiation. IF the main venturi tees are large for small flows, they can become undependable, as the resistance in the mains are marginal to drive the radiation loops from them. This can be corrected by adding a balancing valve between the risers so flow can be adjusted to the radiation above (it is the function of the venturi tee to do that; but if it can't, due to lack of planning the flow, then the balancing valve becomes the main control.) As you don't give gpm flows for that loop and btuh requirements for the radiation above, it is likely you don't have the knowledge to properly size the main and tees for that loop, as the installer didn't have.

Also, venturi circuits become air traps, as there is minimal force to move air downward to the main to be purged at air scoops. Multiple zones have many such air traps, but venturi circuits moreso. Air control and elimination requires care.

http://www.heatpro.us/1pipehwtree/documents/7F98E674CE9051033975E463F8FA6E8E5EF07CF2.html
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#356933 - 03/11/07 07:36 PM Re: Venturi baseboard hot water system not reliabl [Re: HeatPro]
grantnlee Offline
newbie

Registered: 10/19/04
Posts: 41
Loc: Warwick, RI
Thanks HeatPro. I read through your site information on the American style single pipe design. Great background. I've been searching and picking up that information in bits and pieces for months but your site puts it all in one place.

The thing that drives me crazy about this is the way the problem comes and goes. Randomly (once a month or so) we wake up to the room being 50 deg. (when it's 10 deg outside) and then suddenly at 11am the radiation kicks in and starts to warm things up. The circulator is running smoothly the whole time and the main trunk is hot. But its an all or nothing heat question, not like a gradual luke warm flow that finally get's hot.

Is there any chance that this is a bleeding issue? This is the only branch above the main trunk. Could you help me understand the process of bleeding this type of system? I've had the oil company over and the tech did not seem to have experience with this type of design. Should I have some type of bleed valve at the high point of every branch that is above the main trunk? Your pictures don't show one, but the oil tech was saying that each elbow going back down into the floor should have one.

Also one other thing continues to nag me about the design. In the room in question, I've got about 30 ft of radiation and I estimate 75 ft in total circuit length. At the start of the radiation, I've actually got two smaller pipes that come up and join together before entering the larger radiation pipe. This is right within the baseboard itself at beginning. With all the rest of the plumbing hidden in the floor it's difficult to track them down. I was suspecting that maybe they used two venturi diverters feeding such a long run. Have you heard of this type of configuration for a branch? Seems I need to diagram out the plumbing for reference and to sort out what is going on sight unseen in the floor.

Thanks again for the information and any additional help.

Jay

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#356934 - 03/11/07 07:44 PM Re: Venturi baseboard hot water system not reliabl [Re: grantnlee]
HeatPro Offline
Carpal Tunnel

Registered: 07/30/04
Posts: 28022
Loc: South New Jersey
You are on the right track. "the oil tech was saying that each elbow going back down into the floor should have one. " I agree, as each baseboard becomes an air trap above the main.

You probably have 1-inch main and small flow through the 30-footer. 75-feet is really too long a circuit for a single monoflo tee venturi to handle, so two is better there.

Another point is that you probably don't really need the circuit to be one-pipe but would do better to make it a series.
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