Radiant heat in a gravity system

Rob Holzapfel

I have a customer who wants radiant heat in their basement floor. The existing heat is gravity hot water radiators. Can I run a loop on a pump off the bottom with out affecting the gravity system?

Mike T., Swampeast MO

Is it still under gravity circulation (no pump)?

If so how many connections to the boiler? Typically there are three--two for supply and one especially large for return after the two returns have been brought together--but smaller systems may have a single supply and return.

heretic

closely-spaced tees

Hey, funny this should come up.
I was touring an old house recently, and I actually saw one of these! Gravity system with 'powered' low-temp loop coming off it.

This is how they did it:

The low temp loop was tied into the gravity system using closely-spaced tees.
The tees were tapped into the common return (It was like a 6-inch pipe!). I assume they pulled from the return so as not to mess up the gravity supply, and also to get the lower temp return water. Anyways, this 'sub-zone' was clearly a 'slave' to the big gravity zone, scavenging warm water when available. A gravity system is essentially under 'constant circulation' to some extent, so the warm water needed may have been available most of the time. It had a simple 'dumb' 3-way diverter valve setup to provide a fixed temp (when available). The circ was wired to run off a thermostat (not tied to boiler operation at all), but there appeared to be a floor temp sensor (wood floors) acting as a high limit switch. There did not appear to be any provision to protect the boiler from low return temps, but the 'sub-zone' was quite small relative to the entire system (remodeled kitchen, perhaps 12X18 in a massive house), so it was probably unable to flood this [very] high-mass system with enough cold water to matter in this respect.

Reflecting on this setup, I am curious whether feeding that colder water back into the return would essentially 'speed up' the flow in the main gravity loop, but this may again be insignificant given the relative scale of things.

The owner was not around to ask about the system. The contractor did not 'sign' the work.

For your larger load, there may be more issues to consider.

Mike T., Swampeast MO

Corey

Wish you would have seen what I erased! That is EXACTLY was I was going to "guess" if it was still under gravity operation--including the close tees in the SHARED portion of the return main. Tapping in might be an experience!

Gravity systems are mighty forgiving when it comes to low return temps but like Corey noticed and I suspect, I wouldn't try to take too much of a load in this way as you might increase velocity just enough to make the system start favoring the closest radiators.

For tapping you might be able to use the type of system that water departments use to tap into pressurized water mains for a new service. Would certainly save a LOT of work even if you have to rent. I believe they use the pressure inside the pipe to "blow out" the drilling debris through the hole into the new line...

Mark J Strawcutter

gas

It's even more interesting to watch them do a similar "live tap" into a high-pressure gas line :-)

Mark

heretic

Tapped

Yes, they actually tapped a pair of brass nipples (3/4 NPT) into the wall of this big pipe. Not something I have seen before, but I suppose there is plenty of metal for the threads to bite on.

[Deleted User]

pipe tapping

You could use a Victaulic pipe saddle. WE use them on fire sprinkler and hot water pipes all the time. They work well.

Steve Toland

Slantfin Hydronic Boilers

ok... slantfin L - 30 3section castiron has - triple aquastat wired for priority on the main zone wich is monoflo
2nd zone is basement baseboard. the venting - there are 2 3quarter inch tappings on the top and they are vented with #67 autovents. The header on the other hand doesn't have an air scoop or any type of venting on it. demestic hot water runs off the 5 gallon coil with a watts 70A mixing valve. Aquastat settings are at high 160 and low 150. The burner is a Riello F5, has a .65 60 hollow nozzle... Problem - when taking a long shower or the heat has been calling for a while. the boiler overrides and percolates... it's new equipment but it's not the way i would pipe up the boiler. i just went over to look at the problem... my thoughts on how to fix it - i would like to put an airscoop on the header and put a dual aquastat on there and maybe a 30 gallon aquabuster...lol...???? i don't know... thank you

Steve Toland

Slantfin Hydronic Boilers

ok... slantfin L - 30 3section castiron has - triple aquastat wired for priority on the main zone wich is monoflo
2nd zone is basement baseboard. the venting - there are 2 3quarter inch tappings on the top and they are vented with #67 autovents. The header on the other hand doesn't have an air scoop or any type of venting on it. demestic hot water runs off the 5 gallon coil with a watts 70A mixing valve. Aquastat settings are at high 160 and low 150. The burner is a Riello F5, has a .65 60 hollow nozzle... Problem - when taking a long shower or the heat has been calling for a while. the boiler overrides and percolates... it's new equipment but it's not the way i would pipe up the boiler. i just went over to look at the problem... my thoughts on how to fix it - i would like to put an airscoop on the header and put a dual aquastat on there and maybe a 30 gallon aquabuster...lol...???? i don't know... thank you

Rob Holzapfel

Yes, it is still under gravity circulation and there is a single feed and return (both 2 inch). I need to heat 500 square feet of basement floor with 800 feet of 1/2 inch pex tubing (4 loops). The existing heat in the house is beautifully balanced the old timers who built the system were talented.

Mike T., Swampeast MO

The cautionary part of my is seeing a bunch of red flags popping. I'm seeing too much circulation.

Higher circulation will result in a lowered supply temp and the boiler will be running more frequently but for shorter periods. Delta-t will increase and return temp will drop--possibly below the temperature needed to heat the radiant in the basement. Temperature balance in the house may be adversely affected as well.

The "fun" part of me says, "try it"--that is as long as the homeowner understands there is a good chance that the entire system will have to be converted to forced circulation and the near-boiler piping reworked. You MIGHT be able to tap into the mains with a second circulator for the radiant without going to full-blown primary secondary but it's all going to depend on what kind of water temperature you have after it is converted to forced circulation. Remember--gravity systems converted to forced circulation typically operate at VERY low temperatures.

Rob Holzapfel

I think running the radiant heat off of a separarte hot water heater will expose the home owner to the least financial risk. I won't have to convert the milli-volt system over to 24V, repipe the boiler and rebalance the system.