Conflicting Opinions - a Low Loss Header or not?

[mysteryman]

Mrmethane is missing the whole point of weather compensation. You don't reduce the flow rate and increase the Delta T to get a lower return temperature. You reduce the flow temperature by modulating the burner output. You get full condensation on natural gas when the flow temp is below 53C. You do not need a low loss header to achieve this. Yours is not an unusually large system. Don't over-specify - keep it simple!

 

[heatingman]

You have a very old system with unknown piping in a selection of materials which is not a good start in life for a modern boiler. The system works satisfactorily with the present boiler (good flow rate h/ex)

Install a plate heat ex to protect your new boiler and to maintain adequate flow rates on both sides. It will still W/C.

Can't really comment on cost of LLH but it does sound a bit like the "bend over and tell me when it hurts" pricing method.

 

[Mrmethane]

Mmmm

I dont remember saying anything about weather compensation, or did I, I don't recall.
The problem with this forum is you get advice from people who really don't understand heating systems, flow rates etc etc.
Mysteryman is right about one thing your system is not too large, however a heat load of 30kw is not small either.
And yes low less headers are uncommon in domestic heating systems but should be introduced into the larger homes as they are in commercial applications.
Also the concept of condensing boilers is misunderstood causing confusion and incorrect installation of the appliances.
Let us ask mysteryman to explain the concept of condensing appliances and also tell us what the flow rate should be around a heating system supplying 30kw of heat to radiators. It could be in litres per second or minutes, but we need to know what it is, also what happens to that flow rate has trvs start to close.

 

[ajrobb]

The design flow rate for your radiators will typically be for an 11°C drop from 75°C to 64°C (non-condensing), averaging about 50°C above room temperature (some of your old radiators may be designed for 85°C flow):

30 kW ÷ 11 °C ÷ 4.1 kJ/litre/°C × 60 s/min = 40 litre/min

If you are prepared to de-rate your radiators for a 20°C drop from 75°C to 55°C (barely condensing), averaging about 45°C above room temperature, giving about 27 kW (30 * 45/50):

27 ÷ 20 ÷ 4.1 × 60 = 20 litre/min

If you drop the flow and return temperatures to 70°C and 50°C to get moderate condensing savings, averaging about 40°C above room temperature, giving about 24 kW (30 * 40/50):

24 ÷ 20 ÷ 4.1 × 60 = 17.5 litre/min

You have to decide how cold you are prepared to be when we get weather again like last winter. I'd be nervous about dropping the flow below 40 litre/min. That is with all the TRVs open, once the TRVs are at their set point, your flow will drop anyway.

 

[D_Hailsham]

The design flow rate for your radiators will typically be for an 11°C drop from 75°C to 64°C (non-condensing), averaging about 50°C above room temperature (some of your old radiators may be designed for 85°C flow).

It's nothing to do with "design flow". It's just the temperatures used to measure radiator outputs changed in 1997. Up to that date radiator outputs were measured in accordance with British Standards BS3528, which specified a Flow temperature of 90°C, Return of 70°C and Room Temperature of 20°C.

In 1997 BS EN442 replaced BS3528. bringing the UK in line with the rest of Europe. The new standard specifies Flow of 75°C, Return of 65°C (a 10°C differential) and room temp of 20°C.

The effect of this was that, overnight, radiator outputs reduced by 21%. The radiators were exactly the same!

If you are prepared to de-rate your radiators for a 20°C drop from 75°C to 55°C (barely condensing), averaging about 45°C above room temperature, giving about 27 kW (30 * 45/50)

The reduction in rad output is not directly proportional to the change in the difference between the mean rad temperature and the room temperature; it's a logarithmic ratio. (Newton is to blame. )


A radiator set to 75/55/20 will produce 85% of the heat compared to 75/65/20. So your 30kW reduces to 25.5, not 27kW.

If you drop the flow and return temperatures to 70°C and 50°C to get moderate condensing savings, averaging about 40°C above room temperature, giving about 24 kW (30 * 40/50)

For the same reason, this is incorrect. The actual output will be about 21.8kW, not 24kW.

 

[Chris43]

The boiler manufacturer replied to my question about the need for a low loss header.

You might be interested in what was said.

"I believe the reason you have been quoted for a low loss header is because you have a one pipe central heating system. The reason for this is because on a one pipe system the water passes around the system too quickly for the boiler to take a reading of the temperature and can affect the boiler’s performance, and the low loss header slows this down.

For this reason we do not usually advise the use of our boiler on a one pipe system. The main reason is because when they were designed they were only tested on the standard 2 pipe systems therefore we can not guarantee how well the boiler will perform when used on a one pipe system."

Seems to me that if the water temperature was fluctuating widely the boiler might have trouble if the flow was faster, but why should it fluctuate so much? Also I think the disclaimer is ludicrous - and not mentioned in their sales literature.

 

[D_Hailsham]

"I believe the reason you have been quoted for a low loss header is because you have a one pipe central heating system. The reason for this is because on a one pipe system the water passes around the system too quickly for the boiler to take a reading of the temperature and can affect the boiler’s performance, and the low loss header slows this down.

The flow rate through the boiler will be exactly the same. It is determined by the temperature drop across the boiler, which will not change just because it is a single pipe system.

The problem with single pipe systems is that they are more complicated to design properly. This is because the flow and return temperatures vary from one rad to the next, which has to be taken into account when sizing the radiators.

 

[ajrobb]

The actual output will be about 21.8kW, not 24kW.

Thanks for correcting my rough estimates. The point is, we both agree that output will be reduced with flow temperature and flow rate (as ΔT increases). I used 11°C (20°F) and 20°C as they are often stated as target ΔT values for radiator balancing.

While radiator output is slightly more dependent on flow temperature than on return temperature and boiler condensing efficiency is somewhat more dependent on return temperature than flow temperature. Both flow and return temperatures affect both radiator output and boiler efficiency. To get hung up about having a ΔT of exactly 20°C at the boiler is missing the bigger picture; it is somewhat more efficient to modulate flow temperature with boiler output than it is to have a fixed flow temperature.

For someone with gas bills of well over £1,000/annum, it may be worth investing in a system that automatically modulates CH flow temperature, like weather compensation for instance but there are other controller types.

 

[Agile]

There is little point in putting forward examples of temperatures and flows that are not appropriate to the condensing boiler that the OP here needs.

I dont see anywhere that th OP has said that he has a one pipe system although I may have missed that.

Odd though that he seems to accept the "suggestion" that its a one pipe from someone who has not seen it either.

To come to any sensible conclusion a firm understanding is needed of the existing installation as esisting and making wild guesses and assumptions does not help at all.

Tony

 

[Chris43]

Thanks D_Hailsham for your reply.

So basically what the man from the boiler manufacturer was rubbish.

Where is one to go for sound and professional advice? The so called professionals that have quoted appear to be at odds, and I have had at least 4 different views expressed on the forum by various people in response to my original question.

The issue seems to me to be quite simple. I'm happy that a 30kw boiler will do the job, and the radiators and pipework have,, with the old and innefficient Ideal Type 2 CF120 boiler kept the house at an adequate level.

However, because the pipework and radiators are old there is concern about whether the boiler will:
a) have problems because of dirt, and
b) not deliver all the potential fuel savings because of poorly controlled flow and return temperatures.

Three options appear to be possible:
1. Do nothing, just connect it up, with a filter, of course.
2. Use a LLH
3. Use a plate heat exchanger

Probably 2 and 3 are about the same cost, but 3 will totally isolate the boiler and so eliminate problem a). Do both solutions 2 and 3 offer a solution to problem b)?

We are a retired couple, and are glad that this problem arose after the worst of the winter, although our electricity bill will be fairly high, I suspect.

We don't want to risk a winter heating failure such as those that I have read about when a major component fails, so are happy to spend some money on a sensible insurance policy.

Once again, non technical answers are requested. I already know probably more than I ever wanted to know about a heating system - all I wanted to do was pay the bill, have confidence that it is a reliable system, and get the house warm!

 

[ajrobb]

However, because the pipework and radiators are old there is concern about whether the boiler will:
a) have problems because of dirt, and
b) not deliver all the potential fuel savings because of poorly controlled flow and return temperatures.

You missed one, a sealed system's extra pressure may cause your old pipes and radiators to leak, so you might want to keep the header tank for the radiators. (If you already have any leaks, the header tank will be compensating.)

 

[Chris43]

I wasn't planning on changing the system to a closed one. The plan is for it to stay an open system. Is that OK with all of the 3 options?

 

[D_Hailsham]

I don't see anywhere that the OP has said that he has a one pipe system although I may have missed that.

Too true!

See //www.diynot.com/forums/viewtopic.php?p=1937062#1937062 second paragraph. The fact that some of the pipes are iron made me ask the question.

 

[ajrobb]

I wasn't planning on changing the system to a closed one. The plan is for it to stay an open system. Is that OK with all of the 3 options?

With a plate heat exchanger you'll probably have two independent circuits; the original radiators on the open-vented circuit and the boiler together with the hot water cylinder on a new sealed circuit.

 

[D_Hailsham]

Three options appear to be possible:
1. Do nothing, just connect it up, with a filter, of course.
2. Use a LLH
3. Use a plate heat exchanger

There may be a fourth option - if it is practically feasible: convert the system to twin pipe, replacing all the iron pipes at the same time.