Tankless coil not working well

[DavidP]

I currently have hot water supplied through a tankless coil, located in the furnace that heats the house (hot water radiators). When the heating system is not in use, all is well; there is enough hot water for a shower or to do laundry. During the winter, though, there is often not enough hot water. You start out and get some, but 1/4 or 1/3 of the way through the shower the water becomes quite cool. Usually I hear the burner come on to reheat the water just about the time I get out of the shower. The issue is not temperature; the water is hot enough when there is hot water, whatever the season--in fact I wouldn't want it any hotter. What seems to be happening is that the system does not realize the need to reheat the water as soon as it should. I've tried to see if there is a pattern here (e.g., is there more hot water when the burner has run to heat the radiators recently, or when it has been off for some time?) but haven't figured this out.

I've asked my oil company about this (they installed the furnace and tankless coil when I bought the house). They gave me an answer that didn't seem to make a lot of sense. Please don't ask me to repeat their answer because a) it was a year ago and b) I didn't really understand it even when they told it to me. I'm going to bring this up again when they come next week to do the annual cleaning of the burner. Maybe someone here can explain better how these things work so I can press the technician if he gives me the same unclear answer as before.

[Edit; added the following to clarify what I do and don't understand.] When the thermostat calls for heat, the circulator starts bringing a lot of room temperature water from the radiators into the furnace. At this point in the cycle I understand why the tankless coil isn't going to produce any additional hot water. I think this is what the technician was trying to explain to me in a sort of confused way. The issue is that even at other times (when the radiators are hot, or when they have cooled down but have not yet come on again) there is insufficient hot water. But this never happens when the heat is turned off completely, as in summer.

[phil]

where does the hot water go after the radiators? down the drain? aren't most hot water heating systems a closed loop with some sort of brine in them? seems strange to be using the same hot water to heat the house as you use for your shower? How is this plumbed? Its an old trick to run city water through radiators then down the drain to get free AC I am pretty sure the city doesn't like that though

[DavidP]

No, it's not the same water; you're right that the radiators are a closed loop. A tankless coil is exactly that, a coil of pipe that sits inside the furnace above the burner, surrounded by water. The burner heats this water and the heat is transferred to the hot water inside the coil, which is tied in to the hot water lines upstairs. When the heat is not on, the burner heats only the relatively small amount of water inside the furnace since the circulating pump is not bringing water from the radiators in to be heated.

[Gothichome]

David, I would suggest the temp sensor in the water jacket is not picking up the temp drop soon enough to compensate for the draw down of the jacket. I think first place I would look would be there. If you have access to the setting you might want to raise the burner on temp closer to the desired consumable water temp, or to put it another way lower the temp differential between burner on an and temp drop.

[DavidP]

Yes, that sounds exactly right. Your have expressed it more precisely than I did when talking to the technician last summer and I will use that phrasing when he is here next week. But that does leave the question of why I have the problem only during the heating season (see the paragraph I added to my original post if you didn't already).

[Gothichome]

David, I would suggest during the heating season your capacity is close to max, there is not enough make up energy available in the system to handle the added load of consumable hot water. This might be just a simple adjustment of the valving in the closed loop. This should not be a big issue for the tech to sort out.

[DavidP]

Thank you, gothichome -- interesting idea. I will definitely push the tech if he says there's nothing they can do. I usually leave either two or three out of the four big radiators upstairs turned off. If the furnace was sized correctly for the whole house then I would think there would be enough capacity to heat the house and provide consumable hot water, since the maximum heating capability is generally not used (unless they sold me an undersized furnace).

[Gothichome]

Dave, if the system is that close to max requirement try a flow restricted shower head. This will cut back on the flow out of the boiler on the consumable side.

[DavidP]

A lowflow showerhead would make what hot water there is provide a longer/more comfortable shower -- I had not thought of that. I really want to fix the problem, though, since it affects hot water for laundry and doing dishes as well as showers. We'll see what the tech has to say on Tuesday. If the tankless coil can't be made to work better, I may get a separate hot water tank.

Just to be clear, I keep the upstairs radiators off not because the system can't heat them, but because the upstairs gets way too hot if they are all on (except in extremely cold weather). With most of the upstairs radiators off, there should be capacity to spare.

[SkipW]

When we bought our house, we had the domestic hot water coils in the boiler similar to yours. What I found was the boiler was running VERY frequently due to hot water demand from showers, the dishwasher, laundry, sinks, etc. in all seasons.

We switched to a remote (as in right next to the boiler) domestic water heater that is a zone as part of the system. In other words, it is treated as a heating zone as far as the boiler is concerned. The big difference is that it has it's own circulation pump and has been programmed to have priority over any heating zone. Thus we NEVER run out of hot water.

It may be at some expense, if you cannot resolve the issue you currently have, but you could most likely have this installed in your system and you would also find that your boiler would run much less frequently trying to keep up with domestic hot water demand, especially in the summer months.