Monitor MZ 20-40 replacement

Ctax

Hello. I bought a house with a Monitor MZ 20-40C and 40 gal indirect water tank 11 years ago.

The Goodman furnace was converted to an air handler by inserting a hot water coil. All in all over the years, the contractor who came up with this did a pretty good job. I've had to replace some parts on the MZ but it is a pretty simple and reliable boiler.

The MZ is a two stage boiler, about 70k btuh per stage. The house is a 1980's center hall colonial in NJ. I am very satisfied, it has a built-in grundfos pump putting out around 10 gpm per the graph in the manual.

The unit was installed in 2007 so approaching 20 years old. I would like to get ahead of it and purchase a new wall hung unit of similar size to minimize installation.

Looking for a unit with modulation to get me down to a min firing rate to around 50k. This is due to my only complaint that when the temp drops below 15 degF, I will hear the unit shut-off and then restart in about 5 minutes. While I wouldn't say it is short cycling, I would rather drop the firing rate and keep the boiler / air handler running.

Any suggestions from the knowledgeable folks here? I know having a good contractor is key and I do have one, he is excellent. Lets just say that my house is the only combi in the neighborhood

Big Ed_4

I am assuming oil , small to size , three pass cast-iron , push nipple construction boiler .. Simple and will last you and serve you well for life…

hot_rod

I think the MZ was gas or lp. If so there are dozens of mod con boilers out that would replace that.

Most all of the mod cons have 10-1 turndown, and the modulate instead of a two stage fire like the MZ

A 140,000 would turn down to 14,000 btu/hr on low fire

Ideally you would perform a heat load to see what size boiler the home actually needs. Could be a 120 or even smaller would cover the load

What type of heat emitters? Just a coil in the air handler? The boiler would want to match the coil capacity. That info may be on the coil or literature

Ctax

The MZ I have is running nat gas. Other then a hot water coil in AHU for zone 2 there is a 40 gal triangle tube indirect water heater.

140k with 10:1 turndown sounds good.

Now, just need to find a reliable unit with parts readily available in North east. Something simple to set-up and commission.

I have been reading the vitoden manuals on how to configure the burner to take advantage of the firing rate vs OA-T with their rate curves. Seems pretty flexible to configure. There must be other similar. Any ideas?

hot_rod

Lochinvar, IBC, Viessmann, NTI, Triangle Tube are some of the brands that the pros on this list refer over and over

Any of the high efficiency boilers need proper installation and to be fine tuned with specific meters, combustion analyzer and gauges to measure gas pressure, for example

Water quality needs to be spot on to maintain warranty. That MZ came with a special inhibitor for the aluminum. That needs to be flushed out and the system piping cleaned.

Shop for a qualified installer and consider the brand that they are familiar with, and have support and parts availability in your area

clammy

If you have a reliable contractor that you trust then you should have them pick a unit which he is familiar w and has already installed and serviced and has a little history with not like the first one they installed . There's a lot of mod con boiler as stated out there some w lower btu outputs to 8 k and all have about the same features . Proper sizing is important even if a unit that can modulate if it can't get rid of the heat its gonna short cycle all the band aides in the world will not make this disappear . If your 70k monitor short cycles then i would n't be looking at a 140 k replacement more like a 80 k .This being said it all depends and from your description it s one zone heat and one for domestic water . A hydrocoil at 140 k would surely be a min 4 ton drive blower on a 120 k goodman furnace used as hydroair ?

peace and good luck clammy

Ctax

Thanks for all who gave good input.

My Goodman AHU blower is a 3.5 ton with 1200 cfm blower. In heating mode it kicks it down to medium speed so maybe around 800 cfm.

In heating season the second stage kicks on for a while, but then shuts off after maybe 5 minutes and the boiler runs for 20 minutes with the first stage only. So the 70 k first stage might be more like a 50-60k connected heat load when it is 15 degF outside.

I know the brands that have been suggested are excellent and efficient, but I am not so sure about the cost of replacement parts with those advanced controls come many sensors and points of failures. One of the things I liked about the Monitor was that it was so simple and made from reliable parts.

I haven't heard anyone mention W-M boilers. Does anyone have any experience with their newer wall hung boilers? The last W-M I installed in another home 20 years ago was very reliable.