Vaillant Ecotec Plus 637 or Worcester Bosch 8000 35kW

[windsortg]

I had another heating engineer come over this morning. He examined each of the rooms, radiators, existing pipework and boiler. His recommendation was to go with a Vaillant ecotec plus 430 (30kW) or a Worcester Bosch 8000 35kW boiler. He strongly recommended an aluminium heat exchanger (as opposed to stainless steel) and because I generally heat up the hot water for a couple of hours early morning before the main central heating comes on, he believed a 30kW boiler would be fine.

The H/W does need to be taken into consideration when determining boiler size since it has its own zone value (which the central heating and UFH has too). If a 3-port zone valve was fitted, then the H/W can be ignored when calculating boiler size.

From the detailed examination this engineer carried out and picked out things that others had not (e.g. existing expansion vessel undersized, oversize of existing boiler), I am leaning towards the Vaillant ecotec plus 430

 

[Johntheo5]

Do you mean a stainless steel HEX as opposed to aluminium one?.
If you heat up the HW cylinder a few hours before the CH doesn't that make more of a argument for not needing to take the HW requirement into consideration?.

 

[ScottishGasMan]

Save yourself £100 and get the ecoTEC Plus 424. You've been running at maximum of 24kW for years.

Each engineer has their preference between stainless and aluminium, I prefer a Stainless Heat exchanger. However to go with the 4 series ecoTEC may be a safer option, purely on the fact that you can then keep the larger system pump rather than use the internal pump on the system boiler. Although there's every chance it doesn't need the big pump on a correctly sized boiler, the old 438 needed a lot of pump energy to move the required water through it.

 

[windsortg]

@ScottishGasMan - having had a few heating engineers over, it has been established that a Vaillant ecoTec plus 430 is the right sized boiler for my home. I am about to give the nod to one of the heating engineers to carry out the work as they seemed the most knowledgeable, local, good reviews from their customers, and examined my existing system in detail (yes, I can hear some of you saying that all of this this does not guarantee this engineer to be the right person!). Unlike some of them, he thoroughly recommended a power flush of the system as opposed to a chemical flush that some of the other engineers proposed.

With my existing boiler (Vaillant ecotec plus 438) which is about 13 years old, I found a few years back that I was getting the S.53 status when the central heating was running. I turned down d.0 of the boiler to 24kW which eliminated the S.53 status. My understanding of S.53 is that "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".

When I had the UFH system installed last year in a newlt built orangery room, I found that whenever I tuned on the UFH only, the boiler would continually cycle (modulate) even though the heat demand was enough to warrant continuous firing of the boiler. The way I would only be able to get around this was to turn on the CH so that the boiler would continuously fire and the UFH would remain on. I do not know whether this is the case that the heat demand (when only the UFH was on) is less than the minimum the boiler can deliver.

The other reason I have read where the boiler will keep cycling, is if the water in the system is moving too slow around the heat exchanger which is what the boiler is monitoring to prevent it damaging itself. Before I proceed with changing the boiler, is there a way for the engineer to carry out some tests to ensure there is not poor circulation?

 

[windsortg]

@ScotchGas, @Johntheo5, @ianmcd - I was in discussion with another heating engineer around the issue I have where my existing boiler continuously short cycles when I just have the UFH on. I was under the impression this was done to an issue with the boiler. His response was
that the boiler isn't the issue really, to a short cycling issue, it's how the system is split 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.

He has asked how many loops on the UFH have room thermostats and actuators, and how many radiator have TRV's? He said all make a system smaller. For every 6kW of heat delivered from a boiler, 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.

I mentioned that I do not have any TRVs on the 4 towel radiators, and I have TRVs on all the 17 radiators. I sent him the attached photo of my UFH manifold along with associated components. I am awaiting a response from him.

Would you say I need a buffer cylinder to get round the issue I am having with the UFH when operating on its own?

 

[Johntheo5]

Your UFH has 2 loops, if you assume 45C/37C manifold flow/return temps and each loop circulating 3LPM then the UFH output is 3.35kw and if the boiler flow temp is 70C then the flow rate through the boiler will only be 1.45LPM at a dT of 33C, the dT will probably give a S53 status, if you reduce the boiler flow temp to 60C then the flowrate is 2.09LPM at dT of 23C so probably no S53 but might be difficult for the boiler to get away after cycling as the flowrate of 2.09LPM is quite low and the boiler fires up at ~ 60% (~ 23kw) of its rated output (38kw) despite being range rated to 24kw.
You might note the flowrates of each UFH loop and the manifold flow and return temperatures some time to enable more accurate calculations.

 

[windsortg]

Thanks @Johntheo5. I’m not quite sure what to do next to rectify the issue?

 

[Johntheo5]

You must know the design flow and return temperatures for the UFH and its output and then run it, even with the CH, and note the manifolds flowrate(s) and the manifold flow and return temperatures.

 

[windsortg]

The work was carried out by the builder’s heating engineer so no design work was carried out

 

[Johntheo5]

You can just run it then, the flow temperature, depending on the floor material will be set to a particular temperature with the TMV (thermostatic mixing valve), typicall 35C to 45C, if you do run it, even with some CH, to get it up to temperature then note the flowrates, temperature etc.

Eventually, before you finally select on your new boiler, the minimum output etc can be looked at, some boilers like I think the Viessman Vitodens 200W have a minimum output of 1.9kw, I wouldn't be overly worried even if a cheaper boiler can onl modulate to say 4 or 5kw as long as it can happily cycle. Cycling in itself causes very little loss of efficiency.

 

[windsortg]

Thanks @Johntheo5. Do you think the introduction of a buffer cylinder on the return of the CH circuit will resolve the problem?

The Viessman Vitodens 200W you are suggesting is a system boiler from what I can gather and will have its own internal pump. Would that mean I would need to get rid of the Grundfos 25-80 pump that I only upgraded to last year? Also, the Viessman is around £700-800 more expensive than the Vaillant ecotec plus 430 heat-only boiler I was looking at

 

[Johntheo5]

The return buffer is interesting, I'd like to see a example of one working with the UFH only on as the UFH return temp can be as low as 27C/30C and will still result in a excessive boiler dT without some form of mixing, your engineer will advise, hopefully. The cost of the buffer can then determine if the Viessman is more cost effective but this would still require some form of mixing to keep the boiler dT < 30C.
The minimum output of the Vaillant is ~ 7kw but the return buffer volume will determine the cycling time. There is more control of the boiler flowrates with the heat only boiler (Grundfos 25-80) so overall the Vaillant may be the best choice.

 

[windsortg]

I am just waiting for a heating engineer to come round next week to investigate further on the boiler cycling and boiler options / low loss header. Since yesterday, I could hear my boiler making a strange sound and coincidentally, whilst British Gas were over to carry out their annual boiler service, the noise reoccurred which the British Gas engineer put it down to the boiler 'kettling'.

I think some of you may find this interesting. I had British Gas come over this afternoon to quote for a new boiler. They had a thorough look around as well as do a detailed heat loss calculation using an online product on their iPad as well as taking room size readings, cavity wall insulation, etc. The outcome from the heat loss calculation was a recommendation of a 38-40kW boiler!

Since a Vaillant 38kW cannot be obtained from any merchant that British Gas use, their recommendation is for a 40kW Worcester Bosch boiler. A detailed quote to follow.

Is this a money making scheme for British Gas to exploit or is their heat loss calculation a true reflection on the size of boiler I need?

 

[muggles]

If you've been successfully heating your home with a boiler capped at 24kW there's absolutely no way you need anything bigger than that. I don't know what software BG use to calculate heat loss but it sounds like it's not very accurate. Did you try doing your own using Heat Engineer?

 

[Johntheo5]

The biggest problem with whichever boiler is installed will be running the UFH on its own as its only a 2 loop system but ironically, the easiest to see its heat requirement, all thats needed is the flowrates through the loop flow tubes and the manifold dTs.