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Rinnai tankless running hydro air unit?

heatpro02920 Member Posts: 991
Hello all, this is my first post, but I have read through often. I own an HVAC Mechanical company in RI, and I have been in the trade for longer than I want to admit {hate to show my age here, lol}....

One of my techs recently ran into a service call that stumped him and required me to follow up, the original problem turned out to be a faulty board in a 3ton first co. hydro air unit. {not a big problem, I had 2 instock, so it was back to running that day}. But here is the real issue...

The homeowners are young and very nice kids, they recently had a new heating system replace their oil fired furnace, which is a hydro air unit and heating the water for the coil is a Rinnai condensing tankless water heater.... Which I don't like since it is only set to run at 140 which is just one of the problems... I would love to just replace the rinnai with a gb142 or something that will hang on the wall and run their system correctly but they can not afford that, and they have since finished the basement and there would be no room to throw in a boiler of any kind.... so I am going to have to in one way or another get this to work for them, and I think I have a good idea how, I just want to run it by any other pros that may have experience with this...

The unit is heating the space, but it is costing them a small fortune in propane, because when the heated water retruns from the coil it is too warm and causes the rinnai to modulate down, and as many may know when rinnais mod down to the basement they get very inefficient {something like 45%}.... Plus running 140 through the coil that may be undersized as it is, is causing the unit to run constantly...

OK, here is what I want to do....

Swap out the 007 with a 009 s/s circ {I have4 used ones on the shelf}, then add an aquastat to the return, and a taco sentry 3 way zone valve after the circ in between the supply and return, Im also going to add a psi reducing valve, exp tank, and relief valve since none of which are there currently.... Finally adding a commercial control to the unit to allow 180* water temps...

So my theory is, the rinnai will heat to 180 then circulate through the coil, if the water returns higher than 140 {or where ever I figure on setting the aquastat} it will divert the flow back through the hydro air unit, until finally it return cold enough and the aquastat sends it back through the rinnai to get heated...

I wish I could add a storage tank but there is literally no where it could go....

heres a pic I made up to help explain it <a href="http://i998.photobucket.com/albums/af107/turbobike1/rinnaiheat_zps346ded83.png">http://i998.photobucket.com/albums/af107/turbobike1/rinnaiheat_zps346ded83.png</a>

So what do you think, am I on the rite track?

thanks in advance


  • VictoriaEnergy
    VictoriaEnergy Member Posts: 126
    return temps

    To get the boiler to operate at higher efficiency, you need to get lower return water temps into it, so I don't see pumping more GPM (netting lower delta T) through the same coil helping.

    A coil with more heat transfer surface would be beneficial

    It would have been simpler and more energy efficient to have just installed a forced air gas furnace in the first place.  Replacing it with a gas furnace will probably be your best bet in the end. 

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  • heatpro02920
    heatpro02920 Member Posts: 991

    Thanks for the reply, I am thinking the greater flow from the 009, and using a 3 way diversion valve along with an aquastat will give me control over my retrun temps...

    Im thinking the aquastat will tell the 3 way valve when there is less than a 50* delta t and then divert the return water back to the coil, then when the retrun temp reaches more than a 50* delta t the 3way valve will direct the return water back through the Rinnai... Plus using the mcc 180* control will help, so now I can set the rinnai at 180, then the 3way valve at 130...

    So the t-stat calls for heat, kicks on the 009, the rinnai senses flow and fires, the water circulates through the coil and when it doesnt return cold enough for the aquastat setting, the 3 way valve diverts it back to the air handler bypasing the rinnai..

    The point is to keep the rinnai in high fire for as long of the cycle as possible since this is where it is 95% eff. When they are in low fire they seem to be more like 45% eff.... By increasing the flow and bypassing the high temp return Im thinking I can acheive this... Just want some confirmation from other pros...

    THanks again, if anyone else has any ideas please let me know...

    As for the furnace, they are not going to have the funds for something like that, since they will need to add a case coil and still need to remove walls from the basement to remove the new equipment...
  • CMadatMe
    CMadatMe Member Posts: 3,086
    edited January 2013
    What's the Heat Loss

    For the space that the hydro air is heating? What size is the hydro unit or hw coil? The 009 is a high head low flow pump. There is not much of a pressure drop in a hydro coil so I don't see how changing pump is going to help you.

    Let's say you have 3' of head pressure drop across the coil and 2' of head in piping. That's 5' of head. Say the coil needs 4gpm. Your going to get better flow with a 007 then you would with a 009. Pump is going to operate on its curve.

    I'd also have the unit piped primary secondary not direct. Your dealing with a tankless so your limited in gpm the unit will make for heating. What size unit is it? Most tankless heaters are limited to only being able to provide 4gpm on the heating side. Your flow issue may be that you need more gpm then the unit can provide.

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  • heatpro02920
    heatpro02920 Member Posts: 991
    edited January 2013
    Thanks chris

    I had the 009 so I figured it would do better than the 007, I figured the head at or above 18 since I was under the impression the rinnai unit added 9ft alone and first co told me the head of the hydro coil is over 5, plus Im adding a zone valve, and there are a lot of bends in the piping.... And the 007 is under 1gpm at 11ft.... where the 009 stays above 7 until around 16...

    If you think the 007 is a better choice I will try it again, I am an old timer that is very open to advice from others with more experience in circulators ,Im just stareing at some manual charts with my failing eyes, too many yeas of steering at fire, lol.....

    The unit is 199K btu 9.8gpm 96% eff. per the box, lol...

    I though primary secondary would hurt efficiency with a tankless, I have done it before but when there were multiple zones not just for one small hydro coil? Is my thinking wrong on this? I thought the diversion valve with the aquastat would be more efficient and keep the unit out of low fire {where it is using a lot of fuel and sooting up}....

    Any help is appreciated, thanks for the reply, please tell me more... Im a fast learner....

    Rinnai says to use a pump with 5 gpm and 40-45 foot of head....
  • CMadatMe
    CMadatMe Member Posts: 3,086
    edited January 2013
    I Don't Know

    What unit you have to pull the install manual for the max flow and pressure drop across the hx. Normally tankless do not want more then 4gpm on the heating side. This manual

    http://www.rinnai.us/documentation/downloads/U307-071002.pdf show the pressure drop at about 5.5ft of head for a 4gpm flow rate.

    What size is the air handler coil? How many btu/hr do you need to get out lf it? What is the model number?

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  • icesailor
    icesailor Member Posts: 7,265
    Wrong Application:

    Everything that Chris and others say is true with their figures. I'll cut to the chase.

    Whomever decided to install that Rannai, installed an appliance that is not suitable for that application. Last I heard, that was a Potable Water heater, not a heating boiler.

    These heating guys are always trying to be plumbers and supply potable hot water with their heat and are clueless as to how much energy it takes to heat potable water.

    For example, pull up your friendly oil boiler manufacturer and look for a boiler with a tank-less coil. Look at the smallest boiler, and look at the gpm of the tank-less. Look farther until you see one rated for a 4 GPM flow. It will be well over 100,000 BTU's. Is the Rannai rated as high?

    I see these applications all the time and they are not working. Do you tell the customer they got screwed with a misapplication or do you just fade away.

    Its my experience that no matter what you do to try to put band-aids on the infection, it won't heal. Only the properly sized unit will fulfill the demand.

    The reason they have come out with Combi Units is because water heaters don't work as heating devices.

    No matter how hard you try, you will come out badly. They need the proper appliance. You didn't spec or install it. YOU need to fix it and make it proper.
  • heatpro02920
    heatpro02920 Member Posts: 991
    I know its not rite, just want to make it better than terrible...

    I know its not the proper unit to use, but it is what it is. I thank you guys for confirming that it is wrong, I was 100% aware of that and I wish they called me first because they would have a properly sized 95% furnace with a case coil for less than they paid for this mess...

    So I either have to make it work or walk away, so this morning I went and installed the 3 way valve {which my taco rep was nice enough to give me as a demo unit that I dont have to return}, I already had one of my techs throw the 009 in the other day, and I wired in a strap on aquastat with the new mcc control so now it will heat to 180... I also piped in a water feed prv and exp tank so its not running at 60psi...

    I set the mcc at 180 and the aquastat around 135* I ran it a few times and the used 24v transformer I installed stopped working {I am tring to keep their cost down any way I can}, after throwing in a new transformer, it seems to do the trick..

    With my efficiency tester installed I let it run through a few cycles, of course the first cycle went rite to high fire and around 94%eff. then after about 5 minutes the fire started to mod down, {so I adjusted the retrun agaustat} and the zone valve worked properly and diverted flow back to the air handler and the Rinnai unit shut down, then after about 3 minutes the gauge I installed in the return started to show around 140* and the zone valve started to move, then sure enough the rinnai unit went rite to high fire....

    Thats what my goal was to keep it out of low fire where it soots up and get 45%efficiency!!!! So I sat there and watched this thing cycle about 8 times, I showed them the ins and outs of what I did and told them its the best I can do for $450 which was a gift because if I had to use new parts and pay for the zone valve parts would have been almost $1000 before labor....

    I feel so bad for these kids, its their first home and they got hosed by a plumber {not even a master never mind pipefitter}, hopefully this works for 5-10 years which is what the Rinnai rep told me to expect, and Im hoping now that the unit isnt modding down to low fire it will give them 90+ and maybe they will save some money... The unit is definitely enough btu's to heat the space, just want to keep it from running at 45%...

    Thanks for all the replys and all the help
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