What are your goals? It sounds like you would like to conserve energy based on the Solar/Geothermal remarks.

In this case, I would consider a ~200k BTUh condensing tankless heater. The cost for the heater itself would be about the cost of the powervent heater.

I would replace the one that went out first, then the second one when you can.

Natural gas is at record low rates and will stay that way for some time in the future. A condensing tankless will be about 92-95 percent efficient compared to your 65% efficient powervent. The second water heater will use no fuel unless there's actually water used in the guest or basement bath.

You will will want to insure that your gas supply to the water heaters is 3/4" and the supply to the entire house will accommodate the new requirements. These heaters vent with PVC pipe, so venting is cheap - you may be able to reuse existing venting if it's plastic.

It's generally not worth going tankless if you have to increase the gas service to the house.

Thanks for the reply. Yes the goal is to save energy. I keep reading not so good reviews about tankless water heaters. Inconsistent hot water etc. So confused.

I have had a tankless for about 4 years now and it's great.

It takes slightly longer for the water to get hot - but that's not really a problem for me.

The first heaters, particularly the low-end Bosch units, were really bad. In addition, you had installers who didn't know how to spec a system causing problems.

At this point, the units are good and there's a lot more knowledge on the part of installers.

The plan to replace one unit at a time would give you the chance to decide if you are satisfied before you switch the second unit.

Also, if it's important to you that you get faster hot water, and don't ever get a "cold sandwich" a small electric tank heater (5 gal) will eliminate those problems.
That's something that could be added later if you decide that you must have it. I've never felt the need for it... it just hasn't been an issue for me.

I'm a Plumbing & Heating contractor, and the thing about Tankless that wasn't mention was, all manufactures recommend a water softener to go along with their units. Tankless water heaters have a coil inside just like boilers do, and they can and will lime up over time. You can either acid or use another product to flush out this coil, so that is something to keep in mind. There is maintenance with tankless, so please don;t think it's installed and left alone for good.

I'm not a fan of tankless, and wouldn't personally install one in my own home. That's just an opinion so no need for anyone to read into it. I like the concept of tankless, but over-firing a unit to make hot water...that technology has been around for the last 50 years. More bugs need to be worked out with tankless before I would ever consider putting it in my home.

IMO, tankless is meant for small commercial all the way up to industrial applications. However, you do have a 7000 sq foot home, and there will be lots of run off, so a circulating line would be a must if it were my home.

Also keep in mind, if you live in an area where it gets cold during the winter, and you don't have enough hot water, you have to bank another tankless right along side of the other one. You would potentially spend about $6,000.00 (give or take) to have both installed. Also, the payback for the tankless just isn't worth it imo....I'll stick with tanks.

Ozone89,

Let's back up a little - you are mixing opinion with facts that aren't necessarily true. You are certainly entitled to your opinion...

I can say for sure that it's not true that all manufactures recommend a water softener -- because mine didn't. I checked on the new Rheem water heaters and they don't either.

Both tank water heaters and tankless water heaters require maintenance and both are adversely affected by hard water. We deal with limed-up tank heaters in this forum all the time. The good news is that you can reasonably clean tankless heaters with vinegar - try that with a tank heater. I'm on city water and I've cleaned my tankless once in 5 years.

Regarding the technology -- It was reasonable 5 years ago when I purchased mine, at this point it's pretty darn good. The drop in price of condensing units means that you can avoid the expensive stainless steel venting. They have minimum flow rates down (.40 gpm activation and .26 to keep it going on Rheem unit) and the electronics and such are pretty reliable.

As for your last point: I don't know the OP's inlet water temps... but for argument's sake, let's say it's a worst case and the inlet temps are 35 degrees (happened at my house when it was -20 F outside). ONE ~200k heater would provide 5.4 gpm @ 70 degree rise - which is enough hot water for two people to shower in standard showers at the same time and still have .4 gpm left over. In less extreme situations, say 55 degree water inlet temps, the GPM is 7.6 - or three showers going at the same time.

I can tell you that my tankless has already exceeded break-even on the payback - if I had the option of a standard 40 gal gas heater, which I didn't. I would have required a powervent which costs as much as my tankless. I have a $46 monthly gas bill in Indianapolis, and all major appliances are gas. I'm a happy camper.

The OP is in the same situation as I was - the alternative tank heater is a powervent... It's all savings from the first day. They also have power to the water heater and potentially even a usable vent if it's plastic - which will reduce the installation costs.

Thanks for the replies guys. I read similar stuff what Ozone is mentioning, but Jake, your point makes sense too. I do have plastic vents, so installation may not be that expensive. What brand of tankless you have? Thanks!

I have a 5 year old Rheem.

They have newer Rheem units that are condensing models.

These things have been around for a while and are used extensively in the rest of the world - they are just recent to the US in the last 10 years or so.

Be sure to run the numbers for your 10, 15 or 20 year ownership and operation costs. There are helpful tools at the US DOE web sites. "Tankless" is a costly way to heat water when I looked at it. If rebates in your location are good, and your location is good, solar could be better. If you are in a hot, humid climate, a heat pump water heater is good.

Here is a thought. Are you planning other major energy saving changes? Hire an energy evaluator/rater to help you pick the low hanging fruit. They can help you choose a water heater right now.

Ionized:

Just curious, how was Tankless costly when you looked at it?

Where you looking at condensing gas tankless vs gas powervent? the savings is about 30% (.65 to .95) and unless gas service has to be increased in this case, it's about the same installed costs.

Too bad, the gas pipe going to the water heater is 1/2 inch. So I am out as far as tankless goes.

No, you can change the size of the pipe if it's not to big of a deal. The deal killer is when you have to bring new gas service into the house.

Lifetime costs that include: 1) investment in equipment, 2) fuel costs for projected usage, 3) projected lifetime of equipment, 4) opportunity costs for the higher-priced equipment investment.

Tankless DHW is pretty heavy in (1) and thus (4) compared to simple tanked water heaters. It is difficult to catch up with gas as fuel.

Ionized:

In this case they don't have an option for a simple tank heater. They require a powervent heater - which is around $900-1200.

The Rheem 95 condensing tankless is $1300 with the service valves. They already have the power to the heater and they already have plastic venting. This should be a fast, easy and cheap install.

Here is a link that might be useful: Rheem condensing tankless

yes! i also bought one from suntekstore. it seems not bad