Two boilers

[Justin Passing]

The house has an old Profile 100 for the heating, and an old Ultra 50 for the hot water, both sealed, but separate. Both are about the right size for their respective loads, 22 radiators and a high recovery coil.
The thought is to replace both with two (Vaillant?) system boilers, keeping the two systems separate, but with valves and switches/relays fitted to allow cross-coupling, so one boiler can be used if the other fails.

To be able to isolate each boiler and couple the other seems to take half a dozen lever valves, with some complexity on the wiring, which is more awkward if each boiler has compensating controls. It is easier if they don't.

Is there anything available which caters for this sort of arrangement, off the shelf?

 

[ALEC1]

nope, one 30kw boiler with proprietary controls and hot water priority fits the bill

vitodens 200 30 kw or vaillant 631 both with weather compensation

 

[mickyg]

just fit 2 boilers on a header and combine the systems. turn one off in the summer.

 

[Agile]

My simple mind would use one lever valve and two NRVs and leave the wiring alone as they can be operated manually in the very rare event that either failed.

Tony

 

[Justin Passing]

A lot of hot water is used, recently 11 people in the house, so the heating would be off a lot. In coldest weather the CH boiler seems to be working flat out as it is.
In spring /autumn wouldn't the CH and HW need different flow temperatures for best HW reheat and CH efficiency?

Why One lever valve?

Header - single low-loss or just commoned flows and commoned returns?
-Pumps?

 

[dcawkwell]

Firstly you need to do a boiler sizing calculation for the house.

It is unlikely you need two boilers unless you want the backup.

It is possible to connect up two boilers in a boiler cascade with nothing
special. Consult a heating engineer.

You may want 2 pumps as 22 radiators is a bit much for
one pump. One for upstairs one for downstairs.

Working on 1.5kw per radiator gives approx 30kw requirement
so 35kw boiler is going to handle it without a problem.

 

[ALEC1]

and if the hot water usage is high install a large high recovery hot water cylinder...35kws will reheat at about 12l/m... enough to run a shower continuously providing that boiler has hot water priority....

 

[Justin Passing]

Firstly you need to do a boiler sizing calculation for the house.

We know the boiler requirement for the CH, as I have explained.
The online /best practise guide suggests far too little for HW heating n this case.

It is unlikely you need two boilers unless you want the backup.

Yes a single boiler could do it if it were large enough.
I have howerver asked about using two boilers.

It is possible to connect up two boilers in a boiler cascade with nothing special.

Can you give a link to a "nothing special" which addresses the questions?

You may want 2 pumps as 22 radiators is a bit much for
one pump. One for upstairs one for downstairs.

Indeed, though the 15/60 is managing at the moment, obviously Ecotec 6xxs etc have much higher internal resistance than a Profile. The "Pumps?" query was in reference to the suggestion of headers, as the boilers have pumps within. Did Mickyg mean a single low loss header then ..pumps where??... etc.

Working on 1.5kw per radiator gives approx 30kw requirement
so 35kw boiler is going to handle it without a problem.

Except for the hot water, as explained.

and if the hot water usage is high install a large high recovery hot water cylinder...35kws will reheat at about 12l/m... enough to run a shower continuously providing that boiler has hot water priority....

The existing cylinder is Part L compliant and manages to heat enough water fast enough, with the old Ultra. What would be the point of changing it?

 

[Justin Passing]

What's there to design? Why the rolling eyes?

The radiators are not going to be replaced or replumbed, neither is the cylinder.
There are two pairs of F & R pipes, two old boilers and two pumps, simples.
The question is about replacing them with two boilers in a similar but improved way, to provide/retain back-up/reliability, with an awareness of efficiency.

 

[dcawkwell]

I have howerver asked about using two boilers.
Can you give a link to a "nothing special" which addresses the questions?
-----------
If you take a look at the www.warmflow.co.uk website.
It gives a diagram of a simple cascade arrangement.
All that is required is a pipe thermostat to bring on the second boiler
in the cascade if the first boiler isn't keeping up with demand or the
first boiler has failed.
-------------

You may want 2 pumps as 22 radiators is a bit much for
one pump. One for upstairs one for downstairs.

The "Pumps?" query was in reference to the suggestion of headers, as the boilers have pumps within. Did Mickyg mean a single low loss header then ..pumps where??... etc.
-------
If using a low loss header. This is basically a cylinder with flow and returns
for the boiler(s). There are also flow and returns taken off the low loss
header for each pumped circuit. It is simply a connection point where the boilers supply the heat to and the pumps for heating/hot water drag the heat from.
-----------

Working on 1.5kw per radiator gives approx 30kw requirement
so 35kw boiler is going to handle it without a problem. Except for the hot water, as explained.
---------
For a normal house the allowance is usually to allow 2kw for hot water. A 35kW boiler or 40kW boiler will be plenty and allow a generous amount for
hot water.
-----------------

 

[Dan Robinson]

If things are really not changing then.....

two boilers of the required output. Weather compensate the heating boiler and use a boiler that can directly sense the cylinder temperature and operate its flow and return temps accurately.

I wouldn't bother joining anything personally.

 

[Justin Passing]

dcawkwell, Dan_Robinson, thanks.
It seems it's either linked, or compensated, but there may not be a method for doing both readily available.

I don't see at the moment how a LLH helps with that.

Perhaps we can have the boilers set up as though they're servicing both loads, with the plumbing valves "in case", and the control inputs switch-overable or disconnected as necessary. I'm not at all sure one can do that with the controls

 

[Dan Robinson]

It can be done fairly easily, but .....

 

[mickyg]

The suggestion of a LLH wasnt to solve the problem, it was to make the solution work better. I dont see why anyone would mess about with valves etc.. to make what will be a hash of a job. Stick them side by side on the same system sharing the load. OR presonally I would just have one boiler doing the heating and a water heater for the hot water. But my answer was to give you what you originally asked for. Also you dont need to be rude to the other members, they've posted to help in whatever way, show some appreciation.

 

[ChrisR]

It looks to me as though thanks was given where people have been helpful. It's a feature of this forum that people with questions are frequently told to "go ask someone who knows what he's talking about" in a "you wouldn't understand" way. Why bother saying that?
GW asked for it!

I certainly don't have an answer which does exactly what's been asked for off the top of my head, though I expect I could work something out. Part of Alec's was a good answer - "No there isn't anything standard"!


The idea seems to be to have the CH boiler operating at lower temp than the HW one. I dunno how much that would actually save but it seems a fair ask.
Are you really suggesting the guy rips out the cylinder?