Hot Water Recirc with Insulated Lines — is it really that inefficient / expensive?
I’ve read up on hot water recirc and most of the discussions end that its too expensive / inefficient / not a great idea. I’m also guessing that most of these scenarios don’t have fully insulated lines.
By my math, it shouldn’t increase monthly energy costs by THAT much — am I wrong?
Scenario would be:
-new Radiant underfloor heating for main floor and ceiling radiant for basement
-4 bathrooms with up to 8 people in the house at a time
-easiest / least expensive solution is a Combi unit for radiant heat and domestic hot water which would be located across the house from the bathrooms, roughly 45′ away (exploring the possibility of moving all of this to the other side of the house, closer to kitchen/bathrooms but undecided)
-I really don’t want to have to wait 90 or 120 seconds or more for hot water, if I can avoid it
-I have full access to basement ceiling and am running all new PEX domestic water lines, and can insulate them as needed
I am trying to finalize design on an all new heating and hot water system — decisions include:
1) Combi unit vs separate units?
2) HWH across the house, 45′ from bathrooms, or intentionally place it much closer?
3) Recirc hot water or not, depending on items 1 and 2
Is there a great solution to be able to recirc hot water without it costing a lot? Hoping insulated lines would be the game changer here. I’ve also read about optical sensors in bathrooms to trigger the recirc pump on demand which seems smart as well, if needed.
Thanks for any input!
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I have around 70 feet of fully insulated recirculating lines in my hot water system in my home. There is a 1" supply line and a 1/2" return like that I added when I put in the recirculating loop. I have the 1" line fully insulated with the usual split pipe wrap (around R3 or so I think), and the 1/2" line is insulated with the same type insulation and is tightly held against the main line, and slightly under it, using periodic wraps of high quality tape. The idea is that some of the heat loss from the recirculating line will help to somewhat limit the heat loss from the main line. Anyway, I didn't really notice an increase in natural gas use when I added the recirculating line to the system.
I use a themosyphon system, which has no pump. To do this, you have to have the water heater under the living space in home, and you can't have "heat traps" on the hot water outlet from the water heater. You need a swing type check valve on the recirculating line just before it enters the lower part of the tank on the water heater. In my system, I extended out the drain valve a bit and brought the return line into a T before that drain valve. Hot water out the top, returning water in down low. This system has been working well for me for about 7 years now, with zero problems.
All that said, it's still a good idea to try to locate the hot water source as close to the fixtures needing hot water as possible. This limits line length, which will limit thermal losses as well as "time to hot". In my case (an existing home), that wasn't possible, so I didn't really have any choice. If you have the ability to put the hot water source near a cluster of hot water needing things (shower, sinks, etc.), that is a great way to improve system efficiency.
If you go with a recirculating system, keep in mind that the pump uses energy, but can be turned on/off in various ways. A thermosyphon system thus can limit thermal losses somewhat, but not operating continuously, but will also mean that there is the potential to have to wait for the system to run a while before you can have hot water quickly. A thermosyphon system uses no energy to run a pump, but has thermal losses ALL the time it's operating. A thermosyphon system also has no expensive pump that can break one day. There are always tradeoffs, so you have to pick the combination of things that will work best for your specific application.
Bill
In heating season, the heat lost by the hot water pipes goes into heating the house. If you have a combi boiler, the heat for both is coming from the same place, it nets out.
In cooling season the heat lost adds to the cooling load.
Thank you for your input.
Are there concerns with using a recirc setup on a Combi tankless boiler? Or any tankless HWH?
a) stealing priority from radiant heating system
b) short cycling boiler every time the recirc turns on
Or other concerns?
The most compact, inexpensive, easiest to duct route is a combi boiler, if it will do the job.
Its also possible to have a second dedicated hot water heater, with additional cost, additional ducting, etc. if it is a superior solution.
What are the 4 bathrooms? If 4 full baths, a combi is not a good choice. Combis have to perform two jobs: if the DHW (short, sudden loads) are huge, then it’s got to be sized for that. In that case, the central heating (smaller, longer loads) will suffer, because you’ll have a massively oversized boiler. Also, play around with a pipe velocity calculator, you’ll find that a 1/2” pex line moves water quickly. You might not need a recirculate loop at all if 10-15 seconds is acceptable for a shower.