Designing gas central heating for DIY maintenance.

[SteamyTea]

6 hours ago, Temp said:

When the service man rocks up he tells you the contract doesn't cover the first power flush

Why is it called a 'power flush'.

Surely this is just bigging up a system flush.

I have a small electric drill, but it is hardly a power tool, it only has 500W of power.

[PeterW]

Power flush units are pretty powerful - 100 litres a minute is not unusual for the pumps. 

[SteamyTea]

3 minutes ago, PeterW said:

Power flush units are pretty powerful

I still think it is just marketing language.

 

100 litres of water costs me about 80 pence.

 

Edited October 4, 2019 by SteamyTea

[ProDave]

10 hours ago, SteamyTea said:

I still think it is just marketing language.

 

100 litres of water costs me about 80 pence.

 

But you won't get that out of your tap at 100 litres per minute, it needs a pump or something.

[SteamyTea]

20 minutes ago, ProDave said:

But you won't get that out of your tap at 100 litres per minute, it needs a pump or something.

Doubt you would get that volume though a normal CH system either.

 

[PeterW]

3 minutes ago, SteamyTea said:

Doubt you would get that volume though a normal CH system either.

 

 

Most of the big power flush units do upward of this - they really do give the system a good high flow flush to get the crud from radiators and pipework. The downside is a lot of modern boiler heat exchangers have very fine galleries and they can block if you're not careful. 

[joe90]

34 minutes ago, PeterW said:

The downside is a lot of modern boiler heat exchangers have very fine galleries and they can block if you're not careful. 

 

A neighbour had a gas boiler replaced on the cheap and didn’t touch radiators or pipework, boiler lasted about 4 months before major problems and engineer (I had to let him in) said just that, needed a new heat exchanger £££ and no chance of flushing it out.

[epsilonGreedy]

1 hour ago, PeterW said:

The downside is a lot of modern boiler heat exchangers have very fine galleries and they can block if you're not careful. 

 

 

58 minutes ago, joe90 said:

A neighbour had a gas boiler replaced on the cheap and didn’t touch radiators or pipework, boiler lasted about 4 months before major problems and engineer (I had to let him in) said just that, needed a new heat exchanger £££ and no chance of flushing it out.

 

 

This is interesting, I have been looking at boilers and wondered how the same boiler chassis/model could be sold with a rated output from 25kw to 38kw. I wonder if they obtain the higher outputs with more finely channeled galleries? 

 

https://www.vaillant.co.uk/homeowners/products/ecotec-plus-combi-832.html

[Jeremy Harris]

1 minute ago, epsilonGreedy said:

 

 

This is interesting, I have been looking at boilers and wondered how the same boiler chassis/model could be sold with a rated output from 25kw to 38kw. I wonder if they obtain the higher outputs with more finely channeled galleries? 

 

https://www.vaillant.co.uk/homeowners/products/ecotec-plus-combi-832.html

 

 

I'm pretty sure it's just different firmware, so the higher rated models just turn things up a bit more.  These are fully modulating boilers, so they vary their heat output on demand anyway.  The boiler we had at the old house was a Vaillant Ecotec Plus.

[epsilonGreedy]

2 minutes ago, JSHarris said:

I'm pretty sure it's just different firmware, so the higher rated models just turn things up a bit more.  These are fully modulating boilers, so they vary their heat output on demand anyway.  The boiler we had at the old house was a Vaillant Ecotec Plus.

 

 

You answered a future question of mine. I noted that some boilers in the Valilant range had a feature quoted as 1:10 modulation. In other threads UFH experts here had warned that poor modulation leads to excess heating cycles hence a gas boiler can wear out prematurely when coupled to UFH.

 

Will a combi boiler with a 1:10 modulation range cope with UFH?

[PeterW]

The chassis has three things swapped usually - heat exchanger, gas valve and system board (or firmware)  That is how you get more gas into a bigger burner and then into the water. 

 

Some also change the burner assembly for one with more rails but that is less common. 

[Jeremy Harris]

14 minutes ago, epsilonGreedy said:

 

You answered a future question of mine. I noted that some boilers in the Valilant range had a feature quoted as 1:10 modulation. In other threads UFH experts here had warned that poor modulation leads to excess heating cycles hence a gas boiler can wear out prematurely when coupled to UFH.

 

Will a combi boiler with a 1:10 modulation range cope with UFH?

 

 

Depends entirely on the heat demand from the UFH.  Worst case for us, in really cold (-10°C) weather, would be a demand from the UFH of around 1.5 kW, so no boiler would really be able to modulate down that low.  Our normal heating demand of maybe 400W or so would be way lower than any boiler could cope with.

 

I believe that the lowest output that the Ecotec Plus range can modulate down to is around 3.5 kW (might be an idea to check with Vaillant if this is accurate - off the top of my head I think it was set by the d0 variable in the set up menu).  That's probably too high for most UFH systems, I suspect, so a buffer tank would be needed to allow the boiler to run well without excessive short cycling (although the Ecotec Plus has an anti-short cycling time delay, so it would just shut down for a while under these conditions).  In general, condensing boilers are a fair bit more efficient if they can run for a reasonable period of time.  Although the spec gives a high efficiency figure of around 89%, this can only be achieved during a long burn.  A series of short burns will bring this down to around 60% or so.

[Onoff]

My oil boiler must have big galleries. Fitted in 98. Never had any form of service that I'm aware of until 2015 when I replaced a few bits. I keep meaning to add inhibitor. Not a bad advert for Warmflow.

 

 

[PeterW]

2 hours ago, Onoff said:

My oil boiler must have big galleries. Fitted in 98. Never had any form of service that I'm aware of until 2015 when I replaced a few bits. I keep meaning to add inhibitor. Not a bad advert for Warmflow.

 

 

It probably doesn’t have any at all ...

 

oil boilers tend to be coils of steel pipe around the burner gallery or just a stack of baffles -  some have the holding tank sat at the top. 

 

3-4mm is not unusual for thickness hence why a long slow burn is better for heating up on an oil boiler. Gas boiler primary exchangers can be less than 1mm thick. 

[Jeremy Harris]

Many years ago we had an oil boiler that was just a heavy annular steel tank, with a spinning disc burner sat in the bottom.  I don't think anything could have ever blocked the heat exchanger in that.  Took a few hours to get hot, though.  Nice and simple, though, nothing much to go wrong.

[SteamyTea]

34 minutes ago, PeterW said:

3-4mm is not unusual for thickness hence why a long slow burn is better for heating up on an oil boiler. Gas boiler primary exchangers can be less than 1mm thick. 

That would only make a few seconds time difference surely.

[epsilonGreedy]

4 hours ago, JSHarris said:

I believe that the lowest output that the Ecotec Plus range can modulate down to is around 3.5 kW (might be an idea to check with Vaillant if this is accurate - off the top of my head I think it was set by the d0 variable in the set up menu).  That's probably too high for most UFH systems, I suspect, so a buffer tank would be needed to allow the boiler to run well without excessive short cycling (although the Ecotec Plus has an anti-short cycling time delay, so it would just shut down for a while under these conditions).

 

 

Is a buffer tank usually sized as a ratio of the water volume in the UFH pipes or is the tank size calculated to give the boiler a long working burn at its minimum modulated rate?

[Jeremy Harris]

The buffer needs to be sized to try and give the boiler as long a burn time as possible, but will always be a compromise, because of the space that's likely to be available.  If you know your average heating demand in winter, then you can work out the buffer tank size needed to run the heating for a given time.

 

For example, a 70 litre buffer, running between a boiler on temperature of 35°C and a boiler off temperature of 65°C, will store around 2.46 kWh of heat energy between those two temperatures, so would be able to run the heating for a bit over an hour at 2 kW between boiler firing periods.  The boiler would be able to recharge the tank pretty quickly - a 25 kW boiler could do this in roughly 6 minutes, but would probably take a fair bit longer as it would almost certainly modulate down a fair bit.  At a guess you might find that the boiler ran for around 10 to 15 minutes every hour.  If just trying to run the UFH loops directly the chances are that it would cut out after a minute or two, due to the limited heat demand from the floor.

 

We have a 70 litre buffer, and that can usually run our UFH for around 4 hours or so, without being re-heated, and we only run the buffer at 40°C, as it's heated by an ASHP.

[epsilonGreedy]

8 minutes ago, JSHarris said:

For example, a 70 litre buffer, running between a boiler on temperature of 35°C and a boiler off temperature of 65°C, will store around 2.46 kWh of heat energy between those two temperatures, so would be able to run the heating for a bit over an hour at 2 kW between boiler firing periods.  The boiler would be able to recharge the tank pretty quickly - a 25 kW boiler could do this in roughly 6 minutes

 

 

Ah ha, reading that led to a major realization. I thought the buffer tank just bulked up the total water volume flowing around the UFH pipes at whatever temperature is right for a particular slab and house heat loss rate. Given the 30 degree operating range of the buffer tank, somewhere there must a device that somehow leaches heat energy from the buffer tank to the circulating UFH water until the buffer tank drops to the base target circulating temperature, about 35(c) in your example.

 

Thinking some more about this, even when short cycling is happening in the absence of a buffer tank, the operating temp of the circulating water in the UFH pipes must be a bit of thermal roller coaster.

[PeterW]

6 minutes ago, epsilonGreedy said:

Thinking some more about this, even when short cycling is happening in the absence of a buffer tank, the operating temp of the circulating water in the UFH pipes must be a bit of thermal roller coaster.

 

Nope... stays at 33c in and about 17c out when it first kicks in ... manifold blending valve basically takes the water at whatever temperature it comes in at from the buffer and blends with the return flow water. Excess flow goes back to the buffer tank, and over a period the amount of hot drawn from the buffer reduces. 

[Jeremy Harris]

No devices needed, the buffer just has a flow and return from the boiler and a flow and return to the UFH.  The UFH draws heat from the buffer at a fairly steady rate, depending on the heating demand and the boiler tops the buffer up as needed.

[PeterW]

 

@epsilonGreedy this is the current setup I’ve installed (minus the coil in the buffer) 

 

It is wired as W-Plan so the hot water always has priority if both come on together, but the buffer holds enough for around 60-70 mins of running and the thermostat which is 2/3rd down the buffer tank is what controls the ASHP in heating mode. 

[epsilonGreedy]

4 hours ago, JSHarris said:

No devices needed, the buffer just has a flow and return from the boiler and a flow and return to the UFH.  The UFH draws heat from the buffer at a fairly steady rate, depending on the heating demand and the boiler tops the buffer up as needed.

 

 

Ok though I am struggling to picture this drawing off process probably because I know nothing about mixing valves. EDIT: I just reread the @PeterWpost 3 above... starting to make sense.

 

When the buffer tank is at 60(c), the house and slab are at ideal temp and the outside temp is 8 degrees, the there will be little draw from the buffer tank.

 

I need the Janet & John book on how UFH works. This FAQ is useful for a beginner:

 

https://www.underfloorheatingsystems.co.uk/technical-information/questions-and-answers/ 

Edited October 5, 2019 by epsilonGreedy

[epsilonGreedy]

4 hours ago, PeterW said:

 

@epsilonGreedy this is the current setup I’ve installed (minus the coil in the buffer) 

 

 

Is that because the output temp of an ASHP is close to the ideal input circulation temp of UFH pipes? 

[PeterW]

55 minutes ago, epsilonGreedy said:

 

Is that because the output temp of an ASHP is close to the ideal input circulation temp of UFH pipes? 

 

No it’s because the tank I was supplied with had no coil in it .....

 

it basically works as a low loss header, just a 65 litre one ...! It does mean the tank plus UFH all get circulated via the ASHP so having an element of antifreeze is key