What Boiler to prevent short cycling?

[JohnMo]

8 minutes ago, windsor-tg said:

@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. 

No I am not saying that. If you add a LLH you will need another pump in addition to the one you already have.

 

Do the checks and report back.

[windsor-tg]

in the meantime, I have been liasing with another heating engineer. I told them that my concern in replacing my existing boiler with a new boiler will be that when I just turn on the UFH, I will still have the issue where the boiler continues to cycle because the boiler is not able to modulate down far enough. They mentioned that my flow to the UFH manifold is connected to the positive side of the Grundfos pump, and the return is connected to the main return back to the boiler - this is how it should be. My question to them was "how will this prevent the boiler from continually cycling if I just have the UFH turned on".

 

Their response was "it will never be an issue. For instance when thermostats are fitted on radiators they can all shut down. The bypass then opens up so there is still flow around the system. The boiler senses this and modulates down to a low output and shuts off if required. Once valves open back up the boiler fires back up. Same with your underfloor. Your existing boiler isn’t behaving this way because the heat exchanger is blocked". 

 

They suggest I contact the Vaillant technical team to put my mind at rest. More than one engineer has suggested the heat exchanger on my existing boiler is partially blocked. Anyway, I just spoke with Vaillant technical and there were not 100% sure but they did agree on what the engineer said about the bypass. 

 

I am still none the wiser though so I will look to carry out the tests recommended by @SimonD

 

 

[JohnMo]

13 minutes ago, windsor-tg said:

The bypass then opens up so there is still flow around the system

True

 

14 minutes ago, windsor-tg said:

The boiler senses this and modulates down to a low output and shuts off if required

True, but if your min modulation is 7kW and the heating required for your UFH is 1kW, your boiler run is very short, hence it short cycles as it's designed to. Doesn't mean its efficient. And back to your original question.

[SimonD]

21 hours ago, windsor-tg said:

in the meantime, I have been liasing with another heating engineer. I told them that my concern in replacing my existing boiler with a new boiler will be that when I just turn on the UFH, I will still have the issue where the boiler continues to cycle because the boiler is not able to modulate down far enough. They mentioned that my flow to the UFH manifold is connected to the positive side of the Grundfos pump, and the return is connected to the main return back to the boiler - this is how it should be. My question to them was "how will this prevent the boiler from continually cycling if I just have the UFH turned on".

 

Their response was "it will never be an issue. For instance when thermostats are fitted on radiators they can all shut down. The bypass then opens up so there is still flow around the system. The boiler senses this and modulates down to a low output and shuts off if required. Once valves open back up the boiler fires back up. Same with your underfloor. Your existing boiler isn’t behaving this way because the heat exchanger is blocked". 

 

They suggest I contact the Vaillant technical team to put my mind at rest. More than one engineer has suggested the heat exchanger on my existing boiler is partially blocked. Anyway, I just spoke with Vaillant technical and there were not 100% sure but they did agree on what the engineer said about the bypass. 

 

I am still none the wiser though so I will look to carry out the tests recommended by @SimonD

 

 

 

Yes, do the tests and then we have some data, otherwise it's all speculation.

 

Right now we have either/and:

- a problem caused due a flow problem somewhere within the system;

- a simple issue of short-cycling due to low volume of the ufh;

 

The system bypass isn't really relevant as that's just there to prevent a no flow situation through the boiler when trvs are fitted to all rads. Some boiler manufacturers specify a minimum bypass length (e.g. Worcester Bosch say 3m on some models) so that there's a minimum heat loss through the circuit - but that's not relevant right now, nor is it relevant to the boiler short cycling as @JohnMo has said.

 

Other questions:

Does your boiler make any funny noises just before it turns itself off?

Have any of the engineers suggesting a new boiler even mentioned a heat loss calc to correctly size the new unit and mentioned selecting a boiler with good modulation ratio?

Have any of these engineers mentioned that it's the controllers that govern boiler modulation rather than the boiler itself? E.g. the Vaillant VRC 400 weather compensating control for Ecotec boilers that adjusts flow temperatures according to a heat curve and heat demand?

[windsor-tg]

20 minutes ago, SimonD said:

Other questions:

Does your boiler make any funny noises just before it turns itself off?

Have any of the engineers suggesting a new boiler even mentioned a heat loss calc to correctly size the new unit and mentioned selecting a boiler with good modulation ratio?

Have any of these engineers mentioned that it's the controllers that govern boiler modulation rather than the boiler itself? E.g. the Vaillant VRC 400 weather compensating control for Ecotec boilers that adjusts flow temperatures according to a heat curve and heat demand?

thanks @SimonD. To your questions:

1. I do not think so. Do you mean something like a kettling sound? One of the engineers said they could here a noise but I could not. If it is a kellting sound, what does this mean?
2. None of the accredited Vaillant and Worcester Bosch installers have suggested a heat loss calc. Even when I spoke with Vaillant technical, they were shocked that none of these installers recommended a heat loss calc. I may have mentioned it earlier but only one of these installers went into each room to see the size of each radiator to make a mental note of the kW each radiator would be, and hence, they arrived at deciding a 30kW boiler is the right size. I have just been onto the phone with Vaillant technical again and they are saying that the reason my boiler is cycling is because the flow rate through the boiler & system is not enough when only the UFH is operating. For a 24kW boiler, the min flow rate needs to be 17.2L/min, and for a 30kW boiler, the flow rate needs to be minimum 21.5L/min. I have no idea to know what the flow rate is for my UFH. Their recommendation is therefore to install a LLH (rather than boiler cylinder which is for AHSP) along with 1-2 additional pumps. Now I am really confused 😩
3. No. Would this mean I would need to change my controller/thermostats? I am currently using Heatmiser for programming the times the HW, CH and UFH comes on/off, as well as setting the temperature for the CH and UFH. 

[SimonD]

22 minutes ago, windsor-tg said:

thanks @SimonD. To your questions:

1. I do not think so. Do you mean something like a kettling sound? One of the engineers said they could here a noise but I could not. If it is a kellting sound, what does this mean?
2. None of the accredited Vaillant and Worcester Bosch installers have suggested a heat loss calc. Even when I spoke with Vaillant technical, they were shocked that none of these installers recommended a heat loss calc. I may have mentioned it earlier but only one of these installers went into each room to see the size of each radiator to make a mental note of the kW each radiator would be, and hence, they arrived at deciding a 30kW boiler is the right size. I have just been onto the phone with Vaillant technical again and they are saying that the reason my boiler is cycling is because the flow rate through the boiler & system is not enough when only the UFH is operating. For a 24kW boiler, the min flow rate needs to be 17.2L/min, and for a 30kW boiler, the flow rate needs to be minimum 21.5L/min. I have no idea to know what the flow rate is for my UFH. Their recommendation is therefore to install a LLH (rather than boiler cylinder which is for AHSP) along with 1-2 additional pumps. Now I am really confused 😩
3. No. Would this mean I would need to change my controller/thermostats? I am currently using Heatmiser for programming the times the HW, CH and UFH comes on/off, as well as setting the temperature for the CH and UFH. 

 

Yup, it's a bit of a sh*t show.

 

Do the meaurements I suggested so that we know flow and return temperature rise over time and can then assess differential within each circuit.

[HughF]

I think Szymon from Urban Plumbers would cover your way. He'll get this working properly...

[windsor-tg]

thanks @HughF. I just spoke with Szymon; very knowlegable and super busy. From what I described to him, he said that I need a full weather compensation setup with Vaillant controls. Essentially, I need hydraulic seperation (which can be done multiple ways) along with the Vaillant controls so that the Vaiilant boiler can be properly modulated. This, he said, is not cheap 🥴.

 

I will proceed with what @SimonD has suggested but it does appear that I need a hydraulic seperation in place

[TonyT]

1 hour ago, HughF said:

I think Szymon from Urban Plumbers would cover your way. He'll get this working properly...

100%. Very knowledgeable chap comes across well on his videos 

[windsor-tg]

16 hours ago, SimonD said:

 

Yup, it's a bit of a sh*t show.

 

Do the meaurements I suggested so that we know flow and return temperature rise over time and can then assess differential within each circuit.

sorry @SimonD, back to the one of the questions you asked me "Have any of these engineers mentioned that it's the controllers that govern boiler modulation rather than the boiler itself? E.g. the Vaillant VRC 400 weather compensating control for Ecotec boilers that adjusts flow temperatures according to a heat curve and heat demand? " - only the guy, Szymon from Urban Plumbers, who I spoke to yesterday mentioned the Vaillant Weather compensating control and installing all Vaillant controls. Only Vaillant controls can fully modulate the Vaillant boiler, no non-Vaillant controls can, he said.

 

When I spoke with Vaillant technical team yesterday, I do not recall them mentioning I needed a weather compensating control as well as Vaillant controls. 

 

I had another engineer come over last night (recommended to me by a friend). He is going to come back next week to carry out a heat loss calculation (finally!). From what I descibed to him, he said a low loss header is needed, and was very confident that assuming the heat exchanger on the boilre is not blocked, this will solve the problem. 

 

The weather looks to be getting cooler next week so I'll carry out those measurements. 

[JohnMo]

15 hours ago, windsor-tg said:

does appear that I need a hydraulic seperation in place

 

 

You were sure you needed a new boiler at the start of this thread.

 

Many ways to skin a cat, LLH or 4 port buffer is one - they are the same thing, one just a bigger volume than the other), 2 port buffer or volumisers are another.

 

Before you commit to a LLH,  have read up on them, heat geek is a good place to start. Unless designed and installed really well can be less efficient.

 

I too would install weather compensation, but suspect your head and wallet have already gone down the multiple zones and thermostatic valves route.

 

Must say you are asking a lot of questions to a lot of people, I will get my coat at this point, and leave you to it.

[windsor-tg]

37 minutes ago, JohnMo said:

You were sure you needed a new boiler at the start of this thread  --> Answer: based on advice from accredited installers

 

Many ways to skin a cat, LLH or 4 port buffer is one - they are the same thing, one just a bigger volume than the other), 2 port buffer or volumisers are another.

 

Before you commit to a LLH,  have read up on them, heat geek is a good place to start. Unless designed and installed really well can be less efficient.  --> Answer: will do

 

I too would install weather compensation, but suspect your head and wallet have already gone down the multiple zones and thermostatic valves route.  --> Answer: not advised by many including Vaillant technical (experts?) team

 

Must say you are asking a lot of questions to a lot of people, I will get my coat at this point, and leave you to it.  --> Answer: it's the same question "why is my boiler cycling when the UFH is only on? What is the solution". 

 

[TonyT]

Weather compensation all the way!

otherwise it’s just engineers who don’t want to invest time in setting up/commissioning boilers as they will get less call backs

[windsor-tg]

@SimonD - so here are the readings below (I also attached a file showing these readings incase the image below did not come through) . As well as taking the flow & return readings from the boiler, I also took readings from the UFH manifold. The blending valve on the UFH is set around 47 degrees.

 

I took the readings across 5 minute intervals. What I found is that when I had both, CH and UFH turned on, the boiler was happily firing (without any sound of kettling which there was earlier in the week). The flow meters on the UFH was showing 2.5 (range 0-5). When I turned off the UFH, the flow meters on the UFH went to 5 and these flow meters stayed on 5 when I turned on the UFH and turned off the CH. 

 

After about 10 minutes of the UFH only being on, I could see the boiler short cyling (every 2 mins) and the boiler flow temperature would rocket from 45 to 72 degrees. I hope this helps but happy to do some further tests to get to the root cause of the problem. 

 

[JohnMo]

Simple fix a 2 port buffer of around 25 to 50 L capacity (closer to 50L is better), across the flow and return loop to the boiler or across the flow and return from the UFH, where ever is easiest to install. No need for low loss headers or 4 port buffers and additional pump (s).

 

Parts list

Two Tee pieces,

A buffer or volumiser https://www.theplumbshop.co.uk/products/expansion-vessels-intermediate-buffer, these are cheap enough, you need to insulate yourself.

Two isolation valves

 

[SimonD]

5 hours ago, windsor-tg said:

When I turned off the UFH, the flow meters on the UFH went to 5 and these flow meters stayed on 5 when I turned on the UFH and turned off the CH. 

 

This is a curious thing. Do you have any of the design documentation for the ufh at all? This may have some info on design flow rate for your particular installation. Also, what wiring/control centre does your ufh use? What brand ufh is it?

 

If you run the ufh on its own from a completely cold system, what happens?

 

I'm certainly not inclined to suggest going to the expense of any buffer - plenty of these kinds of systems work just fine without them. Given that you've nearly got 30C flow return differential, you're on the brink of an error code so to meit points to a flow problem.

 

My inclination would be go back to the basics, which is to make sure the ufh has been installed correctly - that all the plumbing bits are where they should be and functioning as they should and the same for all the related electrics and controls. Even really simple stuff like making sure the actuators are really working. Then also make sure it's balanced.

 

UFH systems can behave really strangely if just one thing is off. I recently 'fixed' a whole house system in a relatively new build where the ufh was constantly calling for heat and bringing some rooms up to 38C even with the system set to summer mode, meaning it shouldn't be on at all. I noticed that about 4 actuators had popped off their clips which had broken. Manufacturer tech support said I should test all the room thermostats within the wiring centres etc.to make sure there weren't any errant voltages but I decided it best to get the actuators back in place first and then see what happened - as soon as they were back in place, the whole system worked again as it should (which logically shouldn't have been the solution).

 

None of what is going on suggests a blocked heat exchanger or a problem with your boiler, it's somewhere outside in the heating system. But it may be amplified due to a high output boiler with relay control meaning there's no modulation.

[JohnMo]

@SimonDThere are no actuators. At the start of the thread there is a photo of the manifold. Still don't understand how a pump that size (25-80) is expected to cope with only two circuits of UFH pipe, with a demand flow rate of 5l/min, without causing a bucket load of flow related issues.

 

 

Edited September 15, 2023 by JohnMo

[SimonD]

12 minutes ago, JohnMo said:

@SimonDThere are no actuators. At the start of the thread there is a photo of the manifold. Still don't understand how a pump that size (25-80) is expected to cope with only two circuits of UFH pipe, with a demand flow rate of 5l/min, without causing a bucket load of flow related issues.

 

 

 

Well, with the actuators at least that's one potential issue out of the way! Regarding the pump, as I've learned, it's unfortunately a pretty standard 'answer' lots of heating engineers use to 'resolve' or 'overcome' flow issues in heating systems! "Oh, your radiators aren't getting hot, you need a bigger pump"....Makes you wonder, doesn't it 😊

[JohnMo]

2 minutes ago, SimonD said:

Makes you wonder, doesn't it 😊

Just looked at my boiler pump (32kW) and it max head is half and flow at max head is half the one installed.  

[Lofty718]

You could acheive hydrauilic seperation by using closed couple tees under the boiler, this is what I have on my Vaillant

[JohnMo]

He could, but not sure it would change or stop short cycling when only running UFH. He needs water capacity, to dump heat into, which is what's missing.

[windsor-tg]

Thanks @JohnMo, @SimonD, @Lofty718. I have just come back from an evening out and gone through your responses.

 

@SimonD - I do not have any design information of my UFH. The UFH pipework was layed in the screed by by builder and his heating guy installed the manifold, etc relating to the UFH. The UFH system is unbranded and I use Heatmiser as the main control for the radiator & UFH system. When I run the UFH system on its own from cold, it heats up but then starts cycling. With just the UFH on, I can take the same type of readings and report back if that helps?

 

@Lofty718 - I have read elsewhere about close coupled tees. Why did you use them? Can you send a photo of this?

 

From what I have reported, is there anything else I can do and then report back? I have an heating engineer coming back on Wednesday to run some tests. Are there some tests I can make him do and/or get him to check things and report back? With the boiler making kettling sounds earlier this week, I think the boiler is probably coming to the end of its life but I want to make sure I can somehow cure this problem with the short cycling as well as replace the boiler. 

[windsor-tg]

@SimonD - I decided to run a fresh test today and the readings are shown below. The boiler temp is set at 65 degrees. When the UFH is off, the flow meters are 0 so I must have taken a wrong reading yesterday. Also, on T+40, the flow temp on UFH was either 40 or 50. I wrote down 50 at the time but I think that was wrong and it should have possibly been 40. 

 

Are the readings still strange? No kettling sound from the boiler when I was running these test readings. 

 

I have also attached photos of the main pump and UFH pump when both, the CH and UFH are on

 

[Adsibob]

On 03/09/2023 at 09:16, JohnMo said:

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

A Veissman might. Depends how big those UFH loops are, but a Veissman can modulate down to 1/17th of its maximum output. I have a Veissman and we don’t have short cycling issues - we do however have a low loss header installed.

[windsor-tg]

53 minutes ago, Adsibob said:

A Veissman might. Depends how big those UFH loops are, but a Veissman can modulate down to 1/17th of its maximum output. I have a Veissman and we don’t have short cycling issues - we do however have a low loss header installed.

Yes, I've heard that a Veismman cam modulate down a lot further but none of the geating engineers I have spoken to have suggested a Veissman