help with some fan coils

BobbyBJr

Not sure this is the correct section, but hopefully someone can help. We have inherited a mess and need some advice. It is a hydronic heating system with two fan coils using a on demand water heater to heat the water. The first complaint was that only one zone heats when both are calling. Customer says it worked fine until the boiler was changed to the water heater about two years ago. There is no balancing valve. After some back and forth with the customer, he agreed to let us add a ball valve to balance the water flow and we did so. Now the system heats both zones equally, but he is saying he used to get approximately 130 deg air out the supply grilles and he is only getting 100 to 110 deg now. We have 150 deg water feeding the coils and the LWT is 115 deg. We don't work on enough forced air hydronics to really know what's right and what's not. This is a very old system and we have limited data on the coils. Another contractor changed the B&G pump before we came on the scene thinking it wasn't moving enough water. I know this isn't much detail to go on, but can anyone share some insight? What is the normal outlet temp ya'll see at the grille? My thoughts are he's expecting supply air temps that normally are produced by a gas furnace, so I'm not totally buying what I am being told. Comments?

thanks,

Bobby

jumper

Do the zones get warm enough? There are water heaters that produce hotter water but water heaters are designed for cold water input.

EdTheHeaterMan

First Company (that it the name of the company) came out with a fan coil unit designed to work off of a standard tank-type water heater, over 30 years ago. I installed it in my home with an oil-fired water heater and set the water temperature to 165°. I installed a mixing valve on the domestic hot water outlet to the faucets.
This was a very warm air system. The tank lasted about 11 years and then I redesigned the system with a space heating boiler with an indirect water heater. That lasted much longer. I still have the same First Company air handlers. New AC installed with the same air handler about 16 years ago. (off-topic) time for the next AC System now.

So you can get warmer air if you crank up the temperature, but you may not have that option with your tankless water heater. You are getting a 35° ΔT on the water side of the system. I think you are pulling as much heat from that water as you can expect. The bottom line is what @jumper said: are you heating the zones?

Look to have that water heater fail in the next 3 to 10 years and then do the system right with a space heating boiler.

GroundUp

I'm betting it's still piped direct like it was with the boiler, and the water heater has too much pressure drop through the HX which is reducing the flow to a point that the delta across the coils is excessive, meaning a lower LAT

BobbyBJr

Thanks guys. Your responses have each touched on parts of the conversation I had this morning with the tech actually on the call. We did discuss that 100 to 110 deg air will heat a space and it won't fall off with the outdoor temp like a heat pump, so it might work. Since we doing alot of guessing here, we really don't know how many btu's we actually producing. The tech mentioned that the tankless is not ramping up like one normally does and I mentioned it's designed to heat cold water. Since it's not seeing cold water and probably not the flow it would ordinarily see, it's not ramping up and will never give us full capacity of it's 150,000 btu rating. I hadn't thought about the pressure drop through the water heater, but I am pretty much decided on offering to put about 8 P/T ports in the system. Then we can actually measure pressures and temps and we will have some real data to figure out our flow rate and such. Thanks for the comments. I been doing this a long time, but we don't see as many hydronic systems in upstate SC as we did years ago.

Bobby

BobbyBJr

I have discovered that this is a Rinnai 65e tankless, which appears to be a very small water heater sold at local home improvement stores. We have offered to put gauges in the system in attempt to figure out the flow rate of the water and if accepted will go from there. The customer is adamant that it can't be the water heater, but the pump has already been changed and that apparently made no difference.

Bobby

pecmsg

Ball valves are NOT Balancing valves!

joeba

The GPM flow thru the airhandler coil is one varibale the others are the entering water temperature and the CFM the air handler can deliver at a given static pressure. Firstco Airhandlers datasheets will give you the BTUs delivered based on those inputs. I have 3 airhandlers in my house, one is set up with a very low fan speed so the air coming out of those registers is hotter than the other 2 (which run at a much higher fan speed) (they all see the same GPM flow).

pecmsg

Install the TACO that operates off return water temperature. 

Jamie Hall

You do need to measure the pressure drop across that poor little tankless (which is the wrong equipment for the job, but... hey, it's there).

I'm betting with the guys above that you have much more pressure drop than you expect, and that you need a higher head (not higher flow -- higher head at a given flow) pump to handle the load.

TAG

Does the 65e have any piping setup in the manual for a fan coil?

Some of those on-demands did have piping layouts for a fan coil setup ... my guess as others have said is the pump is not big enough.

Most people don't understand that a water heater is not a boiler

BobbyBJr

Yes, we know that a ball valve isn't ideal, but in this case it has worked to get both coils operating when both are calling for heat.

Bobby

BobbyBJr

TAG said:

Does the 65e have any piping setup in the manual for a fan coil?

Some of those on-demands did have piping layouts for a fan coil setup ... my guess as others have said is the pump is not big enough.

Most people don't understand that a water heater is not a boiler

I haven't seen any, but this is apparently the water heater of choice for one small bath or something along those lines from what I'm being told.

Bobby