IMM wrote in message ...
"John Aston" wrote in message
.. .
Thanks for your help to date. I've distilled the advice from various
threads
in this newsgroup to come up with a possible heating design for my
house.
The drawing HD01 at http://tinyurl.com/3zv2g shows the proposed
hydraulic
design for a large domestic heating system. The boiler and hot water
cylinder is on the left hand side of the drawing, space heating is on
the
right. (You might need to rotate the view so that the drawing is in
landscape orientation in your browser)
Aiden has highlighted some points, so I will not go over them. Some
observations and Qs:
Firstly, what drawing package did you use, Visio?
CorelDraw 6. Not a great program but I'm used to it.
The boiler appears to be a Veissmann with an in-built outside weather
compensator with the temp sensor in the low loss header. If the
compensator
slope is set to the UFH, it will not be suitable for the rads. You would
require the low loss header to be on the minimum temp that the rads take,
which means a higher temperature for the boiler to operate on making it
less efficient. If you set the compensator slope for the higher temp rads,
each the UFH zone will control itself on its own mixer controls, set to
maximum of 55C.
The compensator slope is set for the rads. The UFH is on all the time with a
flow temp of 55°C maximum, modulated down by the thermostatic mixing valve
on each manifold. The radiator temperature is determined by the outside
temperature.
You could use a dual temp boiler as the Eco-Hometec, or a simple boiler
maintaining a hot low loss header at a high temp. This is what is done
with
non-compensating boilers where the return temp "has" to be high. So, as
you
have it the rads will not go about 55C.
So I will avoid the non-compensating boiler and consider a boiler where the
flow temperature is modulated between 50°C and 70°C, say (this can be
adjusted). Therefore, the radiator flow temperature is between 50°C and
70°C. The thermostatic mixers keep the water through the UFH at 55°C or
less.
snip - understood
Do you still intend to have two stage heating? UFH with rads boosting?
Yiou have 5 UFH ziones. Where will the rads fit in relating to these
zones?
Yes. UFH as the primary heat source, with the rads on when it's cold. The
colder it gets, the warmer the flow through the rads.
There are two UFH manifolds (one for the ground floor, one for first floor +
attic). The rads are in every room. When it's reasonably warm outside, only
the UFH is on. When it gets cold, the rads come on. As it gets colder still,
the temperature in the rads starts to increase.
Here are two situations to demonstrate the weather compensation for my house
at 20°C inside:
When the temperature outside is -3°C, the boiler temperature is 70°C, the
mean water-to-air temperature in the radiators is (almost) 40°C and the
water into the UFH manifold is 55°C.
Under these conditions, a heat loss of 31kW from the house of is met by the
UFH output of 17kW plus the combined radiator output of 14kW.
When the temperature outside rises to +3°C, the boiler temperature falls to
55°C, the mean water-to-air temperature in the radiators is (almost) 25°C
and the water into the UFH manifold is 55°C.
Under these conditions, a heat loss of 24kW from the house is met by the UFH
output of 17kW plus the combined radiator output of 7kW. (I derated the rads
by 50% for a 15° fall in mean water-to-air temperature. I hope that's
suitably conservative.)
This means that the temperature of the water flowing into the radiator is
equal to to the temperature of the water flowing into the UFH when it's
above 3°C outside. I'm hoping, therefore, that most of the time I won't have
two different temperatures serving the two types of heat sources.
Back to having differing temps for rads and UFH. As it is, efficiency is
compromised by the high temp rad circuits. You have an efficient expensive
boiler not performing to maximum potential. Look at the low loss header
on
the diagram. Replace this with a heat bank/thermal store. Off the bottom
UFH section of the thermal store have the UFH circuits. Off the high temp
top section have the rads and DHW. You may want to have three sections:
top
DHW, middle rads, bottom UFH. Then you have all circuits coming into a
neutral point, the heat bank.
Now you have greater control of temperatures, dividing and ruling, which
means the boiler will not be running at too high a temperature to suit
only
the rads compromising efficiency. An outside weather compensator can be
on
the UFH section to keep this part of the store at the ideal high efficient
low temperature and prevent boiler cycling. You may want a compensator on
a
heat bank mid section serving the rads (UFH & rads have different slopes).
Using a heat bank, a far cheaper and simpler boiler may be used.
Having three sections means that in summer, only the DHW top section is
heated, not the whole store, saving on standing losses.
Using a heat bank immersions may be fitted in the different temperature
sections. So, if there is a boiler outage you can run the whole system,
heating and DHW off electricity. You can't do that with a boiler
connected
via a low loss heater.
Towel rails: These can be teed in before the diverter valve at the
boiler,
between valve and pump and direct to the return. They will then work in
summer, but only when the cylinder or heat bank is being re-heated, which
is
fine for summer use. If you take a shower and the boiler kicks in the
re-heat you will find the towels are hot on the rails.
I understand. I guess this assumes that the header has been replaced by the
heat store. (The header would be a very low resistance compared to the towel
rails.)
Accumulator: If they will not sell unless they supply the unvented
cylinder
(they would sell one separately to me), keep a tank in the loft and have a
booster pump serving a heat bank. This is a cheaper an simpler option too.
The tank does not need to be in the loft. It can be anywhere as it is
pumped.
It would be interesting to see what the Yanks say. They don't do thermal
stores in a big way there and thinking tends to be 1950ish, so only regard
what they say as interest only.
Simpler Alternative:
1. Low Temp Circuit: One dedicated condensing boiler serving the UFH only
on a weather compensator. Simple, separate and sorted.
2. High temp circuit. Another boiler using a 3-way diverter valve
serving
DHW and rads. You may want a weather compensator switching the boikler to
give the ideal temp for the rads. When DHW is called the boiler runs up
to
max temp. This is similar to normal domestic setup.
3. A controller staging in the UFH and rads to give precise control of
room
temps. UFH 1st stage.
4. Backup: Now you have heating backup if one boiler drops out.
Electrical backup for DHW.
5. Cold water storage tank instead of an accumulator with a booster pump.
Tank can be fitted anywhere.
6. The cylinder can be:
a) An unvented version,
b) A DHW only heat bank, such the DPS Pandora, which does requires an
overflow so can be fitted anywhere in the house.
You will find that two condensing boilers can be had for less than the
price
of the Viessmann, and lots of change too.
Thank you for taking the time to compose this reply, I appreciate your
thoughts. I believe that I understand the principle of the heat store and
the principle of a modulating boiler. They both have their advantages. I'll
get some prices together and put my best foot forward.