Okay all........having read up a little on condensing boilers and seen all the blurb on how fantastic they are   , do they stand up to scrutiny in the real world? Manufacturers and Mr Brown are pushing these boilers in a big way (I know they are now effectively compulsory for new build or replacement) but do they really work and make a huge difference?
As I understand, in order to work properly they need to have a return water temp  below about 55c. Does this occur in most heating systems?
Don't most systems have a flow of about 75c and a return of about 65c?
Therefore, the condensing boiler won't be working at it's optimum and all the figures produced on efficiency and cost saving are called in to question. Therefore, a condensing boiler may only be marginally more efficient than a convention boiler on many domestic systems in use today and the cost of a new condensing boiler may actually not be as cost effective as claimed both in terms of hard cash and damage to the enviroment on macro scale since they doubtless require more energy to produce. So, are they just a good marketing exercise?

Part of my interest has been driven by the fact that we have UFH downstairs and rads upstairs and what the best way to connect it all up is. Should a thermal store/heat bank be used as a "buffer" or not.???

Over to you................

I'm building a new house that will use UFH on both floors. I must have talked to 20 different companies about UFH systems and the need or not for a thermal store. Never managed to get a convincing answer either way. Worse - I was surprised at the lack of technical knowledge most installers had. A surprisingly large number didn't understand the arguments put forward by the companies that make the thermal stores. Is it any wonder so few recommend them.

My understanding is that the return temperature from UFH is low enough that your boiler will operate in condensing mode. In fact I've even been told that you MUST use a condensing boiler because an ordinary non-condensing boiler would be damaged by condensation products that are acidic. Aparently condensing boilers use stainless steel heat exchangers to prevent this where as non-condensing types rely on the exhaust gasses not condensing. Is this a real issue or not? I don't know.

The main advantage claimed for a thermal store seems to be reduced boiler cycling. The water temperature coming out of the boiler is normally too hot to feed directly into the UFH loop. In systems that don't use a store a mixer is used to recycle some of the hot water with cooler water returning from the loop. The thermal store manufacturers claim that in mild weather when only a little heat is required most of the water is recycled in this way. This means that the flow through the boiler is reduced. To heat a small amount of water the boiler only needs to come on for a short period and then go off again. This rapid cycling is claimed to reduce the efficiency of the boiler. The thermal store makers claim that a thermal store with two thermostats can supply the necessary trickle of energy without switching on the boiler. They allow the temperature of the store to drop until it hits the lower thermostat limit and only then does the boiler switch on. The boiler then continues to heat the store until the upper limit is reached. Instead of the boiler cycling the temperature in the store goes up and down. The reduction in boiler cycling is claimed to save money. Is boiler cycling a real problem? I don't know.  

What surprises me most is the cost of thermal stores. They shouldn't really cost much more than a regular insulated hot water cylinder but they seem to.

Are thermal stores an expensive cure for a non-existant problem? If you find out do let us all know!

I agree that with UFH a condensing boiler is the way ahead. You can have a reduced temp flow and return. Where a mix valve uses the return flow to reduce the temp going into the UFH circuit it means that the return to the boiler will be within the condensing temp range required.

Do you need a thermal store or heat bank? Well..........as you said BtK, this will depend on a number of factors. For example, does your UFH cover a large area, do the zones have the same heat up time and how long do they take to heat up? This will lead to whether the return flow temp to the boiler will be below about 55c and therefore keep the boiler in the condensing range for a sufficient period of time.

With only UFH throughout the house I'd go for an unvented cylinder with a boiler flow temp of about 65c. Simpler system and maintains the cylinder above 60c whilst not having an excessively hot flow temp for UFH. The area of floor to be heated will be large enough to ensure that the mix valve will lead to a return boiler temp of below 55c.
Over on the 'other' forum someone has said that Viesmann recommend a flow temp of 40c and a return of 30c? I think I must be missing something but how does it heat the hot water cylinder upto 65-75c?


However, I think that if you have both UFH and rads then you have two options:
1. Accept that when only the rads are working the boiler will be in and out of condensing mode.
or
2. Design a system that ensures the rad return flow is routed in such a way as to reduce the return flow to the boiler below 55c.

1 is fairly easy but I would suggest that there are a few possibilities as to how to accomplish 2, one of which is to use a thermal store or heat bank and effectively split the system. In this instance, the domestic hot water or space heating controls the store and the store controls the boiler.

A store with an upper and lower stat makes sense. For example, Boiler heats up store to 75c. Space heating or DHW draw reduces the store temp. When temp in store reaches 55c boiler refires and reheats store to 75c.

A clearer explaination is provide here


Quote:

Out of interest who actually manufacturers cylinders like this?

Anyway, the thread seems to have been hijacked successfully . Back to the original point are condensing boilers really all they are cracked up to be if used on the majority of systems in use today i.e with rads?

Anyone else got any comments on UFH or thermal stores or condensing boilers?

[quote author=CWatters  link=1116523572/0#5 date=1116606730]

Chelmer make a "Dual Thermostore" with two thermostats..
http://www.chelmerheating.co.uk/dualts.htm

and for info on condensing boilers with rads see this...

http://www.plumbingpages.com/featurepages/Destroymyth.cfm

Small Quote: The system does not need to be designed to condense to achieve the improved efficiency. End Quote




[/quote]


Fit a Thermal store and pay the price, they are known to sludge up and pump failure