2 condensing boilers

Id presume that the boilers are set with the stats at slightly different temps so that both fire initially then one satisfies and shuts down leaving one to run alone.

Should I run the boilers at different temperatures?

I wasn't going to get involved in providing a detailed answer, but I seem be getting into an argument with the usual internet know-alls.

Could I suggest that you look in the manuals or contact Keston and find out whether these boilers can be controlled by an external analogue controller, usually 0-10V? It is fairly pointless to control a modulating boiler with an on/off thermostat.
 
Sponsored Links
A low loss header connected to a condensing boiler needs to be designed so as to prevent return temperature raising. It is not just a box or length of tube.

Yes, but what can be done to the design to lessen that when there is a low demand for heat from the system?

Tony
 
This is a text-book/ideal-world type answer.

Bear in mind that I do not know whether your boiler can be controlled by an external controller or whether there is a domestic, reasonably-priced, controller capable of doing what I describe. Maybe one of the domestic heating engineers can advise.

The Raypak diagram is a good illustration of your system.

• The boilers are controlled to maintain the system/secondary flow temperature, Tf, as measured by the ‘main temperature sensor’ .

• If you have a weather compensation system (highly recommended) then Tf is varied from say 40 to 75 degC as the outside temperature, To, varies between 18 and 0 degC. The temperatures used here are just for illustration, they aren't recommended for your system.

• When To = 18, Tf = 40.
If To = 0, Tf = 75.
If To = 9 degC , Tf =57.5, etc..

• A weather compensation system would need an outdoor temperature sensor (not shown) to measure To.

• Note that, because you are reducing the system flow temperature in mild weather, any TRVs will only be acting as ‘trimmers’ to provide control of the individual room temperatures. They will not be trying to shut down completely, as they might be in a conventional fixed flow temperature system where Tf would be 80 degC at, say To = 16degC.
This will ensure there is always a high flow rate in the secondary system.
This is important; see below.

• If the secondary flow were less than the primary/boiler flow, then the
flow direction will reverse in the common pipework and you will get a mixture of primary flow and secondary return water entering the boilers, raising the boiler return temperature and reducing the efficiency.

• Also, the boiler pumps are intended to provide adequate flow when the boiler is connected directly to a conventional heating system. In your system, the primary/boiler circuit has very little pipework and few fittings compared to such a conventional system. The primary system has a very low hydraulic resistance and the boiler pumps will produce a large flow rate; this is not good, see above.

The flow rate through the boilers needs to be regulated. This would usually be done by reducing the pumps' speeds and fitting a double regulating valve (DRV) to each boiler to impose an additional, adjustable resistance. The DRVs would make the reverse return pipework arrangement unnecessary. The DRVs would be adjusted to provide the recommended temperature differential across the boilers.

• The controller should provide 0 to 100% PI (Proportional & Integral) control of the boilers’ modulating burners to maintain Tf at the required valve. If the demand for heat is 0%, all boilers are off. If the demand for heat is 100% both boilers are firing.

• The lead boiler fires at 0 to 100% as the demand varies from 0 to 50%. Above 50%, both boilers are fired at the same rate. At 60% demand both boilers are fired at 60% output. The idea is to keep the flow temperature of each boiler (and so the standing losses) to the minimum required to maintain Tf at the required value.

• There should ideally also be an indoor temperature (Ti) sensor. This would adjust Tf up or down if the indoor temperature is less than the required value.
If, as above To were 9 degC, then a simple weather compensation system would fix Tf at 57.5 degC. This would be inadequate if Ti were 12 degC; you’d want Tf to be 80 degC until the house had warmed up to 18 or 19 degC or so.

Sorry that’s a bit long-winded; I can’t condense a couple of decades of experience into a smaller package.

I would control this with a BMS system, knowing that the £3kish starting price would not be a significant part of the (commercial) equipment cost.

The control requirements for a two boiler 2,000 kW installation are the same as for a 2 boiler 50 kW job.

I’d also employ a controls contractor, who I’d expect to know much more about this than I do; I have sometimes been disappointed.
 
Cheapest solution; switch one boiler OFF!!

This can't be Primary/Secondary circulation when heat will circulate around both boiler at any given time.
 
Sponsored Links
Can anyone suggest simple ways to switch the second boiler off when the flow temperature gets close to the value as required by the W/C ?

By the way, the answer is not as simple as it looks at first! ( For anyone who understands the problem I dont need to explain that. )

Tony[/u]
 
Cheapest solution; switch one boiler OFF!!

This can't be Primary/Secondary circulation when heat will circulate around both boiler at any given time.

The OP says one boiler is inadequate in cold weather.

There are (primary) pumps inside the boiler casings. You will get flow through a boiler only when it's pump is on. That is unless there are no check valves hidden under the insulation, in which case you'd get reverse flow through one boiler when the other boiler is firing.

Bye.
 
Can anyone suggest simple ways to switch the second boiler off when the flow temperature gets close to the value as required by the W/C ?
Tony[/u]

You'd need an electronic controller of some sort. Any suggestions?

A cheap and cheerful way could be a outside thermostat, to enable the second boiler in cold weather. Or the OP making regular manual adjustments.
 
The OP says one boiler is inadequate in cold weather.
Sorry, yes I see that now.

Weather comp. is going to be the best way then as MM has stated.
 
The Raypak diagram is a good illustration of your system.
Link please.

Note that, because you are reducing the system flow temperature in mild weather, any TRVs will only be acting as ‘trimmers’ to provide control of the individual room temperatures. They will not be trying to shut down completely, as they might be in a conventional fixed flow temperature system where Tf would be 80 degC at, say To = 16degC.
Even on a properly designed and balanced fixed flow temp system a TRV should not be acting as a switch but as a flow regulator. The only times it will act as a switch is if the system is not properly balanced or if a radiator is considerably oversized for the demand.

The controller should provide 0 to 100% PI (Proportional & Integral) control of the boilers’ modulating burners to maintain Tf at the required valve.
So the external sensor just provides the controller with the outside temperature. It's the controller's job to translate this into a flow temperature and maintain it?
 
Can anyone suggest simple ways to switch the second boiler off when the flow temperature gets close to the value as required by the W/C ?
Tony[/u]

You'd need an electronic controller of some sort. Any suggestions?

A cheap and cheerful way could be a outside thermostat, to enable the second boiler in cold weather. Or the OP making regular manual adjustments.

Unfortunately, I cannot think of any better simple solution than an outside thermostat to inhibit the second boiler when the outside temperature is above say 14 degrees.

Its worst than that as I need to build a simple control to over ride the W/C when hot water heating is needed!

Tony
 
The Raypak diagram is a good illustration of your system.
Link please.

Note that, because you are reducing the system flow temperature in mild weather, any TRVs will only be acting as ‘trimmers’ to provide control of the individual room temperatures. They will not be trying to shut down completely, as they might be in a conventional fixed flow temperature system where Tf would be 80 degC at, say To = 16degC.
Even on a properly designed and balanced fixed flow temp system a TRV should not be acting as a switch but as a flow regulator. The only times it will act as a switch is if the system is not properly balanced or if a radiator is considerably oversized for the demand.

The controller should provide 0 to 100% PI (Proportional & Integral) control of the boilers’ modulating burners to maintain Tf at the required valve.
So the external sensor just provides the controller with the outside temperature. It's the controller's job to translate this into a flow temperature and maintain it?

100% agree.
 
The Raypak diagram is a good illustration of your system.
Link please.

Even on a properly designed and balanced fixed flow temp system a TRV should not be acting as a switch but as a flow regulator. The only times it will act as a switch is if the system is not properly balanced or if a radiator is considerably oversized for the demand.

So the external sensor just provides the controller with the outside temperature. It's the controller's job to translate this into a flow temperature and maintain it?

Link on previous page.

The TRVs are flow regulators, they reduce the flow rate; in the extreme, they will try to stop the flow.

The intention is to keep the system flow high by reducing the flow temperature. The TRVs then have to reduce the flow less and the possibility of excess boiler flow mixing with the boiler return is reduced. If the primary (boiler flow) > the secondary (system flow) the excess will go backwards and into the boilers' return connections; we're trying to avoid/minimize this happening.

It also reduces TRV noise, a lot.

The external sensors are NTC thermistors, typically 10K ohms at 25 degC. The controller contains the software control loops with the values of gain, proportional constant, integral constant, etc., set in the software. The controller processes the input signals from the sensors into an ouput signal (usually 0-10V, maybe 4-20 mA) that is compatible with the boilers internal electronics.
 
Its worst than that as I need to build a simple control to over ride the W/C when hot water heating is needed!

Tony

I forgot that. :oops:

The DHW thermostat disables the CH and connects a resistor in parallel with the outside sensor, giving a false reading equivalent to To = 0 degC?

Most boiler manufacturers' WC systems disable the CH whilst there is a DHW demand. The alternative system (e.g., Viessmann) allows the CH and DHW to run simultaneously by having the boiler flow temperature high as needed by the DHW, and having the CH flow supplied through a 3-port mixing valve.
 
The DHW thermostat disables the CH and connects a resistor in parallel with the outside sensor, giving a false reading equivalent to To = 0 degC?

Its actually a resistor in SERIES to increase the resistance to simulate a cold outside.

The Vessmann 200 W has an input for a DHW sensor when the internal HW diverter is enabled so that it will operate in condensing mode while heating the hot water.

In my case I will have to make a simple circuit to only increase the flow temperature on CH and HW when its already below say 70 degrees. With HW only selected then I can add a sensor to the cylinder.

Tony
 

DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.


Are you a trade or supplier? You can create your listing free at DIYnot Local

 
Sponsored Links
Back
Top