Central Heating Design - Help Please

Hi All

I am looking at an upgrade of central heating system to a 4 bed detached house (extended 3 bed). I am sure there will be numerous questions, but the first relates to boiler type and temp drop.

Looking at the rads currently installed (14 IIRC and mostly fairly new), they generally look up to the job if I work on an average surface temperature of 65 deg C. This is based on 70 flow, 60 return 20 room temp and a reduction factor from Ideal Standard catalogue of 0.7 for 45 deg room/rad temp difference.

Sources I have seen state that an existing pumped "S" plan system should be fitted with a boiler to give 82 deg flow and 70 return - 76 mean temp. This seems frighteningly hot for a house with children in. Also it stated that there is no mileage in fitting a condensing boiler into a system to run at these temperatures as it won't condense and therefore will be no more efficient than a non-condensing model.

My questions are:

What actually determines temp drop selected? If I consider condensing boilers, the temps stated are 70 flow 50 return - why 20 deg? The condensing option gives mean temp 60 which puts most rads under size! So if I run at my intermediate temperature of 65 deg C average, will the boiler condensing action work at all? Or should I just forget condensing and go for bog standard unit?

TIA

Phil

Reply to
TheScullster
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82C is the recommended temp. Low Surface Temperature rads are available.

Old wives tales. A condenser is far more efficient than a regular as it has a larger heat exchanger. BY April all boilers sold will be condensesers, so not installing one may be foolish for many reasons.

The larger the temp drop the more heat is being dumped into the house, hence more efficient. Some condensers go to 22, 23, 24C temp differential. They limiting the differential because of domestic heating requirements and design the heat exchanger to suit, without over engineering it.

A condensers does not have an efficiency switch which miraculously is switched on, and then condensing and wonderful efficiency occurs.

The better condensing boilers have load compensating control which detects the heat load of the house and modulates the burner up or down to achieve the house temp and the lowest return temp for max efficiency.

Get a good condensing boiler: Worcester Bosch Greenstar, Glow Worm, Keston, etc.

If you are updating the system consider a heat bank. explanatiion:

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makers make them too, not just this company.

Reply to
IMM

The key factor is the Mean Water to Air Temperature.

The order to work in is first to calculate the heat losses to determine the required radiator outputs.

Then decide if you are going for a conventional or condensing boiler. If it is conventional, you design around 82 flow 70 return, giving MWT of 76 degrees. If you assume you run the rooms at 21 degrees it gives an MWTA of 55 degrees.

Look at the manufacturer's data sheet for your chosen radiators and look up the correction factor for this. Usually it is 0.89 for this MWTA.

You then take your heat loss number, divide by the factor to determine the nominal radiator output at its test temperature and select your radiator(s) for the room.

The principle is the same for condensing boilers. With a new design, it is conventional to go for 70/50 where the derating factor is normally 0.6 implying bigger radiators.

With a conventional boiler you must run at at 82/70 or the heat exchanger and other components are likely to suffer condensation on them and corrode

There are two views on that.

You can run the radiators like this as millions do. The kid might touch one once. It will hurt them a little and they won't do it again. Alternatively you can fit low surface temperature radiators where the hottest parts are encased. They usually look rather institutional IMHO.

This isn't true - there are two misconceptions.

- Heat exchangers in condensing boilers are usually larger than those in conventional ones and run more efficiently anyway at a given temperature

- There isn't a Nirvana which is reached when condensing boilers reach the condensing temperature. The reality is that efficiency increases as return temperature decreases, and it does so more rapidly below the condensing dew point. So you don't halve your gas bill at 51 degrees vs. 56 degrees.

It's basically what the boiler can manage. Conventional boilers are designed specifically to operate at 82/70. Condensing boilers heat exchangers will have a maximum temperature drop that they can manage -

20 is typical, some will do 25. That is really the starting point for the design..

'cos that's the drop that condensing boilers will do. You could run at 60/40 or even 50/30 if you wanted but the radiators get quite big then. Remember that these figures are to provide enough heat for the worst case in the winter. Typically the boiler will modulate right down and drop the radiator temperatures to as little as 40 degrees or even less when little heat is needed.

No, see above regarding radiator sizing.

With a new system you can size for 70/50. Equally with existing radiators or if you want to, you can design around 82/70 if you like. The condensing boiler will reach those temperatures simply because the manufacturers want to address the replacement market.

All that happens is that when it's really cold the boiler will run up to these temperatures, still more efficiently than a standard boiler but will drop down to lower condensing temperatures when it can.

If you *can* go for 70/50 radiator design and condensing boiler this is undoubtedly the best route, but it's probably not worth replacing all radiators in a house to do it - rather just fit the condensing boiler keeping the 82/70 assumption.

When I did mine, I was able to change economically to a 70/50 design by doing a few radiator swaps because some were way oversized to begin with, I could move others and I replaced three or four.

So don't get fixated on whether the boiler reaches condensing mode

*all* the time, it's worth fitting one anyway even if you do nothing to the radiators.

.andy

To email, substitute .nospam with .gl

Reply to
Andy Hall

Not so. Some look like normal rads.

Reply to
IMM

I'm very disappointed with your response, IMM!

The *only* thing you could find to argue about in Andy Hall's extremely well reasoned reply was whether or not low surface temp rads look institutional.

Surely you can do better than that? Whatever happened to the combi (preferably two of them) solution?

Reply to
Set Square

institutional.

He already has an existing system. With two combi's he will need to split it to separet looops of one upstairs and one down. It may be simple to do and worth looking into. If going two combi's he could have the downstairs and the most used taps and shower on a condensing combi. A cheaper regular combi does upstairs and the least used taps/shower. Combine the two combi outlets to fill the bath. Simple, very effective and cost effective too.

There you are.

Reply to
IMM

I don't argue, I put them right.

Reply to
IMM

Ah that's better - I can sleep in peace again now!

Reply to
Set Square

That's not how 99.99% of the contributors to this NG see it!

I suppose you *could* be the only one in step.

Reply to
Set Square

You have it right. 99.99% of the people here are inexperienced amateurs.

Reply to
IMM

Glad to help.

Reply to
IMM

Better an inexperienced amateur than an anonymous wanker.

Reply to
Huge

That I agree with.

Reply to
IMM

Thanks to all for considered and informative responses (particularly Andy Hall again) No doubt this question is the start of what will be an avalanche when I really get into the vagueries of the existing system.

Phil

Reply to
TheScullster

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