New build - tankless hot water heater - recirculation pump needed?
Michael Lamb
6 years ago
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Annette Holbrook(z7a)
6 years agolast modified: 6 years agoPinebaron
6 years agoRelated Discussions
Solar hot water, tankless, gas condensing water heater...?
Comments (2)If you go with any sort of solar system, make sure it has a gas backup and not electric. My energy costs went up $100/month when I added our solar system down here in San Diego. I am now getting it corrected with the installation of a small gas tankless unit, using the solar storage system as a pre-heater. If you're building the home then many of the traditional drawbacks of tankless don't apply to you. You can build in the correct gas line and venting from the start. If I were in your shoes I'd probably go tankless (30 percent tax credit this year -- see the stimulus law or visit www.energystar.gov). If a single unit won't handle your demand then get two. And plumb it with the small tank system (10 gallon) for recirculation and to overcome any sandwich problems. This approach has been detailed in this forum somewhere....See Morerecirculating pump vs. local/point of use tankless heater
Comments (9)Donno if this helps, but we bought a 2 bath house with the old,disconnected hot water tank still in place, very close to the kitchen, and laundry room. it had been replaced with a tankless propane (rural) unit, 30 feet from the kitchen and 50 feet from the bathrooms. It was plumbed to the bathrooms first, and in series, thence to the kitchen. The first fix was to have a tee to the kitchen end installed at the tankless unit. We are leaning towards the electric point of use system for the kitchen, to fill in and then thermally cut out when the tankless gets hot water to it, though the plumber, upon speaking with the electrician, recommended we go with a recirc sysstem, which i think is rediculous, given our main heater is tankless. If I had it to do from the beginning, i’d skip the propane and use a combo solar pre-heat to a heat storage tank in our cellar (we‘ve installed a battery backed up 12 kv solar pv system) and electric point of use units for both the bathrooms and the kitchen and laundry room. As it is, we’re going to go with at least one electric point of use heater....See MoreTankless water heaters and recirculating lines: Good deal or not?
Comments (13)North Texan, I'm sorry, but your point #1 is simply not correct. A gas tankless is about 92 % efficient (condensing model) any time it is running. It is that efficient because it extracts enough heat from the flue gasses that it takes the gases below the point of condensing the water vapor - and thus even gets the latent heat in the water vapor. That's true any time it's running. It's not an average between "low efficiency and off" as you suggested. Condensing gas tankless water heaters extract so much heat that they vent with plastic vent. There are 82% non-condensing units that vent with stainless steel, but the vent is so expensive it makes more sense to use a condensing unit and vent with PVC. BTW: Condensing gas furnaces work on exactly the same principle. Standard gas tank heaters are about 60% efficient and can't get much better than that. The reason is that they must leave the flue gases hot enough to rise up through the atmospheric vent (hot gases rise). Atmospherically vented (standard) gas water heaters also have higher standby losses and air infiltration / exfiltration issues. There is a 3" flue up the middle of the tank that is open to the sky 24/7. They also take their combustion air from the conditioned air in the house and dump it outside requiring makeup air. Most gas tankless units get their combustion air from an outside intake. I think that you are confusing the issue of input requirements for a tankless heater. A tankless heater (gas or electric) has to heat water very quickly - and thus the input requirements are massive (a typical gas tankless is 200k btu/h vs 40k btu/h for standard gas tank). An electric water heater is almost 100% efficient already - and an electric tankless doesn't improve on that any. Electric tankless saves about 3% on standby losses. I have no idea where you got point #2 - but since point #1 isn't right, Point #2 is also not right. There is a significant cost to retrofit to gas tankless. In new construction the cost difference should be about $500 or less. I can't really comment on the payback because the install costs are so out of whack. In my view, atmospherically vented gas water heaters should be off the table - just as atmospherically vented gas furnaces are no longer available. I would never recommend whole house electric tankless. The operating cost savings (3%) aren't enough to justify any of the additional service requirements. There is no discernible payback. They don't generally produce enough GPM either. There are much better solutions for homes without gas - such as hybrid heaters. Electric Point of Use units can make sense in some situations, but not whole house electric tankless....See MoreDoes Tankless Hot Water Heater = Inconsistent Hot Water?
Comments (36)riellebee So I'm going to rough-in some numbers: Range: 15k btu/h Fireplace: 80k btu/h Furnace: 75k btu/h (x2) = 150kbtu/h water heater 200k btu/h (x2) = 400k btu/h Total potential load: 645k btu/h - again, an estimate. You can check your furnaces if you want to be be sure. is it likely that you would have enough things on at the same time to exceed your 425k btu/h service? - Yes, it's quite likely. What happens when you exceed the capacity of the service? The gas pressure drops and the range and the gas logs will just not run at full capacity - no big deal. But the things like the furnace and water heaters will throw error codes showing low gas pressure. The water heaters are likely to have that problem anyway, because at 30' of 3/4" pipe they are only getting about enough gas to run one of those heaters full tilt (the water heaters are variable input). The meter may be 20' away, but in actual length of pipe it's probably close to 30' including elbows. I can see close to 10' of pipe in the photo. The fact that the return line for the circulation isn't warm suggests it's not working. Fix that and insulate your pipes and your immediate problem will probably go away. BTW: Insulating the pipes is not expensive or difficult. Foam pipe insulation comes in 4' sections that just snaps around the pipes and uses self adhesive strips to hold it closed. It cuts with scissors. Even if you just did the exposed pipes in the basement you would reduce a lot of heat loss. My guess is that the builder can't really fix the "over-subscribed" gas service at a reasonable cost. One solution is to use a different water heater. A hybrid tank/tankless would solve the problem. The link goes to an AO Smith Vertex 75 gal water heater 100k btu/h and is 92% efficient and vents with the PVC vent pipe you have. I'm not particularly endorsing that brand - just using it as an example. It would provide you with plenty of hot water (you could run 1 shower continuously) and would reduce your BTU load by 300k BTU/h (gas service problem AND undersized gas pipe to water heater solved). In addition, because it's a tank of hot water - along with about 6' of hot water in the pipe - your wait would be significantly reduced even without a circulation pump. It sucks for your builder because it would cost them about $3500-4000 to fix it. They could probably realize some salvage value from the tankless heaters - say $1000-1500. But If they are only out $2-3k total I'd say they should consider themselves lucky....See Morevinmarks
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