Oversized boiler and downrating it

[Alex100]

I previously posted a thread "New CH system underpowered/ wrong layout?" - and many thanks to everyone who contributed. Issue is now about boiler size.

The installer hasn't once been back to inspect the system since I complained it doesn't heat the house enough, but now he says he just wants to sort this out and put in bigger radiators like I (not he) suggested. He asked me what size ones I want? I suggested he comes and looks in case something else is wrong, especially if it is an oversized boiler, but he just wants to be told what size radiators I want instead (I'm not prepared to take that responsibility).

This installer put a WB Greenstar 38CDi Classic [FONT=arial, sans-serif]combi in two months ago and a whole new system. He put in 9 radiators on two floors with total BTU of about 35,000. However, the boiler does about 38kW or 102,000 BTU. He says he chose the big boiler because I wanted good hot water flow (suggesting it's my fault the boiler that he chose is too big). A heating engineer I met, plus several contributors on this forum, told me that it is essential to have the boiler downrated by the installer to something more appropriate on the heating side (around 10-12kW) - and that it does have the facility to do this without downrating the water heating side.
[/FONT]
We told the installer the boiler keeps cutting out and he said we are getting confused listening to people on plumbing forums, and that the boiler cuts out because the water in the system has reached temperature and everything is okay.[FONT=arial, sans-serif]

Questions:
[/FONT]1. I spoke to Worcester-Bosch today and they said there is no need to downrate the boiler and that it will automatically self modulate down to the 35,000 BTU which is being drawn. Does this sound right to any WB engineers out there?

[FONT=arial, sans-serif]2. [/FONT]Also, downstairs rads "rush" loudly, which makes me think either the pump on default setting 3 is too high (but installer says that's normal combi setting) or else could it be an improperly balanced system causing rushing far from the boiler (which is upstairs and other side of building). Many rads are long long branches from flow & return (up to 8 metres in one case) in 22mm reducing to 15mm - could that cause rushing in rad?

 

[dancinplumba]

1, this is true.

2, sounds like air in rads/system

the installer needs to calculate size of rads.

get a better installer in 🙂

 

[Heating-eng]

The boiler is a great choice, your installer has chosen you a good boiler.
the boiler is not oversized as it will adjust to the load required.
your rooms needs to be sized correctly and then the right size rads can be fitted.

 

[Heating-eng]

Your pump should be inside the boiler on a combi, I do not think you can adjust this on the combi.

 

[Whyme]

Dont the pumps on those modulate ? (good boiler like heating-eng said)

 

[Heating-eng]

I fitted a 38 CDI Combi this week, one of the best boilers on the market.

 

[Heating-eng]

Dont the pumps on those modulate ? (good boiler like heating-eng said)

Yes I think the pumps do.

 

[AWheating]

the pump on this boiler modulates, it can be set up to run at different speeds/parameters but its not normally required tbh.

the noise at the rads may indicate that lockshield valve is closed down, might not hurt to go around your rads and open up the lockshield fully and see how this effects the heating.

setting/range rating the boiler on the heating side can be an advantage but the boiler will modulate on its own to suit also.

 

[Silent Dancer]

The CH of the boiler is from 9.4 to 30kw. Your installer has said you need 10.2kw presumably at -3℃.

When the boiler gets a heating demand it will supply 30kw at first and it will never get the chance to
modulate down to the minimum. Matters may be made worse by software on the boiler that prevents short cycling, the boiler firing to soon.

Setting the CH max to a more sensible level of 15kw should help the problems you describe and ensure the boiler runs at it's most efficient. If you're installer won't help then contact Worcester technical department for assistance.

Regarding the pump the boiler has a modulating pump so there no need to change the speed. The noise is either plumbing related or part of the above story that the pump is working flat out to start off with and never gets the chance to modulate down.

 

[doitmyself]

I previously posted a thread "New CH system underpowered/ wrong layout?"

In that topic I suggested you use the Whole House Boiler Size Calculator to estimate the heating requirement of your house. You would then be able to compare this with the 11kW (37,600BTU) of your radiators to see if they are giving off enough heat. (It won't tell you if each rad is correct; only the total.)

If you want to do a room by room calculation, use this Radiator Calculator. It's more reliable than any other I have found - apart from Stelrad Stars, which is too technical for most people.

 

[Heating-eng]

Just guess them, that's what most people do, I use an app on my ipad ,works really well

 

[snoopy8888]

In that topic I suggested you use the Whole House Boiler Size Calculator to estimate the heating requirement of your house. You would then be able to compare this with the 11kW (37,600BTU) of your radiators to see if they are giving off enough heat. (It won't tell you if each rad is correct; only the total.)

If you want to do a room by room calculation, use this Radiator Calculator. It's more reliable than any other I have found - apart from Stelrad Stars, which is too technical for most people.

I have a question about whole house boiler calculation. Let say i done a calculation for a house and the boiler required is a 24kw. A 24kw combi only give a 9-10 litre hot water flow rate.
Let say the customer wants a high flow rate, to get a F/R of 15ltr, you need a 35kw boiler. But according to the house boiler calculation, this is inefficient. So who do you listen to your customer or part L ?

 

[Heating-eng]

I have a question about whole house boiler calculation. Let say i done a calculation for a house and the boiler required is a 24kw. A 24kw combi only give a 9-10 litre hot water flow rate.
Let say the customer wants a high flow rate, to get a F/R of 15ltr, you need a 35kw boiler. But according to the house boiler calculation, this is inefficient. So who do you listen to your customer or part L ?

you would then do the rest of the tests to find the right boiler, ie check the water pressure and flow, this is what is normally used to size a combi boiler.
the whole house size is not even relervent ,if the size of the house is over 24kw then a combi boiler is not normally the right boiler.

 

[Heating-eng]

you would then do the rest of the tests to find the right boiler, ie check the water pressure and flow, this is what is normally used to size a combi boiler.
the whole house size is not even relervent ,if the size of the house is over 24kw then a combi boiler is not normally the right boiler.

Most combis heat output for the heating is lower than for the hot water, if you get a 35kw boiler for hot water the heating might only be 24kw.

 

[doitmyself]

I have a question about whole house boiler calculation. Let say i done a calculation for a house and the boiler required is a 24kw. A 24kw combi only give a 9-10 litre hot water flow rate.
Let say the customer wants a high flow rate, to get a F/R of 15ltr, you need a 35kw boiler. But according to the house boiler calculation, this is inefficient. So who do you listen to your customer or part L ?

There seems to be some misunderstanding about the relevance of the whole house boiler calculation when a combi boiler is involved.

The WHBC gives the heating requirement. (You should really deduct 2kW from the result as this is the allowance for heating a cylinder).

Now this is the maximum heating output; most of the time the boiler will be producing less than this. So you need to ensure that the boiler can modulate as far below this as possible.

The hot water requirement is calculated from the actual flow rate. Just putting in a 35kW boiler will not give you a 15 litre hot water flow rate if the incoming cold water is flowing at a slower rate. You have to match the boiler to the actual flow rate.

Let's say the hot water requirement is 35kW but the 12kW requirement is only heating. So you need a boiler with a 35kW hot water output which, on the heating side, can modulate as far below 12kW as possible.

 

[eaton]

Nearly every combi is way over sized on heating bits that's because their sized for hw flow rate, most don't need touching as they will modulate and some ramp up from low to high instead of go flat out and ramp down, very rarely have I had to range rate the heating but it has been done.

 

[lame plumber]

Nearly every combi is way over sized on heating bits that's because their sized for hw flow rate, most don't need touching as they will modulate and some ramp up from low to high instead of go flat out and ramp down, very rarely have I had to range rate the heating but it has been done.

the best answer yet

 

[doitmyself]

Nearly every combi is way over sized on heating bits that's because their sized for hw flow rate, most don't need touching as they will modulate and some ramp up from low to high instead of go flat out and ramp down, very rarely have I had to range rate the heating but it has been done.

You need 35kW for hot water but only 8kW for heating. You install a 35kW combi boiler which, for heating, it can only modulate down to 12kW. Result: the boiler spends all its time, when on heating, in on-off mode. If you're lucky and the boiler always starts at minimum output, that's not so bad.

But if, on the other hand, you install a boiler which always starts at max output and then modulates down, then you are in trouble. It will light and go to max output; the flow and return temperatures will rise rapidly; the boiler will modulate down in an attempt to maintain the correct flow temp; when it reaches minimum modulation and the flow temp still rises it will turn off and wait for the anti-cycling delay time; during this time the flow temperature will drop, possibly as much as 20 or 30 degrees. The boiler then starts again and the cycle repeats. Result: the mean flow temperature will be about 45C; the radiators never reach the correct working temperature; the house takes hours to heat up.

 

[SafeGasInstall]

i am just signing my self to follow this post for later read

 

[Heating-eng]

You need 35kW for hot water but only 8kW for heating. You install a 35kW combi boiler which, for heating, it can only modulate down to 12kW. Result: the boiler spends all its time, when on heating, in on-off mode. If you're lucky and the boiler always starts at minimum output, that's not so bad.

But if, on the other hand, you install a boiler which always starts at max output and then modulates down, then you are in trouble. It will light and go to max output; the flow and return temperatures will rise rapidly; the boiler will modulate down in an attempt to maintain the correct flow temp; when it reaches minimum modulation and the flow temp still rises it will turn off and wait for the anti-cycling delay time; during this time the flow temperature will drop, possibly as much as 20 or 30 degrees. The boiler then starts again and the cycle repeats. Result: the mean flow temperature will be about 45C; the radiators never reach the correct working temperature; the house takes hours to heat up.

You are talking total rubbish. For one it would need to be a very small house to only need 8kw for heating, and if it was so small then it would not need 35kw for hot water as the house would only have one bathroom tops.

 

[Heating-eng]

Example. Ideal logic 35 kw combi boiler . Heating min is about 7kw.
so would work well in nearly any house.

 

[zzzjim]

the house takes hours to heat up.

Could poor pipe layout /sizing contribute to boiler ability stay within operating limits

(Every install has its own personality )

 

[Heating-eng]

Could poor pipe layout /sizing contribute to boiler ability stay within operating limits

(Every install has its own personality )

yes maybe his pipework is wrong, will not be the boiler.

 

[doitmyself]

You are talking total rubbish. For one it would need to be a very small house to only need 8kw for heating, and if it was so small then it would not need 35kw for hot water as the house would only have one bathroom tops.

8kW may be a bit low, but you need to take a look at new build houses. They are so well insulated that a four bed, two bathroom house will have a 12kW system boiler and a large unvented cylinder. If a combi boiler was installed you would get exactly the situation I described.

 

[Heating-eng]

8kW may be a bit low, but you need to take a look at new build houses. They are so well insulated that a four bed, two bathroom house will have a 12kW system boiler and a large unvented cylinder. If a combi boiler was installed you would get exactly the situation I described.

no you wouldn't , your totally wrong.
a 24kw combi will work from about 4 kw for heating.

not sure you would want a combi in that type of house.

 

[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.