What is the correct design/operation of a circ pump on a domestic hot water system?
Admittedly, I am not a plumber.
The way I see it:
Domestic hot water is run out/away from the hot water tank to the home. A return line brings the water back and dumps it into the inlet of hot water tank.
Circ pump moves the water; controlled by a thermostat.
The home I am working on has the domestic hot water circ pump wired together with the heating circ pumps. Domestic hot circ pump only runs when boiler runs.
Solution: place a thermostat (clamp-on type) on the hot return line to control the domestic hot water circ pump?
Is this correct?
Thanks all.
Jim
Replies
First, are you sure that you're looking at a DHW circulator and not the pump required for an indirect water heater, heated by the boiler? If your answer to this question is "I'm not sure", then you might reconsider your "working" on it.
OTOH, there are several ways that a DHW recirc system can be setup. The way you describe is one, the term is "aquastat", and when the return loop falls below a preset temp, it starts the circ pump. Some systems, especially in two story structures, simply use gravity to provide the water movement. Some systems use constant circulation (or manual control) and a common proctice is to route the return loop through the drain connection instead of the CW supply. IF the return is through the CW supply, a check valve should be installed.
Thanks for the input.The system heats the DHW with a coil in tank set-up (indirect).What looks like the DHW return line is set up with a check valve, circ pump and dumps into the cold water feed for the DHW tank.Should the DHW circ pump be controlled separately (with an aquastat) from the boiler circ pumps?This home is less than 5 years old and the pump "has never worked," according to the GC and homeowners. Guess that statement got me looking at system setup/design.This is rural Alaska, and anything can happen. And usually does.Jim
Just wanted to be clear before we got in to more details. So the IDWH works and makes hot water on demand, using the boiler as a heat source and were talking about a separate, convenience recirculation loop.
The recirc pump should be controlled by an aquastat such as http://customer.honeywell.com/Honeywell/ProductInfo.aspx/L4008A1015
set about 110 to 130 degF, on the recirculation/return line far enough away from the water heater so that it is not affected by conduction. The temp of the recirculation line reaches the set point, the contacts open and the pump stops (you wire the pump power feed directly through the normally-closed SPST contacts). There is a "differential" setting that is adjustable from 5 to 30 degF as well. 15 to 20 degF is a good place to start.
The control of the recirculation pump should completely separate from the heating system pumps, with a disconnect or master switch to turn it off when desired.
Great answer. Well writen and easy to understand.Makes total sense. This DHW pump has no switch of any kind. Runs only with the boiler circ pumps.Also interesting. the boiler zone valves (radiant heat) are on the supply side.
I always thought placing them on the return was industry standard.Thanks,
J
AKJim mentioned the comment below
"Also interesting. the boiler zone valves (radiant heat) are on the supply side. I always thought placing them on the return was industry standard."
Does anyone have the correct answer?
My two zone valves for hydronic baseboard heating are on the supply side, of the boiler, with the pump on the return. I don't have check valves but have them ready to install.
Is there a benefit to having the zone valves on the return side?
Pump the supply or pump the return? Control on the supply or control on the return? Which is correct? Both, if done properly.
Current practice is to pump away from the boiler with an air separator and expansion tank/prv/feed valve between the boiler and the pump.
Pumping the return was done in an effort to eliminate pump cavitation and "was the way it was always done". Controls on the return side of terminal devices (baseboads, convectors, coils, radiators, etc) is, as far as I can determine, a hold over from the old days of gravity circulation and steam systems, where a lot of the controlling and all of the pumping (if required) was done on the condensate or return side of the system. Also, with the classic hot water design of 180 degF (or greater)supply temperatures, the pressure drop across a control valve could (and often would) cause the fluid to flash much easier than the return side. Flashing was bad and caused control problems and valve damage. Components in the return side of the system are subjected to lower temperatures and smaller differentials, tending to improve longevity.
I have seen a lot of older hot water systems in commercial and institutional buidings and most were piped contrary to current "good practice". All worked well (in the absence of any other "issues"). As far as 2-way zone valves are concerned (these are on/off non-modulating), the location is not critical, but if installed on the suppy side can help to eliminate "ghost" circulation flow. There are many reasons for professionals opinions and preferences on this issue, but I believe there is no "right or wrong" approach. This is a subject that would be more fully explored at HeatingHelp.com's The Wall, section.
Thanks for the excellent info. I believe I may have some of those 'ghost' circulation flows thats why I was looking to put in the check valves.
I do have an expansion tank right after the boiler supply but no air separator. I have been looking at one of those after changing a couple of TACO cartridges right after some pipe modifications which I wasn't able to get all the air out of the pipes. ( I was sure that I had).
I'll check out the HeatingHelp web site. Thanks
My hot water heater is in the basement and I have 3 different recirculating zones and none have a pump. It works just great.I think the most important part is the check valve on the hot water heater supply line just prior to the return junctions.
Just get one of these; timer and thermostat built in.
http://www.grundfos.com/Web/HomeUs.nsf/Webopslag/PAVA-53MKRN
Do you have a grundfos up10 ?
I have a taco with a clip on aquastat and it never has worked consistantly. Mike
Trust in God, but row away from the rocks.
"Do you have a grundfos up10 ?"
I do not have one yet, but I have installed for friends and family.
No problems reported.
I do think that using the cold line for the return is a questionable practice.-The poster formerly known as csnow
I do think that using the cold line for the return is a questionable practice.
Yeah, I have a return line to my boiler, I am going to price the new pump after the weekend. Mike
Trust in God, but row away from the rocks.