An idea, good or bad

Tom_133

Wallies,

I have a customer who really wants to use a Rinnai E50C, but needs a bit more hot water. I suggested an electric 40 gallon as a storage for the DHW. He liked the idea of the E50C DHW piped into the cold water feed of a Rheem Hybrid 50 gallon water heater, with the flow restrictor removed from the Rinnai. I have my reservations, but can't really see why it won't work well. The house requires around 30K in heat loss, and has two showers. My thinking is when the hot water kicks on the Rinnai will dump 90-100 degree water into the Rheem and that the heat pump can bring that up the rest of the way to top it off pretty quickly. I appreciate any constructive criticism or suggestions.

Snowmelt

Sounds good to me

SWEI

The heat pump will run much more efficiently if you pipe it as a preheater.

Tom_133

Thanks for the responses, I was trying to avoid using the Rheem water heater as the initial source because the job is on natural gas and using the Rinnai to heat the large share of water would be cost effective. Honestly, I don't know enough about heat pumps, so I will listen.

SWEI

What are your net prices for gas and electricity (divide the total at the bottom of your bill -- with all taxes and charges and such by the quantity used)?

Tom_133

My concern is purely on the enough hot water, or does anyone see any glaring problems. The customer can decide which bill hurts his pocket more. I think this idea will work and I don't see any major problems, but just wanted to put it out there in case I missed something.

Ironman

SWEI said:

The heat pump will run much more efficiently if you pipe it as a preheater.

Agreed. But, will the tankless like the warmer water being supplied to it? Will that shorten its life span?

I can see the argument from both sides.

I wonder what either manufacturer would say.

icesailor

Well, if thought is put into the operation, the way I always connected water heater tanks to indirect tankless coils will work fine by connecting the DHW part of the Mod-Con boiler and use it as the source of hot water. A circulator pump will initiate flow if the pump comes on and mimics the cold water inlet when under use. You just have to be sure that the lead free circulator has enough head/flow pressure to initiate high fire on the pressure flow switch. If it is too low, it may not ever go into high fire. Only when there is a hot water usage call.

IMO.

SWEI

The real ROI for any capital expenditure must take into account the number of hours it will actually be used. A heatpump is both capital intensive and complicated. If you're only going to run it once in awhile, it may not be the best choice. Once we know the energy rates and something about the use patterns, I might suggest a tankless electric. Or not. It depends.

Tom_133

Swei, I hear you and if it was my project I would want to know ROI and length it would take to recover it.

In this scenario the E50C will do a pretty good job of handling the heat load properly but will fall short of DHW load. I typically install an on demand with a small boiler but this job is economically challenged. The E50C is has a good price point, the right heating Btu but I need a bit more hot water and right now the State of Vt is giving healthy rebates on this particular tank so I wanted to match them together. So I am looking for the best way do it. I like Ice's suggestion if I can get the right flow it would also work well, probably could use any electric then though, the benefit of the Rheem tank is I could use the heat pump system in the summer.

Mark Eatherton

"I have a customer who really wants to use a Rinnai E50C, but needs a bit more hot water. I suggested an electric 40 gallon as a storage for the DHW. He liked the idea of the E50C DHW piped into the cold water feed of a Rheem Hybrid 50 gallon water heater, with the flow restrictor removed from the Rinnai. I have my reservations, but can't really see why it won't work well. The house requires around 30K in heat loss, and has two showers. My thinking is when the hot water kicks on the Rinnai will dump 90-100 degree water into the Rheem and that the heat pump can bring that up the rest of the way to top it off pretty quickly. I appreciate any constructive criticism or suggestions."

How is it anyone with a lick of common sense believes that the energy the HP is scrubbing out of the mechanical room is "FREE" energy? Unless your mechanical room is physically located outside of the buildings conditioned envelope, with the exception of Summer, someone had to pay for the energy that is in the air, no?

Mechanically, it makes sense to preheat the water (assuming the tankless can handle preheated hot water and not all do...) but physically and economically, I find it hard to justify. As Kurt pointed out, I think you'd be better off preheating with the hybrid and final heating with the thankless. I think you have more btuH output from the thankless than you do the hybrid.

If it (hybrid) were ducted to the outside, using outside air, then in the words of the late great Gilda Radner, "NEVAH MIND..."

Check the thankless and make sure it can handle preheated incoming hot water. If not, it will short cycle.

ME

Tom_133

Thanks Mark, I am not very well informed with new hybrid heaters and will take any info I can to make sure I don't get involved in a bad install. Though I may be highjacking my own thread anyone want to list some pros and cons to the hybrid water heaters?

Larry Weingarten

Hello: Things to know, in no particular order: Some of the newer heat pump water heaters can be ducted, which allows you to do a little spot cooling or discard the coolth. If the house needs cooling, it can help. ~~ Heat pumps move a lot of air which gets filtered. Dirty filters can have a dramatic affect on performance. ~~ They are noisy. ~~ You need to make sure you have easy access to the anode rod in the tank, (if it's glass lined). ~~ They don't have a long track record. Time will tell if there are serious problems with durability. ~~ Work is being done on gas fired heat pumps. If the COP remains around two, this would make for really inexpensive hot water. ~~ Carbon dioxide heat pumps are available elsewhere around the world and are being looked at for importing to the US, so there is a lot of change in this arena. For another example, the Nexus unit from Australia is a water to water heat pump with a claimed COP of four! ~~ It really depends on your client's hot water usage. If it's low, don't bother. Efficiency in hot water use with low volume plumbing/fixtures and/or a drain water heat exchanger might save your client from buying a lot of equipment.

Yours, Larry

SWEI

Look carefully at the performance numbers for hybrid water water heaters. The 'hybrid' part of their name means that they use electric resistance to meet part of the load. Like all heatpumps, the COP goes down as the source air gets colder and as the output water temp gets higher. Again, I will ask about your gas and electricity rates? There is almost certainly a best answer to this but lacking any info on cost we can only guess.

Tom_133

Swei,

I appreciate the help, my original question wasn't about efficiencies, I really wanted to know if the idea was plausible or even a good one. I need to do a bit more research.

I know we have beat this question to death but what are you guys doing for small heat loads like 20K Btu with two bathrooms?
Typically I size the smallest boiler and install an on demand but this job needs an economic stimulus package!

My other thought was small boiler, big indirect?

j a_2

What type of heat emitters, are there? What is the calc. flow rate required based on your worksheet? Give the customer your recommendations, based on your worksheet…anything else can lead to trouble back on your end…Don’t try to create your own system….It is what it is….

psb75

I just installed a Rinnai E50C in north central VT. It is a very nice unit in the right application. The house had one and a 1/2 baths. So only one bathing apparatus. These units are only meant for small heating loads, like tight houses and apts with single baths. This unit's DHW production would be challenged being installed in a two-bath house/apt. Would need some add. help.
There is a very good rebate on the Rheem elect. hybrid tank DHW in VT. No one mentioned the dehumidification benefits along with conditioning the air. Who cares if the unit steals some heat from the utility area (basement?). Noise IS a factor. Also this unit is VERY tall. 6' and with annode rod.
Only problem I had w/ DHW was the anti scald, mixing valve I installed had "stiff" check valves on the hot and cold intake ports. After customer complaints of "spotty" and delayed DHW delivery,
I had to remove the checkvalve in the hotside port--because of the coldwater flow restrictor required on the E50C 1/2" supply line, incoming flow was not opening the spring-check in the mixing valve.
Not sure if the E50C would do well receiving pre-heated water. Call Rinnai. I'd strongly consider placing the Rheem downstream.

icesailor

I'm not disagreeing with anything. But, my experience is that regardless whet the recovery rate or how slow it is, with time, the tank will always recover. I saw it over and over with boiler tankless coils that were slow recovering because the coil might be dirty. But connected to a storage tank, it didn't matter. Because if you are using hot water, the cold inlet is whatever the incoming water is. But, once the flow stops, you are heating "Preheated" water. I'm only promoting the concept. If you need more hot water, and the source can't deliver it in the amount you need in a given time, make water and store it. You do it with well water pumps all the time.

Heatinghelp.com keeps locking up and crashing my computer. It just stops. Sometimes it saves what has been written, sometimes not. I don't remember my thoughts when this one locked up and crashed.

After this latest cold winter fiasco, any one that has a 20,000 BTU heat load and has two bathrooms in a house, needs to put electric heat in because it is the cheapest, and put in a water heater. Because the DHW load will far exceed the heating load in July AND January.

If you want to heat water, put in a water heater. If you want to heat a building, put in a boiler or WA furnace.

In spite of what they might say, the Media Prognosticators aren't telling you that Europeans expect their boilers to die in 5 years and be replaced, but drive their cars for 10 to 20 years or until they die. Americans expect their boilers to last 20 to 30 years and buy new cars before 5 years.

SWEI

Tom said:

I appreciate the help, my original question wasn't about efficiencies, I really wanted to know if the idea was plausible or even a good one. I need to do a bit more research.

I was working towards a tankless electric providing make-up post heat to the undersized combi. Depending on the fuel costs and use patterns, it may well be the best option, especially when space is limited (which we see driving much of the demand for combi units.) First cost will be less than an electric tank plus a pump, and for the case where the combi deficiency only occurs once or twice a week for an hour or less, it may well be the least expensive option to own & operate. Or not. It depends.

I know we have beat this question to death but what are you guys doing for small heat loads like 20K Btu with two bathrooms?

For a customer on LPG or oil, a 6 KW electric boiler (specifically, a Thermolec TMB-6.) Onboard ODR, no gas line or venting required, half the space (and one third the cost) of a mod/con.

My other thought was small boiler, big indirect?

For customers on natural gas here who are willing to spend the extra cash in exchange for the lower OpEx, that sometimes works. Most of the time, a separate gas tankless will cost less to install than the indirect, but with our hard water (and for customers who are too cheap or lazy to maintain one) we sometimes suggest the indirect.

Jack

I think I would install the E50 as Rinnai suggests and see how it works. There may be occasions where someone is inconvenienced for a few minutes, but you say this job is $ challenged. I'd want to see how it works before adding additional equipment. Do lay-out the job for the additional equipment and pipe it to accommodate whatever supplement you go with. You might find that it works well enough for the homeowner.

Another option at 30kbtu heat load would be an ES38 Energysaver (30kbtu output) and a tankless for the DHW. 25 yrs after their introduction I estimate that almost 5% of all the homes and apts in VT have one installed.

Jack

One other thing. I do not know what the flow/pressure drop would look like through the 50 without the flow restrictors.

Tom_133

One other option I was considering was the HTP Versa hydro. It would give the hot water I need and with it's 10-1 heating turn down ratio it would very seldomly short cycle if at all. I have used HTP in the past and like their products, but never this particular model. This house is small, the design temp for the system with plates is 130 which would be good for this unit as well. Any thoughts on the Hydro?

SWEI

Viessmann 222-F? Minimum firing rate of 11k and nicely packaged.

psb75

Viessmann 222-F is a fine unit. Price-point is prob. not suitable here.