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: The thermal store makers claim that a thermal store with two thermostats can supply the necessary trickle of energy without switching on the boiler
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?