I have oil furnace for baseboard heating and domestic hot water (no hot water tank). For a long time it has been difficult to get enough hot water to run the shower continuously without the temp dropping to cool. It will start hot, but over the course of 2-4 minutes slowly cool off to where I have to keep increasing the temp on the single handle faucet to maintain temp until it is all the way up and I cannot increase anymore at which point the water runs lukewarm for an extended period until (I presume) the system somehow catches up. If I run the shower long enough (10-15 minutes), the temp does come back up and actually stay hot (although I have not kept going for long beyond to see if the same sequence occurs again). I have had three technicians from our various "oil company du jour" come out and mess with the furnace settings and such... I have had one tell me that if the furnace can only do one thing at a time ( ie: if house thermostat is calling for heat, the domestic water coil will not be getting heated). Some have suggested a coil problem (although the furnace is only about 5 years old now). The best thing we have been able to do so far (and it does not work that great) is to partially close the valve on the hot water line in basement to slow the flow of hot water leaving the furnace tank. We only run one thing (shower, dishwasher, washing machine, etc) at a time as we know we have this issue. Our shower head is 2.5 GPM. My question is this. Is the type of unit I have EVEN INTENDED to run a continuous hot shower? Is the statement about only domestic water coil OR coil for baseboard heat running at any one time accurate? Have thought about just getting a 40 gallon electric water tank installed, but would still really like to know if my furnace is operating as it should or is there perhaps some issue with the plumbing. Home new in 1991, and the layout of the plumbing sure looks very well done/neat. Original owner had a tankless furnace also which we replaced as it was getting old and burner can had crack. Had same issue with it for the few years we had it, hence my wondering if they just not intended to run a shower or if the plumbing is somehow messing things up.

Hi GP


Is the statement about only domestic water coil OR coil for baseboard heat running at any one time accurate?

No, that statement is not true. It may be true for how your boiler is set up right now, but your tankless coil should have its own AQUASTAT to keep water at a constant temp. of say 180F...a mixing valve then reduces the hot water temp. to 125F maximum.

Choking the water flow in is a quick shot at trying to slow the volume of water into the unit, but it will NOT work as good as if you have a WATTS P3 valve (see image) installed at the cold water inlet to the tankless coil. Here, this valve redcues volume, but not pressure and it is adjustable so after it is installed you can start at high volume and reduce until you find the correct balance for your system. You can check these out here: P3 Multi-Orifice Flow Control Valves for Tankless Heaters , Flow Control Valves, Water Safety & Flow Control - Watts (http://www.watts.com/pages/_products_details.asp?pid=3425) As much as it sounds like it is the same as choking the valve down like you have now, it isn't...much different principle at work! Otherwise, your mixing valve may need to be checked or have the element replaced...another first line of defense against your issue. If that all won't work then you may want to consider installing an exterior storage tank system...expensive, but will put this to bed once and for all!

If you decide to install an electric water heater you can use the tankless coil to "preheat" the water into the tank...will help the electric tank to last a little longer for you.

Mark