Potentially Replacing Tankless Coil

camiarrobino

Hello all -

Seeking some input here. I have a rental property which has a Weil Mclain boiler w/ tankless coil for DHW. It was originally oil, but it now has a Carlin EZ gas conv burner. I understand that these tankless coils tend to develop problems over time and also are not really as capable of providing plenty of consistent hot water. There is a mixing valve off the coil (not a thermostatic one). Back in December, the tenant did complain once that they "weren't really getting hot water to the shower" ....I asked her if she could hear the boiler running, she said yes, so obviously the boiler was heating the water, I am guessing that it had just been run out of HW at that point (maybe from laundry, dishes, previous bath, etc).

No further complaints, so for now I'm just going to keep riding it out, but I'm just looking ahead and anticipating the coil probably getting worse over time due to scale (the water is VERY hard in this area). I do know how to de-scale my wall-hung gas on-demand WH that I have in my home, should I bother cutting in the drain valves necessary to do this on the coil, and try to de-scale it? I have heard that it's possible that that may uncover a pinhole leak that the scale was covering up. If I did that to try to pro-long the serviceable life of the coil (or even if I didn't de-scale it), could also just increase the upper & lower limits of the relay controls right? Thermostatic mixing valve may also help to at least make the hot water temp more consistent (until it runs out) ?

I could install an electric WH myself for pretty much just the cost of the tank and expansion tank and a few fittings (i can borrow a friend's pro-press and also know how to solder, and my cousin is an electrician and can tie the wire into panel for me). I know that operating cost wise, electricity sucks where I'm in MA, but the tenant is paying all the utilities. I'd never touch natural gas myself, so installing a gas-fired tank myself is out of the question, and so is an indirect (don't have the aptitude to do that).

If I did any type of separate water heater & abandoned the coil, should I just cut the cold & hot pipes of the tankless coil, and cap them? as opposed to having the coil removed, and possibly having a leaky boiler after it's put back together? Lastly, would the boiler wiring need to be changed to only fire on a call for heat? If it's a more complicated / longer job, don't mind paying an electrician for that since my cousin is quite busy and it would just be a few mins for him to tie the wire into the panel (I can rough the wire and tie it into the tank)

just looking for some input here to stay ahead of what may come up.

ethicalpaul

Just cut them and don't cap them

I did the same thing myself when I had a rental once. Even for resistive electric heat, it's just not that expensive to heat DHW.

In this case, the cost of heating the water with electric will probably be more than offset by being able to turn off the boiler in the summer.

camiarrobino

ethicalpaul said:

Just cut them and don't cap them

I did the same thing myself when I had a rental once. Even for resistive electric heat, it's just not that expensive to heat DHW.

In this case, the cost of heating the water with electric will probably be more than offset by being able to turn off the boiler in the summer.

Why wouldn't I cap them?

Hear you on costs, like I mentioned though, I'm not on the hook for any utilities for the this property so doesn't really make a difference to me.

ethicalpaul

Why would you cap them? But one reason not to would be you don't want to introduce a high pressure situation in the coil.

I know you aren't on the hook for the costs, but it's a nice landlord who at least considers and thinks about costs that he might be putting onto his tenants (although in this case as I said, I don't think it will)

camiarrobino

ethicalpaul said:

Why would you cap them? But one reason not to would be you don't want to introduce a high pressure situation in the coil.

I know you aren't on the hook for the costs, but it's a nice landlord who at least considers and thinks about costs that he might be putting onto his tenants (although in this case as I said, I don't think it will)

My thought is, let's say that there is a pinhole leak in that coil, or the coil develops a pinole even after it's decommissioned. Theoretically that could allow boiler water into the coil, and if the incoming / outgoing pipes aren't capped, then couldn't that allow the boiler to lose some of it's water? My thought is, with the caps, why not just keep the boiler (including the inactive coil) a 100% closed system. Regarding pressure, isn't that what the relief valve / expansion tank are for?. Correct me if I'm wrong (which I very well might be)

Regarding cost, yeah I do hear you, but from my perspective, if I replace the coil while it's still technically working, but just not working very well, & very quickly running out of hot water, I'd argue I'm being nice enough to do the upgrade, but just my 0.02 there.

But you're right anyways that the coil is pretty much a pig /sieve having to keep the water at 140-180 all summer with a lot of standby loss, even if it's burning an economical fuel like natural gas.

bburd

The best of both worlds would be to install the electric water heater and feed the cold water through the tankless coil first. Also rewire the boiler so the burner will not run unless there is a call for space heating, a.k.a. "cold start".

This way the DHW will be preheated economically by oil when the boiler is running in the heating season. The electric water heater will smooth out temperature fluctuations in the winter and will operate by itself economically in the summer, with the boiler shut down.

Also if either fails to work, there is a back up.

camiarrobino

bburd said:

The best of both worlds would be to install the electric water heater and feed the cold water through the tankless coil first. Also rewire the boiler so the burner will not run unless there is a call for space heating, a.k.a. "cold start".

This way the DHW will be preheated economically by oil when the boiler is running in the heating season. The electric water heater will smooth out temperature fluctuations in the winter and will operate by itself economically in the summer, with the boiler shut down.

Also if either fails to work, there is a back up.

I had considered that too. Only reservation there, is that if the coil develops a pinhole leak, or the scale gets so bad that it kills the water pressure, now I'm going back there again to alter the plumbing to bypass the coil. Not a huge problem, but it does leave the door open for the tankless to cause problems

Jamie Hall

'On capping the pipes. Don't. If you did develop a pinhole leak yes you would notice it. Not a bad thingn. If you cap both of them, though, and there is water in there, you run a very real risk of generating very high pressure in the coil -- and the boiler pressure relief valve wouldn't help a bit. This could lead to loud and disconcerting noises as the coil blew out...

bburd

You can plumb in a coil bypass and shutoff valve when you install the electric water heater. Leaks in the coil itself are uncommon, and if the coil limes up and reduces pressure you can bypass it.

ethicalpaul

I'm thinking of the pressure of the coil itself. Like if you cap it when it's cold, the air inside will come under pressure. Or if it has water in it and you cap it, that water could later boil making the coil into a pipe bomb.

camiarrobino

ethicalpaul said:

I'm thinking of the pressure of the coil itself. Like if you cap it when it's cold, the air inside will come under pressure. Or if it has water in it and you cap it, that water could later boil making the coil into a pipe bomb.

I'm probably wrong as I'm not a professional, but I guess I just don't understand how the caps are any different than the way in which it's plumbed now (which is a 100% closed system, until someone opens a tap, faucet, etc). Theoretically if the boiler is firing now, and everything is closed, then the water in the closed system is also coming under pressure, no? I guess maybe the difference is that right now, there is a PRV coming out of the coil for that reason, and also there's no check valve for the whole house so I guess the pressure could be pushed back out into the street. now with the caps, there's no escape at all.

if everyone is saying just don't cap it then i guess i'll just do that then, if / when the time arises.

ethicalpaul

Let's see what the others say, I might learn something too! I think your description above of all the issues is looking correct.

bburd

The PRV on the coil would relieve any excess pressure. If you remove the piping and cap the coil, that protection is eliminated.

If you have a public water supply, newer water meters contain an internal backflow preventer that would prevent the water from backing into the water main as it expands when heated as it can do now if you have an older meter.

If you have well water, the well tank contains an air cushion that will take up the expansion.

camiarrobino

bburd said:

The PRV on the coil would relieve any excess pressure. If you remove the piping and cap the coil, that protection is eliminated.

If you have a public water supply, newer water meters contain an internal backflow preventer that would prevent the water from backing into the water main as it expands when heated as it can do now if you have an older meter.

If you have well water, the well tank contains an air cushion that will take up the expansion.

So if I decommissioned the coil, but physically left it inside the boiler, do you agree just cut the pipe but don't cap?

Jamie Hall

Yes, just cut the pipe and don't cap.

As to the hot water expansion, it is being taken up somewhere. If you have well water, and no check valves, the well pump's expansion tank is absorbing it. If you have city water and no check valves, it's just backing into the city main.

If you have any check valves before the coil (or water heater) you are overpressuring the hot water side. You MUST have an expansion tank on the hot water line somewhere to take care of that.

camiarrobino

Jamie Hall said:

Yes, just cut the pipe and don't cap.

As to the hot water expansion, it is being taken up somewhere. If you have well water, and no check valves, the well pump's expansion tank is absorbing it. If you have city water and no check valves, it's just backing into the city main.

If you have any check valves before the coil (or water heater) you are overpressuring the hot water side. You MUST have an expansion tank on the hot water line somewhere to take care of that.

I get what you're saying. It's city water, and the town did replace my meter in the basement in 2022, and now I'm wondering if the one that they put in has a check valve in it. The expansion tank on the boiler is just plumbed into the heating zone loop so totally separate from the DHW. There is a PRV immediately off the coil (can't remember if hot or cold pipe). Maybe I need to get over there sooner rather than later and just cut in an expansion tank for the DHW for now....

EBEBRATT-Ed

Tankless coils are pretty cheap. I wouldn't bother descaling it.

Either replace the coil or install the electric water heater.

The determining factor is what shape are the bolts that hold the coil in and the gasket for the tankless. Any signs of leaks or crusty stuff is or could be a problem.

STEVEusaPA

If you’re going thru the work to eliminate the coil, just pull it out and blank it off.

Although I’ve always heard the ‘don’t cap off both pipes on the coil…you’re building a potential bomb'…, I’ve never heard it actually happening, and don’t think it’s more than a theoretically possibility. At worse it would damage the coil and then it couldn’t do any harm. Not saying I’d cap them off.

EBEBRATT-Ed

@STEVEusaPA

Agree. There would have to be water trapped in it.

ethicalpaul

Seems like a lot of places for water to be in there...but as the pressure increases the boiling point goes up so yeah it probably wouldn't burst...probably...or maybe...