Boiler DHW Replacement Advise

SamTheMan

I am posting in the hopes of getting some ideas and direction to an upcoming boiler and DHW replacement. Here's the situation:

Currently have a Weil-McLain GV-5 140K BTU input (122K BTU output) boiler which was installed when the house was built about 1995. It runs 6 zones, one of those being a DHW sidearm 40g tank, one to a hot tub heat exchanger, and one zone does in floor radiant heating (with a separate circulation pump and temp mixer to get down to PEX temps). The GV-5 was fairly efficient for its day (~88%), and is direct vent and air intake.

It has been very reliable over the years, only repair being a new induction blower motor ~5 years ago when the bearings in the old motor were failing. However it is approaching 30 years old, and has started making a blower vibration at cold startup. I could just replace the blower assembly (and igniter and gaskets). Not that hard a repair, but this boiler is getting a bit old. And I had quite a time getting a working blower motor a few years back, took 4 tries to get a working one. And have read online that folks were complaining that their new blower vibrated just like their old one. It would seem WM is/was having some quality control issues. So it's a question whether or not it is worth putting more into this old boiler or put the funds towards a new boiler.

An additional factor on the DHW side is a Solar Hot Water system was added ~14 years ago. The solar has an 80g storage tank and currently feeds its hot water to the input to the boiler sidearm tank. Not the ideal setup but there was no space in the utility room for two tanks, and as a result I often see the sidearm heat zone come on when there is plenty of hot water still in the solar tank. I'd like to fix this inefficiency if I could during this change of boilers. It would seem that the best fix for this is to have one DHW storage tank that both the solar and boiler can heat (although finding the right combination of tank and heat exchangers for this may be difficult).

So the questions are how best accomplish these goals. Namely:

1. Should I fix up the old WM boiler or go for a new one? The old WM is a cast iron boiler and may have more life left, but it's hard to say (would be nice if someone offered boiler water chem analysis so one could see if there was corrosion, oxidation, etc). There actually is not a lot of efficiency to be gained from a new boiler (88% vs 95%). It would be more to avoid a catastrophic failure mid winter or the like in the upcoming years.

2. I had thought of getting a combi to eliminate the sidearm tank, but I take it that folks here prefer a separate sidearm tank and zone to the combi. If I did go with a combi, would have to pick one that would accept hot water in its DHW without throwing codes. Laars Mascot says they will do that (and I assume the HTP and other clones will too). And since this is on well water I would prefer a combi that uses a small internal tank (like the Laars Mascot) to the ones with flat plate heat exchangers which can have deposit issues with hard water.

3. Another solution would be to get a new solar storage tank, one with both an extra heat coil for the boiler zone as well as taps for the solar. And use this tank to replace both the sidearm tank and the solar tank. Main problem with this solution is cost of the new tank, and finding one with the required taps/HXs; it's not a common configuration, and local codes require a dual wall heat exchanger for the boiler loop.


Anyhow, these are the things/issues I have been trying to solve, and the potential solutions I've come up with. I'd be interested in hearing which of these folks here would recommend, and/or if there are other solutions to this.

hot_rod

Not knowing how much solar you get in your location?? 80 gallons of solar pre-heat to a combi is a nice match up. Even without solar a few days the tank will bring water close to room temperature. That helps the combi output a lot!

There is some delay with combi or tankless, if that is a concern.

That GV was a great boiler, we installed quite a few when they first came out. Early versions had a black colored Hoffman mix valve under the hood, I don't know if parts are around for that anymore?

Water quality over the 30 years could have an effect on the block, that may not be available anymore either? If it leaks.

Just curious on what type of flue? Stainless or that high temperature plastic that was popular on that vintage boiler.

SamTheMan

Thanks. The GV has the plastic 3" flue (and intake).

I am surprised that there are no labs doing boiler water analysis. On my vehicles I can send an oil sample in and they will tell me about contaminates like antifreeze, bearing material etc in the oil; helps to get view into the internals of the engine without pulling it apart. I would have thought someone is doing the same with boiler water and could tell you about rust, corrosion levels and the like, but have not found anyone.

I have heard about the delay with combi/tankless, but am curious what that might look like. Right now we have to run the hot water for a while (10-15sec) on the faucets which are a ways from the heater. I would guess that it might be similar with a start of hot water then cold for a bit finally getting back to hot. I would assume it might be a bit better with the solar preheat, and the Laars small tank combi. Is this about right, how much delay might one expect?

And we typically get quite a bit of sun. Solar hot water tank is often 140F+ by the end of the day in the summer, and could get to 160F often (but it has controls which will limit that). Even in winter tank temps will be 100F+ on sunny days,

Jamie Hall

Boiler water chemistry concerns come in two flavours: low pressure steam or hot water (mostly residential, small commercial) closed systems, such as yours, and high pressure (power) boilers.

The reason you can't find someone to do boiler water analysis is simply because in your type of application there is no nee for it -- and for power applications the firms that make boiler chemicals are happy to do it, for a price.

What you need to know is pH (near neutral to slightly basic), chlorides (the lower the better; zero is nice but hard to get without RO or distilled water, hardness (reasonable) and that's about it -- and your local public health department either can do that or direct you to a local lab which can.

If you have hot water heating, and are running glycol, your boiler technician can test for the condition and amount of that.

Now if you are having to add makeup water, all bets are off -- and there's no point to testing. The makeup water will contain oxygen, which will kill your boiler.

hot_rod

Some, maybe all the hydronic chemical suppliers offer test kits or will do a test on a mail in sample.
I'd guess finding a lab in any major city would be fairly easy. Universities often have fluid labs. Just let them know what the fluid is used for to get the correct testing.

The do it yourself kits are fine, unless you have a specific issue.

ethicalpaul

> What you need to know is pH (near neutral to slightly basic)

Well the steam boiler manufacturers say 7-10 ph, right?

SamTheMan

Spent the last few weeks talking to install companies and getting some quotes. But have run into some questions on the proposed systems I am getting.

The biggest difference is several installers won't pipe hot water from a solar DHW tank into a Combi DHW input. They claim it won't work well. That the Combi's are designed for max temp rise whenever they detect DHW flow, and that they won't turn down the heat enough (or at all) and will be switching their 3-way valve a bunch and even cycling heat on/off. I did call one manufacture (Triangle Tube) and they said more or less the same thing.

So my question here is, is this true for all Combi's, or are there some folks know of which would work well with pre-heated input water? That is can work with pre-heated water, which may not require full boiler output (or maybe even any boiler heat) to bring up to DHW temps. Which brand/model would that be?

An alternative would be to put in a new solar tank with an additional heat exchanger for the boiler, which some have proposed. Problems with that are space for that tank (and that I just got a new solar tank a year ago which doesn't have a boiler heat exchanger on it). Also, it isn't clear to me if the folks recommending this new solar tank are just trying to sell more equipment, or if there really isn't a Combi which will work for what I would like to do.

Thoughts on this?

mattmia2

SamTheMan said:

Spent the last few weeks talking to install companies and getting some quotes. But have run into some questions on the proposed systems I am getting.


So my question here is, is this true for all Combi's, or are there some folks know of which would work well with pre-heated input water? That is can work with pre-heated water, which may not require full boiler output (or maybe even any boiler heat) to bring up to DHW temps. Which brand/model would that be?

I think you would be better off using an appliance that is just a boiler to heat system water then using either a heat exchanger and a nonferrous circulator to circulate and heat the water in the solar tank or replacing the tank with a tank with a HX for the boiler. If scaling of a brazed plate HX is a concern you could use a shell and tube or a small indirect as the HX. It may cost less to replace the tank than the other options. You also have better options to control it if it is not the combi HX built in to a boiler. @hot_rod probably knows of the controls that are available. You could also solve the problem with the current boiler and install it in a way that it could be connected to the new boiler whenever the current boiler fails, assuming that the solar stuff isn't worn out by the time that happens.

hot_rod

I would confirm that with the manufacturer of the combi boiler you are considering.
Most trigger on with a flow switch and a sensor watches the DHW outlet and modulates the burner to prevent over-shoot.

It is possible the solar hot water could get to 160F or warmer. That may trip off the combi on some limit? If so a thermostatic valve on the solar tank would cover that.

Caleffi builds a 262 mixing valve specifically for that application. It pulls from the solar tank until it drops down to say 110F, then switches flow to the combi or tankless. A third mixer protects the combi or tankless outlet temperature.

Its basically 3 mixing valves in one forging. You could buy 2 or 3 off the shelf mix valves and build your own.

Beware with hard water you have 3 mix valves to keep clean and delimed

I'd make sure the combi would not work fine without valves first.

I know early tankless DHW heaters would not work well with pre-heated water. Takagi finally came out with a solar specific tankless. I think all the manufacturers adjusted control logic to work with solar pre-heat now.

In Hawaii, solar DHW is required in all new residential. So there must be tankless appliances that work with solar pre-heated water?

Attached is an example how to build this assembly with 3 mix valves.