Oversized boiler and downrating it

[Heating-eng]

Combi boiler.
Central heating (CH) output is fully modulating with a range of:
24 kw 4.8 to 24.2kW (16,500 to 82,600 Btu/h)
30 kw 6.1 to 24.2kW (20,700 to 82,600 Btu/h)
35 kw 7.1 to 24.2kW (24,100 to 82,600 Btu/h)

 

[eaton]

Funny how I've done hundreds of boilers in new builds and never had an issue

 

[vern]

Funny how combi's are EXEMPT from whole house heat calcs.......they are sized for hot water! Software on Cdi WB can adjust to any situation, there are 2 hidden menus....RTFM :vanish:

 

[eaton]

Atag a325ec will modulate to 5kw in heating, plus its starts in low fire and modulates up, big fan of this boiler, not seen anything else with 18lpm at tap, 14c incoming cold, 50c hot and flue temp of under 30c.

 

[doitmyself]

Atag a325ec will modulate to 5kw in heating, plus its starts in low fire and modulates up, big fan of this boiler, not seen anything else with 18lpm at tap, 14c incoming cold, 50c hot and flue temp of under 30c.

My house (4 bed detached), if built to current Building Regs, would only require 4.5kW, so even the Atag would be running in on/off mode all the time. Starting at low fire would be an advantage.

 

[SafeGasInstall]

so why not use 2 x 2.5kw electrical removable heaters/radiators Doitmyself ??? this will do you bang on and you can save thousands on ch system

 

[eaton]

Cant please some!

 

[Nostrum]

Funny how combi's are EXEMPT from whole house heat calcs.......they are sized for hot water! Software on Cdi WB can adjust to any situation, there are 2 hidden menus....RTFM :vanish:

Whats the turn down ratio of the gas valve vern?

i do agree with doityourself in some respects, I've seen 37cdi's fitted in new build 2 bed flats for 'optimum hot water' needless to say the boiler short cycled badly.

 

[vern]

Whats the turn down ratio of the gas valve vern?

i do agree with doityourself in some respects, I've seen 37cdi's fitted in new build 2 bed flats for 'optimum hot water' needless to say the boiler short cycled badly.

4:1 meaning 9 kw min..........
This means very little if you know your way around the hidden menus and they are set up correctly, you could set on min kw, alter the max flow temp, alter the anti cycle delay, alter the pump overrun, set a delay after hot water draw off, alter the pump curve from modulating, as high back pressure will result from a small rad system, like I have said before, a monkey can fit a white box on the wall, but its the setting up and commissioning on the job which is out of the norm is where the skill is.....

 

[Heating-eng]

Whats the turn down ratio of the gas valve vern?

i do agree with doityourself in some respects, I've seen 37cdi's fitted in new build 2 bed flats for 'optimum hot water' needless to say the boiler short cycled badly.

only if there not set up correctly.

 

[Nostrum]

Still wont stop the boiler producing less than 9kw though. Reducing the flow temp will just mean the boiler will stop firing quicker, all the other bits will help but you can't get away from the fact the boiler is producing more heat than is required or is capable of dissipating. Pump over runs and delays will just mean the pump is circulating unheated water around the system for longer. Lowering flow temp will increase heat up times and rad sizes will be increased.

It'll work, but it won't be efficient in comparison to a boiler that is able to modulate down and tick over all day long.

heating-eng - don't agree. The flat in question probably had a heat loss requirements of about 4kw at -3 outside temperature. How about at 5-10 degrees through the average temps, 3kw?

If this wasn't an issue, why are manufacturers now introducing boilers with a 1:7 turn down ratio?

as said, they'll always work I'm not disputing that, but they are not operating as they are designed for maximum efficiency, hence the reason why no one saves much money when changing to a condensing combi boiler, as you said vern, poorly commissioned and often poorly specced appliances all over the place fitted by box hangers.

Most of Europe don't use combi's and invest heavily in well designed heating systems sized correctly, whatever the fuel. We seem miles behind, too interested in gaining an extra cupboard to pile crap into!

 

[hammers4spanner]

excuse me for being thick

turn down ratio????

 

[Roger Welton]

Difference between hi and low fire rates


Sent from my iPhone using Tapatalk 2

 

[dancinplumba]

Europe is swarming with combi's millions an millions of them

 

[Nostrum]

Yeah, so most boiler can only reduce by a certain amount, which is governed by the gas valves ability and range. Most boiler are 1:4 so 25% of maximum rate ish.

I know the ideal vogue and the Baxi GA have a 1:7 ratio gas valve :smug:

 

[vern]

Yeah, so most boiler can only reduce by a certain amount, which is governed by the gas valves ability and range. Most boiler are 1:4 so 25% of maximum rate ish.

I know the ideal vogue and the Baxi GA have a 1:7 ratio gas valve :smug:

Baxi GA lmao because they work not lol!

Some of what you say Nostrum is true, but being specific to your example of the 37cdi with 2 or 3 rads, it has not got a return sensor, so calculates the temp rise v time via the flow thermistor and board software, this can also be tweaked if needed, I agree the boiler will be circulating burner off more, so whats the problem? Rads heat up ultra quick, burners off saving gas, circulated water is hot as min delivery is above heat dissipation, so it won't short cycle......rads rapidly dump heat into the air, this is picked up by the room stat, programable stat etc, which is interlocked, boiler turns off demand for heating, set up right it will hardly be on! Job done.

i look after 12 x 42CDi's in luxury apartments, with 2 rads and a towel rail, never been a problem, as they have all been set up

Dont get hung up on turn down ratios they are only as good as the sensors and software controlling them and also the system design and environment they are installed within.

 

[Nostrum]

Vern, what you describe IS short cycling. The boiler will fire. A slug of hot water will make its way around the system and the flow temp will rise very quickly. The boiler will see this and turn off burner which then initiates the anti short cycling. Boiler will then sit for 3 mins or whatever pumping the water which is now cooling quickly as the MWT drops. Boiler then fires shortly again, temp rises quickly, process repeats until rooms stat satisfied.

this is still short cycling. Also all boilers fire on a higher than minimum rate, then wind down after ignition so the problem is made slighty worse by this fact.

again, it will work, yes. But this isn't how these appliances are tested and SEDBUK rated. They won't condense hardly at all so running like this I would guess and say they are a similar efficiency to a S.E.

Just to add, we are all doing it, its the combi's that we all love that make it hard to be able to size the boilers correctly. If I specced a heat only and cylinder every time someone wanted a combi I wouldn't win much work either so I'm not trying to pretend I size every boiler perfectly as its impossible when the customers are experts and know exactly what they need.


you can get away with it a lot easier with gas, less so with oil, solid fuel etc where the boiler doesn't modulate etc

 

[zzzjim]

They won't condense hardly at all so running like this I would guess and say they are a similar efficiency to a S.E.

I would still expect Lower flue temperature and some condensing to take place
as the circulating water return temp will be nice and low .
Surely an improvement on SE

 

[Nostrum]

Can't say as honestly I don't know. Have you ever seen a boiler plume when you first fire up though after filling up? I know I haven't.

 

[zzzjim]

Can't say as honestly I don't know. Have you ever seen a boiler plume when you first fire up though after filling up? I know I haven't.

Cheers see where you are coming from ,fins.(or whaterver-inside)may not have got saturated enough to mist well ,but some vapour is probably leaving more slowly.

 

[vern]

Can't say as honestly I don't know. Have you ever seen a boiler plume when you first fire up though after filling up? I know I haven't.

Yes it will as long as the return is below 55 degs, even more on fill up as the water is stone cold!

 

[vern]

Vern, what you describe IS short cycling. The boiler will fire. A slug of hot water will make its way around the system and the flow temp will rise very quickly. The boiler will see this and turn off burner which then initiates the anti short cycling. Boiler will then sit for 3 mins or whatever pumping the water which is now cooling quickly as the MWT drops. Boiler then fires shortly again, temp rises quickly, process repeats until rooms stat satisfied.

this is still short cycling. Also all boilers fire on a higher than minimum rate, then wind down after ignition so the problem is made slighty worse by this fact.

again, it will work, yes. But this isn't how these appliances are tested and SEDBUK rated. They won't condense hardly at all so running like this I would guess and say they are a similar efficiency to a S.E.

Just to add, we are all doing it, its the combi's that we all love that make it hard to be able to size the boilers correctly. If I specced a heat only and cylinder every time someone wanted a combi I wouldn't win much work either so I'm not trying to pretend I size every boiler perfectly as its impossible when the customers are experts and know exactly what they need.


you can get away with it a lot easier with gas, less so with oil, solid fuel etc where the boiler doesn't modulate etc

Boilers with a return sensor may slug, but it aint like that on the cdi, if set up right it won't be a problem, an ABV at the end of the circuit can also be set up to help if needed.

 

[Nostrum]

Ok, so why have manufacturers invested money into developing weather compensation units, as you know they are designed to match the boiler output to the heat loss depending on outside weather temperature. As we know, the differential temperature effects the buildings heat loss rate so in summer the fabric losses are less. The weather compensation tries to keep the boiler running for as long as possible by matching the heat loss, which in turn makes it efficient.

There is no way of sugar coating it, no matter what you stick on the system or in the boilers software, 9kw output for 5kw heat load is not going to work in the same manner.

Sometimes you can talk yourself into believing something is ok, but look at the science behind it and tell me where the the other 4kw are going? The only place they can be going is straight back to the boiler raising the return temp prematurely.

In an ideal world the boilers return temp would only start to rise as the differential temp between radiator and room dropped, heat transfer would start to slow down as heat loss decreased.

The long an short is, it'll work, everyone will be lovely and hot and no complaints, but you won't be achieving efficiencies as stated by the manufacturers. I suppose in exceptional circumstances you may have heat up problems. The only time I've seen this is where there has been from circulation problems.