Replace A/B valve with pump

pch

My boiler (Monitor MZ) has a priority domestic hot water mode. It came with an A/B valve to support this. I would like to change to a pump as I have read that A/B valves can sometimes be problematic and I have recently been having occasional problems with mine. DHW works great, but semitones I don't get heat; I have to go down and "speak politely" to the valve so that it will switch to heat mode.

I understand that I can use the existing wiring from the boiler to control the pump by connecting to a 24 VAC relay which will signal a pump controller to run the pump. However, the boiler also powers a high temp air handler fan coil as well as several low temp in-floor radiant zones. My understanding is that when there is not a call for DHW but there is a call for heat, water may still flow through the offline pump to the DHW tank. This flow may be at a lower temp than the DHW tank wants to maintain, and therefore may slightly cool the water in the tank, potentially triggering an unnecessary priority DHW call. My questions are... 1. Will this happen? I am not sure if the tank would be subject to siphoning which would pull the water through the pump; 2. If so, how do I pipe the pump to prevent this?

Might be important to mention that the MZ boiler has a built-in circulator, so flow is pushed from the boiler as well as pulled by the other pumps in the system.

Jamie Hall

The line to the domestic hot water heater will flow quite happily. A centrifugal pump, such as you would use, is no obstacle to the flow of water.

You need a valve. In fact two valves-- one of the heating side and one for the domestic side -- to work. Whether it will be more reliable than the changeover A/B valve is doubtful. I'd simply replace the existing valve -- or maybe just clean and repair it -- and carry on.

EdTheHeaterMan

There is a diagram in your I/O manual that shows how to use circulators for the different zones you describe. This option uses the boiler pump on the primary loop of the system with each zone having a separate circulator for the secondary loops to each zone (or each temperature) . There is a air handler circulator, a radiant circulator and a DHW circulator. This would be the best for what you describe, however it may involve repiping to a Primary/Secondary design

In this diagram it shows the AB valve kit (however it does not show the two different space heating temperatures). You can replace the AB valve with 2 circulator pumps with internal flow check valves. The relay that you would connect to the AB valve controller would then select which circulator pump would operate. The two additional pumps would run in series with the boiler pump.
I'm not sure you need the other diagram or can use this diagram, based on your piping design.

pch

Thanks very much for your reply, which of course triggered more questions. 🤔

The initial MZ installation supported only the high temp fan coil and DHW, without a primary loop. I am planning to repipe so that I can add primary & secondary loops with a mixing valve per the diagram you pointed to. Currently the MZ cycles on/off about once per minute or less. The fan coil zone holds only about 8 gal. and the additional radiant tubing will add another ~15 (The repiping will add probably another few gallons). So, I am planning to also add a 30 gal. Boiler Buddy to provide both hydraulic separation and thermal mass. However, if the boiler supply flow is piped first through the BB, then a call for DHW will heat the 30 gal. BB as well as the DHW tank. Seems like a lot of standing hot water for half of the year.

So, I guess, in my case, I need to go with the dual check-valve pumps and place the BB downstream of the circulator on the heat side of the 3 way valve? Is the 3 way valve then normally closed for heat and opened by a call for DHW? Since the MZ has a circulator, can the 'extra' pump on the DHW side of the 3 way valve be eliminated?

The MZ is nearing its life expectancy and has already been repaired once (bad ignitor). In the next year or so I will probably replace it (assuming we get through this winter first!). I'm trying to do the repiping so that swap is relatively easy. 😂😂😂

EdTheHeaterMan

During the repipe, you can certainly add the 30 gallon buffer tank to reduce short cycling, That sounds rather large and may be a source of down time heat loss. Since you are considering replacement in the next few years, I assume you will be using a Modulating boiler to replace the MZ. That will reduce the need for the tank, ao maybe you should skip the tank for now and save on parts and ultimate fuel waste for as long as that tank is included in your system.

If you are considering a modulating boiler, @hot_rod would be the go to guy for that pipe design if you want to get ready now for the future boiler. He needs to know that you have low temp and high temp zones, along with the indirect.

pch

Thanks for the feedback! I think I have pretty solid plan now, but I certainly will reach out when I inevitably hit a snag.

hot_rod

Sounds like you have a combi MX, does it have a S in the model number? If so it has a tiny indirect inside.
It toggles from heat to DHW via a 3 way valve.

It may be easier to replace the 3 way valve. A lot less piping changing. Not much room under the hood of those. that does heating or DHW vis a 3 way valve?

pch

Looks like the model # is 225C. The diverter is external to the boiler; it was supplied with the boiler. It occasionally sticks in DHW mode, so no heat. I did some checking online yesterday and apparently I can get a replacement cartridge for it. That seems like the best way to go.

As for the piping, that still needs to be redone. When I initally installed the system I only put in a high temp (air handler) zone and the DHW. I am adding infloor radiant, so I need to move to a primary, secondary loop layout, but I am now convinced that I should keep the 3 way valve for DHW.

pch

Twitchy fingers... model # is 25C

hot_rod

Yes the "C" version was boiler only.
I'd also look at the flow switch or whatever trips the 3 way valve. More often those small flow switches get sticky after years of hard water deposits.
How old is that MZ?

pch

How old is that MZ? I installed it in 2003. Ran it the first winter on 20lb. propane tanks. When it was really cold and the boiler was pulling a lot of gas, the tank would ice over. I had to keep another (warm) one ready to swap out. Good times!

look at the flow switch or whatever trips the 3 way valve The valve actuator (VC8114-11) is wired to the MZ. From what I can tell, it is OK; I tested for current at the actuator when the DHW call finished. I think the valve is gummed up. The DHW mode opens the valve, but when the call is finished (no more power to the actuator), the valve stays open. I can remove the actuator and manually pull the valve closed.

hot_rod

Is it a spring return valve or power open, power close? Got a pic?

pch

The vavle is a Honywell 3 way valve. It is normally closed (NC) and the actuator just opens it.