how do boilers with TRVs work?

weil_fail

how do boilers work with TRVs? as each valve closes, is it the back-pressure that tells the boiler to slow down or shut off? is it the return-side temp? if return-side temp, do you have to have some kind of bypass for when all of the TRVs close that just puts the output water back toward to the input? is it output temp, so that if the flow is slow, the temp on the output will gradually rise and tell it to back off? is it some combination of these?

edit: hot water

thanks

EBEBRATT-Ed

Hot water or Steam???

I am assuming hot water. If the heat loss is done and you have the right amount of radiation in each space you shouldn't need TRVs. But, some areas benefit from TRVs to keep the space from overheating and allow thermostatic control in all rooms.

A lot depends on the type of system and how it is piped. If the TRVs close down you still need to have minimum flow through a pump.

This can be accomplished by using a water differential bypass valve or a smart pump with "delta P" which will sense the pressure and slow down the pump. Depends if every room has a trv or not

If you have a mod con boiler that senses return and supply water temp the boiler will modulate down. Conventional boilers usually do not

weil_fail

what is the common way of running a boiler with TRVs? a mod-con boiler will modulate down, and a delta-P pump will slow down, but how does it stop/start in the first place? do the all-in-one units typically have the capability to operate with TRVs? taking a random example: link

would such a boiler be able to operate with TRVs? or does the model have to be specifically designed for it?

hot_rod

With a delta P circulator, when a TRVs starts to open the circulator wakes up. Some brands have an end switch on the circ which will call on the boiler

A conventional boiler will fire and run until it reaching the operating control temperature

EdTheHeaterMan

in the 1950's a boiler might have an analogue outdoor reset aquastat that maintains a certain boiler temperature based on outdoor temperature. If the outdoor temp gets above say 65° the burners would stop firing.

The circulator(s) would be on constant run. The boiler room would have a bypass pipe with a differential pressure valve. If all the TRVs are closed, the bypass pipe would allow some water to bleed via the bypass to keep the pump from dead heading. That is the way I was taught to design a system with TRVs

In short>>> the boiler is always on maintaining a temperature and the circulator is always on using electricity and pumping the heated water. The manual switch at the boiler room door turns the system on and off.

Today there are modulating, electronic, digital microprocessor, WiFi connect, vari input, pressure differential, mumbo jumbo systems that can be created on the idea of some egghead at a drafting table with a load of college degree letters after his name.

So the question is How do YOU want to start a boiler for your project?

There are many answers! Some are better than others.

weil_fail

haha, great reply EdTheHeaterMan. if you were to design a TRV system today with a condensing boiler, how would it be done? are there units that do everything all in one, or is it something where you need to put together a variety of parts?

PC7060

I have Honeywell Braumann TRV on all 19 cast iron rads in my system.  Several are left full open, others are set based on room.   The boiler is a modcon with ODR and circulator set to constant velocity. The boiler runs a low temp curve with call for heat controlled via single thermostat.   

System work well with long heating cycles.   

EdTheHeaterMan

my first question would be, are you starting from scratch, or is this an existing home with old radiators that you are adding TRVs to those existing radiators?

Let's say it is a retrofit. I would select the hardest room to heat and place a regular thermostat there. that rooms radiator would not have a TRV since it is the coldest room it will always call for heat based on the thermostat setting. This rooms radiator will also act as the bypass as the only radiator with no automatic shut off.

Now add TRVs to all the other radiators. as those rooms will shut down the boiler will modulate based on the internal thermistors on the supply and return inside the boiler and any field install thermistors suggested by the boiler manufacturer. Now the wall thermostat is your automatic boiler operator and the TRVs keep everything even.

If this system is from scratch, you are specifying the boiler, radiators and piping design, then you could just put a jumper across the boiler thermostat input and be sure to set the outdoor reset parameters to maintain constant operation as close as possible to the needs of the building. This will take some adjusting and tweaking to get it close, but it is possible.

If you go this route, you may not want to use any night setback because the boiler may not have the ability to recover. This is because your boiler will be set to only provide enough capacity to maintain temperature, not enough for temperature recovery. Some boilers microprocessors may have "Recovery" capacity for setback. Read your boilers specifications carefully

Does this help?

Mr. Ed

hot_rod

What type of boiler? I would not let a cast iron boiler just run hot all winter at a fixed temperature. And a boiler bypass even a PAB is a parasitic device. You have heat loss and pumping power being consumed.

With a mod con, use the ODR function included, a delta P circ and let the TRVs do what they are designed to do, respond to a call for heat based on temperature. They are modulating, non-electric control valves that work perfectly with variable speed, delta p, circulators. This way everything modulates together, the boiler, the pump, and the TRV control valves, and you should get near constant circulation.

A plumber friend in Alaska does a lot of TRV systems with an Alpha pump and an induction relay to call on the boiler. As soon as the pump ramps up, draws current, it calls the boiler.

Properly dialed in the ODR should fire the boiler at the beginning of heating season and modulate until WWSD occurs.
This is how most or the European systems, predominately TRVs, work, no such thing as a TT or boiler call for heat