What Boiler to prevent short cycling?

[windsor-tg]

I currently have a Vaillant ecoTEC Plus 438 system boiler which is 13 years old. I have been having problems with the boiler which potentially boils down to the heat exchanger so I am now looking to bite the bullet and have a new boiler installed.

My set up at home is that I have a 4-bed detached with 21 radiators (includes 4 towel rails), a 6m x 4m orangery/conservatory that was built last year with under-floor heating (UFH), and a Megaflo for storing hot water. Last year, I had all my radiators changed (unfortunately I did not have my system thoroughly flushed) and the main pump upgraded to a Grundfos UPS2 25-80 pump. I control the hot water, central heating, and UFH through the Heatmiser app. The boiler is located in the garage whilst the pump, 3 individual zone valves and megaflo are all located in an airing cupboard on the landing. 

Quite a few years ago, I was getting an S.53 code on my boiler "the flow-return temperature difference is too great OR Boiler is in waiting period of modulation of block function (or operation block function) due to water shortage". I turned down d.0 of the boiler to 24kW which eliminated the S.53 status.

Last winter, I had 2 issues: 

1. I found the radiators furthest away from the boiler were hardly heating up. I had an engineer come round who eventaully found the issue - debris in the main pump. After removing this, these radiators heated up perfectly well. 
2. when I only had the UFH turned ON, I found the boiler would fire for a minute and then stop. This would be continuous (short firing). The only way I would get round this would be to turn on the central heating so that the UFH would remain on. The same engineer looked into this and believed there was a problem but as he had not installed the UFH, he was not 100% sure where the issue lay. He said there could be a blockage somewhere and/or the heat exchanger could be partially blocked. He suggested a low loss header to resolve the issue since it would elimate the problem of return & flow tempaerates being too great, and therefore the boiler turning off. 

Since my boiler is 13 years old, I am happy to replace by boiler but I obviously want to avoid spending lots of money but still find I am in the same position where I cannot run my UFH independently. I have had a few heating engineers come over to quote for a new boiler. The majority have recommended the Vaillant EcoTEC Plus 430 (30kW) heat-only boiler, and believe the installation of a heat-loss header is an overkill. They pretty much believe the heat exchanger on my existing boiler is partially blocked. A power flush is highly recommended. 

Can anyone recommend if the Vaillant EcoTEC Plus 430 will be suitable? I do have a heating engineer in mind who will be returning again next week to carry out a thorough examination of my system. Is there anything I should make sure he looks into?

 

I am assuming I need to get a boiler where it can go below the minimum required for the UFH to work on its own?

[JohnMo]

The boiler isn't the issue really, to a short cycling issue it's how you split the system up. A boiler can only deliver a set min load. If the system isn't big enough to absorb the heat the boiler manages this by shutdown. 

 

You may have 3 zones, but how many loops on the UFH have room thermostats and actuators and radiator TRV's? All make a system smaller. For every 6kW of heat delivered from a boiler or heat pump you need approx 50L of water engaged to stop short cycling.

 

If you want to operate on small circuits you need a buffer cylinder in the return of the CH circuit 

Edited September 2, 2023 by JohnMo

[windsor-tg]

Thanks @JohnMo  I have just one room that has UFH (all the rest of the rooms have radiators) so I have just one room thermostat to control the UFH. I believe I have 2 loops on the UFH and 2 actuators. I have attached a photo of my UFH manifold and associated components. 
 

None of the 4 towel radiators have TRVs. All the other 17 radiators have TRVs. 
 

Based on this, is the only solution is to have a buffer cylinder in the return on the CH circuit?

[JohnMo]

No boiler would be able to support the UFH without a high risk of short cycling.

 

So you have to options.

1. Leave everything as it is add a buffer, so if your UFH has 160m of pipe that's about 25l of water, so you need a 25l buffer.

2. Don't install a buffer, but have a rationalisation of the TRV's and zoning. So you always have a decent circuit volume open. You would need to do some balancing of the radiators to keep the room from overheating. Having the towel rads as a buffer my be the easiest solution. Have them so they are always on if anywhere else calls for heat why

 

If you are going new boiler I would suggest the minimum.

 

Have it set up to run weather compensation, to do this, it will need to be a boiler that will do one temperature for cylinder heating and a different temperature for CH.

 

Don't go low loss header.

 

Get rid of most of your TRV's so the system runs open circuit, but on weather compensation, with a temperature setback overnight.

 

Get the whole system flushed.

[SimonD]

14 hours ago, windsor-tg said:

elimate the problem of return & flow tempaerates being too great, and therefore the boiler turning off. 

 

High difference between flow and return temperatures is usually caused by a blockage of flow in your system somewhere, not the boiler - the boiler is shutting off due to the low flow. Short-cycling is caused when you don't have enough temperature difference between the flow and return but the flow is sufficient through the boiler. They are different problems that need to be separated. Get the flow problem understood and resolved first before spending money on a new boiler or a low loss header.

 

Nowdays a heating engineer should really be doing a heat loss calculation to appropriately size the boiler as part of the replacement job.

[windsor-tg]

Thanks @JohnMo - regarding the options you have provided:

1. If I were to add a buffer, is this adding a buffer cylinder in the return on the CH circuit? If so, is there any downside to this, and is this likely to be costly?
2. Maybe I am misunderstanding the 2nd option but the issue I have is with the boiler short cycling when I only have the UFH on. I don't see how the radiators come into when I have my central heating turned off? When I only have my UFH on, if I were to have to change all the pipework to have the towel rads connected to the UFH, that would be a major upheaval in the house. All my bathrooms where 3 of the 4 towel rails are located have a tiled flooring and we recently had all the upstairs recarpeted so having to lift up a lot of the carpets will not go down well with the mrs! I cannot see any other way of having the towel rads to be connected to the UFH system (and acting as a buffer) without a major overhaul. 

For a new boiler, I was being recommended the Vaillant ecotec plus 430 but I am not sure what the minimum load on this boiler is. I have had a look at a PDF from Vaillant and unless I am looking at the wrong place, the min load is possibly 7.2 - 7.5kW (https://www.vaillant.co.uk/downloads/aproducts/ecotec-pure/tech-sheets-3/ecotec-plus-open-vent-boiler-1786922.pdf). Also, I do not know if this boiler has the ability to run weather compensation.

 

Also, I am not totally clear where you say "Get rid of most of your TRV's so the system runs open circuit, but on weather compensation, with a temperature setback overnight" - I do not see how this helps when I am just running the UFH on its own. 

 

Why do you not recommend Low Loss Header? 

 

If I do replace the boiler, I will certainly have the system power flushed (not sure if that would generally include the UFH plastic pipes). 

 

@SimonD - thanks for this. What is the best way to get the flow problem sorted out? When we talk about the flow and blockage, is this referring to the pipework in the UFH system or the central heating system I have had the pipework running between the boiler (in the garage) and the main pump (in the airing cupboard) flushed out. I have attached some photos pof the zone vavles, pipework, Grundfos pump, etc in the airing cupboard. 

 

On the picture of the zone valves, they are labelled 1 - HW, 2 - UFH, 3 - CH. I will send more photos on seperate replies as there is a limit on file sizes 

 

 

[windsor-tg]

more photos. On the photo 'pipework around pump' - the two pipes labelled '1' & '2' in blue are the 28mm pipes running to & fro from the boiler. The 2 white pipes are the UFH pipes.

Edited September 3, 2023 by windsor-tg

[JohnMo]

6 hours ago, windsor-tg said:

Maybe I am misunderstanding the 2nd option but the issue I have is with the boiler short cycling when I only have the UFH on

Yes that's the issue. You need more system engaged to get away from short cycling.

 

If it's not easy to get say the towel rads on, the only option is a small buffer. There are no real disadvantages having a buffer in this situation, as long as it's used in a two port configuration. The water from the heating circuit at or close to boiler goes into the top port, the water from the buffer to boiler out the bottom port. The buffer is basically acting as a volumiser add water volume to the system, so when a small volume circuit like your UFH is only on the boiler has a meaningful volume of water to heat up.

 

An analogy-  Just like a kettle, you put a small amount of water in, it heats up very quickly, you get one cup out, but not enough for the wife, so you put a bit more water and boiler again, get another cup and so on. Fill the kettle up it takes a while to boil, you, your wife and a couple of others get a hot drink. 

 

The boiler is just the same, the hot water in the buffer/volumiser, gets heated with central heating or UFH,  it carries on being circulated after the boiler has stopped heating, the heat gets used in the system, when it's cool enough the boiler starts again..

 

 

[windsor-tg]

Many thanks @JohnMo. Would you be able to share a link to one of these buffer cylinders? Would it need to be a particular size?

 

Since my boiler is in the garage but all the other central heating pipes, zone valves and main pump are in the airing cupboard, would the best place for the buffer cylinder be in the garage or the airing cupboard (depending on the size of the cylinder)?  Having a diagram on how the setup of the buffer cylinder in relation to the boiler would be great 😊 

 

Are you still suggesting I get rid of my TRVs on the radiators or do I not really need to do this if I install the buffer cylinder?

[JohnMo]

Loads of threads on here - herex is one

 

https://forum.buildhub.org.uk/topic/26090-underfloor-heating-and-radiators-mix/

 

 

[windsor-tg]

thanks @JohnMo. I did have a read of that thread you provided but I cannot really tell how this would work in my situation, what size I would need, and how it would be configured 😕

[SimonD]

21 hours ago, windsor-tg said:

 

@SimonD - thanks for this. What is the best way to get the flow problem sorted out? When we talk about the flow and blockage, is this referring to the pipework in the UFH system or the central heating system I have had the pipework running between the boiler (in the garage) and the main pump (in the airing cupboard) flushed out. I have attached some photos pof the zone vavles, pipework, Grundfos pump, etc in the airing cupboard.

 

First you have to diagnose the cause of the error message and that's only by a process of elimination. A competent heating engineer should be able to do this, but it does take some time. Then, if indeed a flow problem has been found somewhere on the system then this gets rectified accordingly. Sometimes it can be caused by a faulty fitting, sometimes due to crap in the system. Again, it's a process of elimination. For me the best tool I have for this is an infrared camera that immediately picks up temperature differentials in the system which will helps to zone in on where the problem lies.

 

You need a cogent explanation of what is going on which means a proper assessment of the functioning of the whole system. Only then do you proceed with spending money on new bits.

[JohnMo]

25 minutes ago, SimonD said:

 

First you have to diagnose the cause of the error message and that's only by a process of elimination. A competent heating engineer should be able to do this, but it does take some time. Then, if indeed a flow problem has been found somewhere on the system then this gets rectified accordingly. Sometimes it can be caused by a faulty fitting, sometimes due to crap in the system. Again, it's a process of elimination. For me the best tool I have for this is an infrared camera that immediately picks up temperature differentials in the system which will helps to zone in on where the problem lies.

 

You need a cogent explanation of what is going on which means a proper assessment of the functioning of the whole system. Only then do you proceed with spending money on new bits.

Aren't the error messages a thing that occured several years ago and are now fixed? Needless info in the first post?

 

The point of the post is when only UFH is engaged the boiler only runs for a minute then switches off - repeat, when he engaged the radiator also the problem goes away.

 

Two UFH loops alone (possibly 4 to 5 l/min) will not keep a boiler happy, hence the short cycling. My boiler was looking for around 18l/min. To support a boiler that can turn down to say 6 kW you need around 50L of water (the posters boiler looks to have a min out put of around 7kW), those two loops will be around 25L. So you have a low flow rate, would suspect the water is just short circuiting via min flow valve and no water volume.

[windsor-tg]

thanks @JohnMo & @SimonD. The S.53 message that I got a few years back was resolved by turning down the power of the boiler to 24kW (d0 = 24). Whether this is/was related to a flow/blockage problem, I am not sure. When the UFH was recently turned on (to test what is happening with the boiler), the S.53 did appear again but went away. 

 

 I checked with my builder who layed the UFH pipework in the extension and he believes each of the 2 loops were around 60m long. I am not sure what this equates to in terms of litres/min. If I do have a low flow rate, is the only solution is to have a buffer cylinder of x litres in size to allow for the UFH to run on its own without the boiler continually cycling?

[JohnMo]

Here is a link to kensa heat pump talking about buffers, have a read.  Don't worry it's talking ground source heat pumps, it's also relevant here. 

 

If you install per the guidance on the web paged link below, it will act as a min flow device and volumiser, which is what you need. (Ignore my previous advice of installing in the return line only as it does get you away from the min flow required.

 

You should aim to install between 15 and 25L sized buffer/volumiser.

 

https://www.kensaheatpumps.com/installer/buff-up-your-knowledge-of-buffer-tanks/

 

Something like this could be ideal (below), tee from flow from boiler, into top connection, and a tee from return in the bottom connection.

 

 

 

[windsor-tg]

thanks @JohnMo. From your description as well as the web link page, am I right to assume this buffer cylinder should be positioned as close to the boiler as possible? 

 

How can I determine the exact size of the cylinder needed?

 

When the central heating is ON, will this cylinder be ignored or does it still have an act to play?

[JohnMo]

Basically if you hook it up as per Kensa, only excess flow goes through the buffer. So when CH is on, the flow will more or less sail straight past the buffer, but will still heat the volume of water. When you only have UFH heating on, quite a lot of the flow will go through the buffer, but that is why its there.  20 to 25L should be an OK size as a minimum.  The bigger the buffer the longer the run time of the boiler, the more efficency gained.

[SimonD]

13 hours ago, JohnMo said:

Aren't the error messages a thing that occured several years ago and are now fixed? Needless info in the first post?

 

The point of the post is when only UFH is engaged the boiler only runs for a minute then switches off - repeat, when he engaged the radiator also the problem goes away.

 

The op says about engineer looking at the ufh:

 

"he was not 100% sure where the issue lay. He said there could be a blockage somewhere and/or the heat exchanger could be partially blocked."

 

In any case, the flow rates need to be checked and confirmed and in relation to the temperature drop across the system - is the boiler cycling because there is too much temperature differential still (and corresponding low flow rate) or because the temperature differential is too small? I'd want to know this information.

 

In each case a buffer cylinder could be of benefit, but it may need to be implemented differently is each instance. In one it may be necessary to differentiate flow rates between heat source and heat emitter where they need to be different, and/or to buffer the excess output from heat source that can't modulate down enough for the current heat loss.

[windsor-tg]

thanks @JohnMo.

 

thanks @SimonD - I will doible check when the engineer is here is a couple of weeks but I am pretty certain the boiler cycles when there is too much temperature differential. The flow temperature rockets up quickly when the boiler fires rather than the temp going up gradually. Assuming this is the case, how would the buffer cylinder be implemented in this instance? 

 

Also, I have had some people mention about Low Low Header - how is the LLH different to the buffer cylinder? Are the supply & installation of either of these roughly the same? The engineer who will be visting in a couple of weeks said a LLH would be in the region of £1,000 for installation 😟

[JohnMo]

A buffer and LLH can be the same thing depending on how they are plumbed. A LLH provides hydraulic separation between the heat source and the heating system.  It needs to sized and the pump rates either side also need to sized correctly, otherwise huge efficiency hits are seen. You also have a secondary pump in the system, which runs when ever the heating is on.  The solution I provided, does not need the secondary pump.

[SimonD]

10 hours ago, windsor-tg said:

thanks @SimonD - I will doible check when the engineer is here is a couple of weeks but I am pretty certain the boiler cycles when there is too much temperature differential. The flow temperature rockets up quickly when the boiler fires rather than the temp going up gradually. Assuming this is the case, how would the buffer cylinder be implemented in this instance? 

 

Also, I have had some people mention about Low Low Header - how is the LLH different to the buffer cylinder? Are the supply & installation of either of these roughly the same? The engineer who will be visting in a couple of weeks said a LLH would be in the region of £1,000 for installation 😟

First do the investigations to get the data that supports the decisions. You can get the flow and return temperatures from your boiler yourself  - d.40 for flow, d.41 for return temp. Run your system fully open and log readings over a period of time, then methodically close parts of it down while taking recordings over time, including the ufh only.

[windsor-tg]

Thanks @SimonD. Where you say run the system fully open, do you mean having the hot water, central heating and UFH all turned ON and then taking d.40 and d.41 readings? How many readings would you say I should take in this state and would 5 minute intervals be sufficient?

 

Should I then turn off the hot water, and then followed by the central heating such that only the UFH remains turned on?

[SimonD]

27 minutes ago, windsor-tg said:

Thanks @SimonD. Where you say run the system fully open, do you mean having the hot water, central heating and UFH all turned ON and then taking d.40 and d.41 readings? How many readings would you say I should take in this state and would 5 minute intervals be sufficient?

 

Should I then turn off the hot water, and then followed by the central heating such that only the UFH remains turned on?

 

I think you can probably ignore the hot water system for now and just run the heating system so you have all rads plus ufh, then all rads no ufh and then no rads and just ufh basically so you're monitoring each zone. That should give you a reasonable picture to start with. Every 5 min or so should be fine. You want to be taking these readings until you see the boiler cycling. It's a bit tedious I know...

[windsor-tg]

20 hours ago, JohnMo said:

A buffer and LLH can be the same thing depending on how they are plumbed. A LLH provides hydraulic separation between the heat source and the heating system.  It needs to sized and the pump rates either side also need to sized correctly, otherwise huge efficiency hits are seen. You also have a secondary pump in the system, which runs when ever the heating is on.  The solution I provided, does not need the secondary pump.

@JohnMo - are you saying that if I went with a buffer or cylinder, I will no longer need my Grundfos 25-80 pump? I would have thought this is still needed to pump the water around the radiators. 

[windsor-tg]

11 hours ago, SimonD said:

 

I think you can probably ignore the hot water system for now and just run the heating system so you have all rads plus ufh, then all rads no ufh and then no rads and just ufh basically so you're monitoring each zone. That should give you a reasonable picture to start with. Every 5 min or so should be fine. You want to be taking these readings until you see the boiler cycling. It's a bit tedious I know...

thanks @SimonD - I will look to do this when the weather cools down a bit. Otherwise, my house will turn into a sauna which my mrs would love! Would 5 readings in each state be sufficient, i.e. 5 readings with CH + UFH on, 5 readings with CH only on, and 5 readings (or however many readings until the boiler is cyling) with UFH only on?