System upgrade - Combi, Unvented Tank or Gravity fed system - HELP!

coffeehound

FreeBear said:

Section62 said: The impetus to replace old boilers is very much sold on the newer ones being more efficient.  But against that you need to consider whether the capital cost of a new boiler (and installation) will be recovered in savings from a more efficient system before it becomes life expired.  Sadly I expect a lot of people have had new boilers thinking it would save them money, but in reality what they have spent in having the replacement will never be recovered by savings in energy use.

Went from an ancient Baxi back boiler with a SAP rating of 65% efficiency to a Viessmann 050 combi (SAP rating 89%). Had been using around 3300kWh per year of gas with the Baxi, and now looking at just 2800kWh. A £20 per year saving - That is going to take a very, very long time to recover the cost of having a new boiler (like, never).

But having replumbed the system and fitted larger radiators, the place is easier to heat. Also in a better position for the day when heat pumps are the only viable option.

I suspect larger radiators and pipework will work against overall efficiency.  It's adding considerable extra thermal mass that must be dragged up to temperature each morning.  Also AIUI the mandatory modern control systems might run the boiler at full pelt during this phase, so potentially further harmiing efficency (I haven't seen proof, but it seems likely that a boiler running at low power will waste less).  Then once setpoint is reached, there will be excess heat left in the circuit.  So running low rad temperatures with the old rads might work out to be more efficient overall?

Albermarle

coffeehound said:

FreeBear said:

Section62 said: The impetus to replace old boilers is very much sold on the newer ones being more efficient.  But against that you need to consider whether the capital cost of a new boiler (and installation) will be recovered in savings from a more efficient system before it becomes life expired.  Sadly I expect a lot of people have had new boilers thinking it would save them money, but in reality what they have spent in having the replacement will never be recovered by savings in energy use.

Went from an ancient Baxi back boiler with a SAP rating of 65% efficiency to a Viessmann 050 combi (SAP rating 89%). Had been using around 3300kWh per year of gas with the Baxi, and now looking at just 2800kWh. A £20 per year saving - That is going to take a very, very long time to recover the cost of having a new boiler (like, never).

But having replumbed the system and fitted larger radiators, the place is easier to heat. Also in a better position for the day when heat pumps are the only viable option.

I suspect larger radiators and pipework will work against overall efficiency.  It's adding considerable extra thermal mass that must be dragged up to temperature each morning.  Also AIUI the mandatory modern control systems might run the boiler at full pelt during this phase, so potentially further harmiing efficency (I haven't seen proof, but it seems likely that a boiler running at low power will waste less).  Then once setpoint is reached, there will be excess heat left in the circuit.  So running low rad temperatures with the old rads might work out to be more efficient overall?

The problem with this ( in bold) is the house would take a long time to heat up and/or may struggle to maintain a comfortable room temperature in the Winter.

Vitor

I doubt you'd get the grant, which is madness, but installing an air sourced heat pump and keeping the system oil burner for the coldest days in the winter would made most sense. It's well known that heat-pumps are only viable in properties designed for 55 degree water in rads/underfloor heating and with extensive insulation. 

FreeBear

coffeehound said:

FreeBear said:

Section62 said: The impetus to replace old boilers is very much sold on the newer ones being more efficient.  But against that you need to consider whether the capital cost of a new boiler (and installation) will be recovered in savings from a more efficient system before it becomes life expired.  Sadly I expect a lot of people have had new boilers thinking it would save them money, but in reality what they have spent in having the replacement will never be recovered by savings in energy use.

Went from an ancient Baxi back boiler with a SAP rating of 65% efficiency to a Viessmann 050 combi (SAP rating 89%). Had been using around 3300kWh per year of gas with the Baxi, and now looking at just 2800kWh. A £20 per year saving - That is going to take a very, very long time to recover the cost of having a new boiler (like, never).

But having replumbed the system and fitted larger radiators, the place is easier to heat. Also in a better position for the day when heat pumps are the only viable option.

I suspect larger radiators and pipework will work against overall efficiency.  It's adding considerable extra thermal mass that must be dragged up to temperature each morning.  Also AIUI the mandatory modern control systems might run the boiler at full pelt during this phase, so potentially further harmiing efficency (I haven't seen proof, but it seems likely that a boiler running at low power will waste less).  Then once setpoint is reached, there will be excess heat left in the circuit.  So running low rad temperatures with the old rads might work out to be more efficient overall?

I think your suspicions are way off. Although my boiler is rated at 24kW for central heating, and I have a total radiator load of ~11kW @ Δt 50°C (70°C flow temp), it has never run at full power. During the initial firing up, it hit ~9.5kW for 5-20 mins before throttling back to 3-5kW. I fitted a heat meter to monitor actual CH power in April just as the heating season wound down. Haven't really had much chance to log data for the really cold days.

Modern controls will throttle back boiler output as the house gets up to temperature. The boiler will also modulate down to keep the water in the system at a set temperature. Both of these go a long way to improving efficiency. Having larger radiators means you can run at lower flow temperatures without compromising the speed at which the house heats up. This equates to a lower return temperature which pushes the boiler further in to condensing mode (crucial for higher efficiency). Having a boiler that can modulate down very low (3kW is very good) means that it won't be short cycling (bad for efficiency) when heat demand is low.

Old installations will have probably been designed based on a flow temperature of 80°C, so the radiators will not perform as well at lower flow temperatures. And with low flow temperatures, you need to pump a larger volume of water around the system to transfer the same amount of heat - Small bore pipes restrict the water flow, so places a limit on pumped volumes.

But some bad news for the OP. There are very few oil fired boilers that will modulate, so it will be a balancing act to find a flow temperature that keeps short cycling to a minimum and still heat the property effectively.

Vitor said:

I doubt you'd get the grant, which is madness, but installing an air sourced heat pump and keeping the oil burner for the coldest days in the winter would made most sense. It's well known that heat-pumps are only viable in properties designed for 55 degree water in rads/underfloor heating and with extensive insulation. 

Your thinking is quite a bit out there. The Boiler Upgrade Scheme is open to anyone replacing a fossil fuel heating system (and that includes electric boilers). Yes, UFH is an ideal companion for heat pumps, but it is not essential, nor do you need a super insulated property. What is essential is that the system is properly specified and set up for the property in question. Done right, with suitably sized radiators, a heat pump can work. Insulation and draught proofing will go a long way in reducing the heat losses, and we should all be striving to do the best we can on that point regardless of the heat source.

If you want other viewpoints on heat pumps, pop over here -> https://forums.moneysavingexpert.com/categories/heat-pumps

coffeehound

^ I think the points about the extra thermal inertia still hold, so not too way off.  I've run three very different properties with LRT regime using the standard rads over the past seven years and all have worked out fine.  As @Albermarle suggests, one property -- an old solid-wall stonebuilt -- took about 10 days to reach thermal equilibrium, but was fine after that.  (It had been stood empty for a couple of weeks in November.)  

One flat had only four rads, all on the small side, but once it was draughtproofed, it never needed to go above 55º peak flow temp over the course of three winters.  Though that was with running the C/H longer hours, and even overnight when near zero outside.  Incidentally a further benefit (of quite a few) with running longer hours is that the boiler (at least the one I tested) will always run at minimum modulation while it is working within the 15º water temperature envelope, compared with the higher outputs when facing a bigger temp difference from setpoint in start-stop operation.

But yes in general, the argument for needing oversize rads in order to run LRT is often over-stated here

Kiran

I used to have an oil combi, other than the old age related issues I never had a problem, recently replaced with a heat only unvented system and tank. Ordinarily if its just 2 people and 1 bathroom etc. then a combi would be your best option. You can however now make use of renewables, I have a solar thermal coil in my tank and we don't use any fuel between around May and October as it generates more than enough hot water. During the winter months my solar PV generates the electricity that the immersion uses to heat the water so I only really use the oil for heating at that point. That is offset by a woodstove too so massively reduces the dependency on oil. If you wanted to go down that type of route you would need a tank to store the hot water in.

As for your pipes buried in the concrete, when they are replaced the pipes need protecting from the concrete, the old school, and still very effective way, is to wrap them in Denso tape. it prevents the corrosion and also stops pin holes forming when the pipes expand and contract.